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Charles2

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What is your local 99% outside design temp? I'm guessing it's in the 20F range give or take a few, but it matters, since the max capacity at 10F is often quite a bit lower than at 25F.
The BBNW tool defaults say Heating 99% Dry Bulb 25°F and Cooling 1% Dry Bulb 92°F.

You can always experiment with turning the strip heat completely off to determine the outdoor temp at which the strip heat is really needed for comfort.
Somewhere I got the idea that these Fujitsus heat so well at low temp that strip heat is not required at all. Is that wrong?

If whole system drop on the air handler is <<0.8" w.c. (should be, if the ducts & filters were designed correctly)
I think maybe not. The filter is 16" x 20", which I'm pretty sure is undersized. Behind the filter the plenum briefly narrows to a 14"x18" oval before assuming the 17"x20" dimensions of the coil. Can we not split the existing system into 2 systems, something like a ton for each floor? Remember that the closet may not be large enough to accommodate a larger R-410A coil.

Often the returns will need some attention when going from a 1 or 2 speed PSC type blower to a modulating or multi-stage ECM (high efficiency) blower motor.
Our current motor is 1 speed. I'm curious about those Tamarack return air pathways you linked. The Tamarack website shows a variety of wall-mounted pathway sizes, but doesn't say how many CFM each accommodates. So how do you choose?

High static pressures can burn out ECM motors pretty quickly
I would trust Nate on that but not so sure about his claim that high blood pressure causes heart attacks. I could see hemorrhagic stroke (what my Dad died of), but not heart attacks.

Are all of the Fujitsu model numbers you gave available under the Rheem brand? I sure wish we could get those SEER and HSPF numbers up more. Recall that my conflicting goals are to minimize carbon footprint and lifetime cost, but maximize comfort and reliability. Which of these goals seem achievable to you now, and which must be sacrificed?
 

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The BBNW tool defaults say Heating 99% Dry Bulb 25°F and Cooling 1% Dry Bulb 92°F.


Somewhere I got the idea that these Fujitsus heat so well at low temp that strip heat is not required at all. Is that wrong?

The Fujitsus (and many cold climate heat pumps with similar "hyper heating" compressor designs) heat great at low temp and maintain (or even increase) their capacity when it drops into single digits F, but if the model is undersized for your design load you'll either have to install some strip heat or bump up to the next larger size. At present your 99% design load is 29KBTU/hr @ +25F, so to fully heat your house with heat pump alone (no strip heat) you have to pick a model that delivers at least 29K @ +25F. The extended temperature capacity tables for the -RGLX series starts on page 21(PDF pagination).

At 70F indoors the 2.5 ton Fujitsu ARU30RGLX puts out 27-28K (see p.24), and would almost certainly make it without strip heat until temps drops below 10F. Note that a -5F it's capacity increases to more than 29K, but your load at -5F is much more than 29K. If you ever see single digits in your neighborhood it's worth installing some strip heat, or moving the ducts into conditioned space which drops the 99% load to 25.7K. With ducts in conditioned space it would only need strip heat to cover the worst hours of the coldest Polar Vortex disturbance cold snaps.

The 2 ton only puts out about 20KBTU/hr @ +25F (p.23) which pretty much covers your post-DER load, but doesn't quite cover the 25.7K load of the house with the ducts moved into conditioned space with the house in it's current condition. Even with ducts in conditioned space it would DEFINITELY need some strip heat to stay comfortable during a Polar Vortex disturbance events pre-DER, but would modulate well and be more comfortable than the 2.5 tonner during the shoulder seasons than the 2.5 tonner. Due to it's better modulation range on the low end delivering much higher average duty cycles, even with the use of some strip heat during bitter weather it's likely to use less electricity for the whole season than the 2.5 tonner. (If only modulation ranges were infinite...)

The filter is 16" x 20", which I'm pretty sure is undersized. Behind the filter the plenum briefly narrows to a 14"x18" oval before assuming the 17"x20" dimensions of the coil. Can we not split the existing system into 2 systems, something like a ton for each floor? Remember that the closet may not be large enough to accommodate a larger R-410A coil.

The mini-duct cassettes aren't using the A-coil form factor. The dimensions of the cabinet of 18 though 30RGLX models we have been discussing are ~12" thick x ~40" wide x 29" long. (See the bottom of p.10, PDF pagination.) Does it fit?

The plenum attachment to the cassette would then be 12" x 40" = 480 square inches, or 3.3 square feet. At the max 800 cfm of the 2 tonner the duct velocity in the plenum would then be 800/3.3= 267 fpm, a very modest velocity that would not impart much static pressure on it's own, and only about 200 fpm for the 1.5 tonner. Even the 2.5 tonner's maximum 1000 cfm only delivers 300 fpm in the plenum. These are all very modest duct velocities.

The 17" x 20" existing plenum is 340 square inches or 2.35 square feet. At 800 cfm that's a duct velocity of 800/2.35= 340 cfm- still no sweat, but at 1000 cfm you're looking at 425 fpm, which is about the maximum you'd want to see in a low-static system (mind you, the RGLX are midstatic, and will have NO problem driving that, assuming duct velocities in the branches aren't higher than that.)

As long as it's a conical taper to the short ovaled out narrow 14" x 18" section it's not going to impart much static pressure. If it's a sharp edged transfer it'll add some, but still not a big deal. That's going to be roughly 1.4-1.5 square feet which at 800 cfm is 570 fpm, but only 430 fpm at 600 cfm. For a short straight section this is not a big deal.

The 16" x 20" filter is 2.2 square feet, so the velocity at the filter at 800 cfm is 365 fpm. That's not a big deal until the filter gets dirty or if you install a MERV 8 or higher 1" thick filter , at which point static pressures induced by the filter go stratospheric. But if you cut into that plenum and installed the ~5" thick pleated media in the 16" x 20" Honeywell Home F1001620/U or similar, going MERV 8-10 or higher isn't going to impart a huge static pressure at <400 fpm, since the total surface area of the filter itself is many times that of a pleated 1" thick filter. According to the short spec on this site MERV11 would impart 0.23" of static pressure @ 500 cfm, which is fine for the RGLX units, but would eat up half the static pressure budget for the low-static 18 RLF (which still might make it, depending on the rest of the duct system.)


Our current motor is 1 speed. I'm curious about those Tamarack return air pathways you linked. The Tamarack website shows a variety of wall-mounted pathway sizes, but doesn't say how many CFM each accommodates. So how do you choose?

Start with how many cfm the supply registers are delivering to the room, and go at least to the next size bigger on the Tamarack unit.

I would trust Nate on that but not so sure about his claim that high blood pressure causes heart attacks. I could see hemorrhagic stroke (what my Dad died of), but not heart attacks.

Note, it's no Dr. Nate, MD- don't take his analogies literally.

Are all of the Fujitsu model numbers you gave available under the Rheem brand? I sure wish we could get those SEER and HSPF numbers up more. Recall that my conflicting goals are to minimize carbon footprint and lifetime cost, but maximize comfort and reliability. Which of these goals seem achievable to you now, and which must be sacrificed?

I'm pretty sure NONE of the Fujitsu model numbers are available under the Rheem nameplate. SFAIK Fujitsu hasn't OEM'ed any of their mini-splits to others, though there was a time when some of the internal component pieces such as variable refrigerant valves, etc were sold to other vendors.
 

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"hyper heating" compressor designs . . . heat great at low temp and maintain (or even increase) their capacity when it drops into single digits F
I assume that the only way it can increase output at a lower temperature is to switch to a different operating mode that has a lower COP? Is the COP still bigger than 1, or is it just saving you the trouble of adding strip heat?

Cheers, Wayne
 

Charles2

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If you ever see single digits in your neighborhood it's worth installing some strip heat
I've lived here for 52 years and the lowest I've ever seen is 5°F. But why not just run space heaters, or the oven, or the wood stove for such extremes? Is there a legal requirement for strip heat?

The dimensions of the cabinet of 18 though 30RGLX models we have been discussing are ~12" thick x ~40" wide x 29" long. (See the bottom of p.10, PDF pagination.) Does it fit?
I think you mean page 9. I'm having trouble visualizing how the unit would be oriented. What I can tell you with certainty is that the closet door opening is 19" wide and 48" tall. The interior of the closet is 29" wide and 32" deep.

Interestingly, I cannot figure out how the existing unit supplies the downstairs because the supply plenum has no ducts coming off of it in the closet. Which means that the downstairs ducts must descend from the attic thru a chase, but I can't imagine where that chase could be located. Which implies that it's impossible to abandon all of the attic ducts unless a separate system is installed downstairs. But I really like the idea of 2 separate systems, because smaller systems are more efficient than larger systems, and because the downstairs and upstairs are so rarely occupied simultaneously.

As long as it's a conical taper to the short ovaled out narrow 14" x 18" section it's not going to impart much static pressure. If it's a sharp edged transfer it'll add some, but still not a big deal.
It's sharp-edged, no taper. Here's a photo:
upload_2021-8-26_21-46-25.jpeg

That's not a big deal until the filter gets dirty or if you install a MERV 8 or higher 1" thick filter
I'm not a fan of high MERV filters. I only want to keep the coil clean, I don't care about allergies. So I don't go higher than MERV 8, 1" pleated.

SFAIK Fujitsu hasn't OEM'ed any of their mini-splits to others
Then I'm confused. I though Fujitsu was partnered with Rheem, like Mitsubishi is partnered with Trane?

Start with how many cfm the supply registers are delivering to the room, and go at least to the next size bigger on the Tamarack unit.
Sorry, I don't understand. Do you know what ELI5 means?
 

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I assume that the only way it can increase output at a lower temperature is to switch to a different operating mode that has a lower COP? Is the COP still bigger than 1, or is it just saving you the trouble of adding strip heat?

Cheers, Wayne

Hyper heating compressors gain both capacity and efficiency at lower temps by using a clever vapor reinjection scheme. With R410A refrigerant with a well designed vapor reinjection compressor the COP is >1 even at -30F (well below the normal rated operating temp).

Just because it's possible to spec this type of heat pump that would cover the load at temperatures much lower than the 99% outside design temp to avoid using strip heat, it's not always the best strategy for saving energy. If the capacity is high enough to cover the load at -5F it has way more capacity than necessary at a 99% design temp of +25F, and even more excess capacity at the average seasonal outside temperature. The modulation range isn't infinite for a reason- at some lower speed the efficiency falls off a cliff. When it's oversized and spending more time cycling on/off than modulating it's speed relative to the load, it's spinning up compressors & blowers multiple times per hour, taking a hit in efficiency with every start cycle.

As a rule of thumb, if the capacity at the local 99% outside design temp is more than 1.5x the heat load at that temp it's already losing seasonal efficiency. Holding the line at 1.2x is better, and may still have enough capacity to fully cover the average cold snap that drops below the 99% design temp. Being undersized at only 0.95x the capacity to cover the 99% load and covering the remaining 0.05x (or more) with strip heat is not an efficiency disaster- most of the heat is still being delivered by the heat pump at a COP better than 1. Most current model Fujitsu mid-static ducted units are running a COP of ~2 at max speed at +5F outdoors, but running closer to 3-3.5 when running at mid to high (but not max) speed at +25F, and 3.5-4 at low to mid speed @ +47F. The COPs & capacities at different speeds and temperatures for the 2 ton ARU24RGLX are on the NEEP page for that unit.

Note that at +47F at it's minimum speed the -24RGLX only has a COP of a bit under 3, whereas when running in it's mid range it's COP is nearly 4, (but delivering a whopping 27KBTU/hr at that mid range, which WAY more heat than the home's load at that that temp.) This is a characteristic of the technology across model sizes, and a reason why they don't just ramp it down further. Going up a size or two to cover extreme cold snap loads without strip heat guarantees that it's never running at highest efficiency during average heating season temperatures due to the fact that it's just cycling on/off at it's minimum speed most of the season, and only rarely modulating in the sweet-spot efficiency range.
 

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I've lived here for 52 years and the lowest I've ever seen is 5°F. But why not just run space heaters, or the oven, or the wood stove for such extremes? Is there a legal requirement for strip heat?

There is often a code requirement that the heating system needs to be able to automatically (= no wood stoves, no plug in space heaters) heat every occupied room to a minimum of 68F indoors at the 99% outside design temp. If the capacity of the heat pump isn't quite covering it, strip heat (or other automatically engaged auxiliary heat) is engaged. This may or may not be a requirement in your local codes. (In NY state there is even a minimum oversizing factor.)

I think you mean page 9. I'm having trouble visualizing how the unit would be oriented. What I can tell you with certainty is that the closet door opening is 19" wide and 48" tall. The interior of the closet is 29" wide and 32" deep.


The Fujitsu ducted cassettes can be oriented in either a horizontal or vertical (upflow only) position. In an upflow orientation it can be pushed back to the back or side wall, but the ___RGLX needs at least 40" of closet width (or depth) to fit, but only takes up 29" of vertical space, with plenty of room for a plenum on top and plenum on the bottom. But with only a 29" x 32" closet footprint even the physically smaller 18RLF won't fit that space.

Interestingly, I cannot figure out how the existing unit supplies the downstairs because the supply plenum has no ducts coming off of it in the closet. Which means that the downstairs ducts must descend from the attic thru a chase, but I can't imagine where that chase could be located. Which implies that it's impossible to abandon all of the attic ducts unless a separate system is installed downstairs. But I really like the idea of 2 separate systems, because smaller systems are more efficient than larger systems, and because the downstairs and upstairs are so rarely occupied simultaneously.
From a total capacity perspective a pair of 3/4 ton ARU9RLFs would cover even your pre-improvement heat loads with margin, but run the zone loads to make sure the top floor wouldn't need to be the 1-tonner. Since this would need modifcations to the duct system it would be pretty straightforward to design for the 0.5" w.c. maximum of the xxRLF series.


I'm not a fan of high MERV filters. I only want to keep the coil clean, I don't care about allergies. So I don't go higher than MERV 8, 1" pleated.

Going with a thicker pleated filter still imparts lower static pressures, and it's possible to get MERV 8 media for any of them. The best thing about it is you only change filters once per year or so. I have the 16" x 28" x 6" AprilAire 2400 on my system, and I have to write the date on a piece of tape the units door or I'll forget, but it went nearly 2 years at one point. The tighter your house, the less dust there will be, and the longer the filters can go (at any MERV rating.)

Then I'm confused. I though Fujitsu was partnered with Rheem, like Mitsubishi is partnered with Trane?

Couldn't find any compact ducted (or ductless) Rheems listed on the NEEP heat pump website (which is fairly comprehensive) but plenty of Fujtisus. Rheem's 2-ton mini-duct cassette is 43" wide, wider than any of the Fujitsus, but pretty much exactly the width of the 2 ton Midea/Carrier. The 2-ton Midea/Carrier 40MBDQ24---3 is good for over 0.6" w.c., and delivers more than 25,000 BTU/hr @ +17F, and like the Fujitsu units can also be mounted in an upflow orientation.

Sorry, I don't understand. Do you know what ELI5 means?

No idea what ELI5 means.

To figure out your max for the room, divide the room load by the total zone load, and multiply by the max cfm of the unit. Eg, say the room's maximum load is 1857 BTU/hr and the total zone load is 10,771 BTU/hr, and you're going with a 9RLF with a max cfm of 353 cfm (per the ARHI Submittal sheet), the room would need it's proportional share of the total flow, which is 353 x 1857/10,771= 61 cfm. The number will be different if a different cassette is used.
 

Charles2

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There is often a code requirement that the heating system needs to be able to automatically (= no wood stoves, no plug in space heaters) heat every occupied room to a minimum of 68F indoors at the 99% outside design temp.
I guess one good reason for that requirement is to keep water pipes from freezing when the house is unoccupied.

But with only a 29" x 32" closet footprint even the physically smaller 18RLF won't fit that space.
I'm confused. What is the width and depth of the system shown in the Berkeley photos you posted?

run the zone loads to make sure the top floor wouldn't need to be the 1-tonner.
According to the BBNW tool, the ground floor has a heating load of 8,752 btuh and a cooling load of 6,495 btuh. The top floor has a heating load of 16,965 btuh and a cooling load of 12,022 btuh. This is assuming all ducts are in conditioned space.

Going with a thicker pleated filter still imparts lower static pressures
I guess if we're going to the trouble of framing new duct soffits, a thicker filter wouldn't be much extra. I've got no problem with embiggening the filter to keep the blower ECM happy.

The tighter your house, the less dust there will be, and the longer the filters can go (at any MERV rating.)
Forgive me, but that strikes me as a trivial improvement. Dust inside houses is composed mostly of dead skin and fabric shedding, possibly including pet dander or wood ash during winter, not particulates from outside. Do you have any data that shows otherwise?

No idea what ELI5 means.
It means "explain like I'm 5" and you did! Actually the BBNW tool calculates the individual room CFM's for you. The problem is that I failed to notice Tamarack's PDF specifications download.
 
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Dana

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I guess one good reason for that requirement is to keep water pipes from freezing when the house is unoccupied.


I'm confused. What is the width and depth of the system shown in the Berkeley photos you posted?

That unit in Berkeley is the -18RLFCD, mounted vertically. The orientation in the picture is the same as in the "Top View" on p.2 of the submittal sheet. The cabinet is about 33" wide, the mounting tabs make it 37" wide, but you also have to add a few inches to accommodate the refrigerant line connections. Figure at least 41" of closet interor space to avoid having to do a "sledgehammer fit". In the picture it's the somewhat thinner center box below the very short supply plenum, above the return plenum with the large grille:

2ffa6e108a7ded9f51130ff14126239b275b1244b7d53138beb63b4182d68f13.jpg


Since it's useful to have service access to the condensate drain and refrigerant lines, where space is ultra-tight it's good to include a narrow side door in that area to be able to get

According to the BBNW tool, the ground floor has a heating load of 8,752 btuh and a cooling load of 6,495 btuh. The top floor has a heating load of 16,965 btuh and a cooling load of 12,022 btuh. This is assuming all ducts are in conditioned space.

The ground floor loads can be handled comfortably by the (narrower) -9RLFCD, which is only about 30" wide (+ 4" for connections). The 12RLFCD is "rated" at 16,000 BTU/hr, but that is the modulation level at which it's efficiency was tested for AHRI, not it's maximum. Running full-out it can deliver 17.6K @ +17F, and would work fine for a load of 17KBTU/hr @ +25F, but would be needing the toaster to fully heat to 70F during cold snaps below 20F or so. The form factor on the 12RLF is the same as the 9RLF, significantly narrower than the 18RLF in the Berkeley house.

Forgive me, but that strikes me as a trivial improvement. Dust inside houses is composed mostly of dead skin and fabric shedding, possibly including pet dander or wood ash during winter, not particulates from outside. Do you have any data that shows otherwise?

Yes those are all common sources of dust, but in an air leaky house you'll have plenty of sticky pollen when things are in bloom, as well as much larger-than-PM 2.5 aerosolized particles, such as mold spores, or even aerosolized clay during windy drought conditions. The tighter the house, the less gets in from the outside, of course.

A wood stove puts out a lot of particulates of various sizes. At low duct velocities even a MERV 13 will grab a large chunk of the PM2.5, but not as much as a HEPA filter. The lower the velocity at the filter the better it works. MERV ratings are all tested at much higher velocities, and a MERV 8 at <200fpm can work as well or better than a MERV 10 at 600fpm. More on static pressures & filter performance here.
 

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Figure at least 41" of closet interor space to avoid having to do a "sledgehammer fit".
In your photo it doesn't look more than 30" wide, but I'll take your word for it. Fortunately it would fit very well downstairs in the kitchen against an exterior wall. Do you suppose it's the sort of thing that contractors around here have ever done? If not, do I really want to be the first? Could something similar be done with Mitsubishi equipment?

The form factor on the 12RLF is the same as the 9RLF, significantly narrower than the 18RLF in the Berkeley house.
All of the units you've been considering are around 20 SEER. Is there no way to get up around 30 SEER?

The lower the velocity at the filter the better it works.
For 10 years I lived in a leaky upstairs apartment where I only used those ultra-cheap, non-pleated fiberglass filters. Surprisingly, when it came time to replace the system, the coil looked perfectly clean. Do you suppose I can get away with that for the new system(s)?

I also have a question about linesets. Smaller systems require smaller diameter linesets. Upstairs we'd be going from 2.5 tons to 1 ton. I don't think it will be possible to replace the existing lineset with a smaller one without tearing into ceiling drywall, which I am loath to do. The ceiling has popcorn which can never be patched invisibly.
 
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Dana

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In your photo it doesn't look more than 30" wide, but I'll take your word for it. Fortunately it would fit very well downstairs in the kitchen against an exterior wall. Do you suppose it's the sort of thing that contractors around here have ever done? If not, do I really want to be the first? Could something similar be done with Mitsubishi equipment?

Like most mini-split compact duct cassettes, Mitsubishi compact duct cassettes all need to be mounted horizontally, not vertically in an upflow orientation. SFAIK only Fujitsu and Midea/Carrier mini-duct cassettes can be mounted vertically like that.

Without knowing exactly where "here" is it's hard to speculate who in your neighborhood would be up to the task. In the Atlanta area I would call Allison Bailes @ Energy Vanguard, who is fully capable of specifying the equipment (including complete Manual-D duct design) and I suspect could steer you toward someone competent to implement their designs. (He has a PhD in physics- I see there is a "Book Dr. Bailes" link at the top of that page, but I think that is for his training courses. Don't try to book him for a splenectomy or anything... ;))

All of the units you've been considering are around 20 SEER. Is there no way to get up around 30 SEER?

With blower motors made of unobtanium and frictionless ducts you might hit SEER 30, but I don't know of any of those for sale in the real world. With the high summertime dew points your area you'll be running it in "DRY" mode some fraction of the time anyway, which could knock 5-10 SEER off any 30 SEER mini-split's "as-used" performance, but much less of a hit on 20 SEER goods.

For 10 years I lived in a leaky upstairs apartment where I only used those ultra-cheap, non-pleated fiberglass filters. Surprisingly, when it came time to replace the system, the coil looked perfectly clean. Do you suppose I can get away with that for the new system(s)?

Yes, you can "get away with" ultra-cheap 1" filters, which many people are forced to do for static pressure reasons in order to make their old ducts work with a low-static cassette. But as a general rule I would normally spec an oversized deeper pleated filter just in case you're not inclined to change them every month.

I also have a question about linesets. Smaller systems require smaller diameter linesets. Upstairs we'd be going from 2.5 tons to 1 ton. I don't think it will be possible to replace the existing lineset with a smaller one without tearing into ceiling drywall, which I am loath to do. The ceiling has popcorn which can never be patched invisibly.

Oversizing a refrigerant line isn't a problem as long as the installer does the math to know whether and how much refrigerant would need to be added to accommodate the higher volume. If the old heat pump is NOT running R410A (if it's more than 15 years old it's probably R22) be strongly advised to change out the linesets no matter what, since the lubricants used with older refrigerants are not compatible with R410A and even small amounts of contamination with those lubricants will degrade the equipment. There are some installers willing to run some type of cleaner through the lines and punt with fingers crossed, but I personally wouldn't risk new equipment on that. YMMV
 

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I would normally spec an oversized deeper pleated filter just in case you're not inclined to change them every month.
Seems to me that the longer a filter goes unchanged, the better it filters. What is the depth of the filter in your Berkeley photo?

If the old heat pump is NOT running R410A (if it's more than 15 years old it's probably R22) be strongly advised to change out the linesets no matter what, since the lubricants used with older refrigerants are not compatible with R410A and even small amounts of contamination with those lubricants will degrade the equipment.
As I stated last month, the current system does run on R22. Seems like somebody would have invented a good cleaning system by now. When R410A is outlawed, will the lineset need to be changed again?

Do you happen to know why a lineset would audibly vibrate in heating mode but not cooling mode?

The BBNW tool calls for minimum possible (3 inch) ducts to 3 out of 5 rooms. That seems awfully small to me, appropriate only for bathrooms. Does it seem small to you?

The BBNW tool says that the bedroom over the garage has a heating load of 8,345 btuh and a cooling load of 4,313 btuh. Do you still think it's a bad idea to give it its own individual 1/2 ton Mitsubishi minisplit? If so, suppose that room's occupant refuses to dress appropriately for the seasons and insists on wearing summer clothes in winter and winter clothes in summer. Could they be satisfied by embiggening the duct diameter to that bedroom?
 
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Dana

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Seems to me that the longer a filter goes unchanged, the better it filters. What is the depth of the filter in your Berkeley photo?

I don't know the particulars about the filter in the Berkeley photo (not my photo, not my project.)

While it's true that the dirtier the filter, the better it filters (particulates above some size), the dirtier it gets the greater the static pressure that it imposes on the system, and the lower the total air flow in the system. Dirty filters are a common reason for air conditioner evaporator coil icing, low efficiency, and lower cooling capacity in cooling systems. Also, filters that are unable to filter out things as tiny as mold spores can grow mold and mildew in the material that it is clogged with, spreading those spores around the house.

As I stated last month, the current system does run on R22. Seems like somebody would have invented a good cleaning system by now. When R410A is outlawed, will the lineset need to be changed again?

I depends on what replaces the R410A. Lubricants compatible with R410A are compatible with R32 (which has only about 1/3t the 100 year global warming potential of R410A, and one of the likely contenders), given that R410A is simply a 50/50 mix of R32 + R125. Originally R32 was the logical higher efficiency choice for replacing R22, but part of the industry balked at the (albeit low) flammability of R32. Mixing it with R125 retained most of the properties as a refrigerant, but also suppresses the flammability.

Do you happen to know why a lineset would audibly vibrate in heating mode but not cooling mode?

Yes.

I suppose you want to know too, eh? Very well...

In heating mode the system is sending hot liquids directly into the indoor coil via the usually fatter "suction line" in a cooling only application and the flow is metered after the coil, sending the post-metering gas down the usually skinnier "liquid line" in a cooling only application. The mass and velocity of what is in each line changes depending on which mode it's operating in, so it's not surprising that it might vibrate more or differently with the flow in one direction than when flowing in the other.

refrigerant-flow.jpg
heating mode

_____________________________________________
refrigerant-flow2.jpg
cooling mode

_____________________________________________

The BBNW tool calls for minimum possible (3 inch) ducts to 3 out of 5 rooms. That seems awfully small to me, appropriate only for bathrooms. Does it seem small to you?

That would be on the small side for a low-static air handler system, but common enough in high-static high velocity systems with moderate to low room loads. Minimum is minimum, not optimum. It's generally good practice to keep duct velocities <<400 fpm wherever/whenever possible to keep the static pressures and noise at bay. Low duct velocity increases the duct size. While there is such a thing as too low a duct velocity, the duct would have to be pretty ridiculously oversized to get there. More here (from the folks in Decatur, GA).

The duct design part of the BBNW tool isn't the greatest tool for newbies, but people who have run their fair share of Manual-D by the book can make it useful.

The BBNW tool says that the bedroom over the garage has a heating load of 8,345 btuh and a cooling load of 4,313 btuh. Do you still think it's a bad idea to give it its own individual 1/2 ton Mitsubishi minisplit? If so, suppose that room's occupant refuses to dress appropriately for the seasons and insists on wearing summer clothes in winter and winter clothes in summer. Could they be satisfied by embiggening the duct diameter to that bedroom?

With a design heating load of 8345 BTU/hr @ +25F an -FH06 or -FS06 is a decent choice due to it's very low (but still COP 4+ efficiency) 1600 BTU/hr minimum modulation level @ +47F, with plenty of spare capacity at 25F.

Bonus rooms over unconditioned garages are often hard to make work well when zoned with the rest of the house, and as long as there is sufficient load to make it run efficiently (=TRUE, in this case) a standalone single zone minisplit works. However, and FS06 married to a MXZ multi-split in this situation is usually a recipe for comfort &/or efficiency failure unless all other zones cassettes/heads are similarly appropriately sized relative to their loads (=FALSE, in the "ductless head for every room" approach.)

Back on the filters & static pressure issues, this guy is a bit hard to listen to (unless you are accustomed to sipping from a firehose- feel free to pause, back up, and grab an espresso before restarting), but even a MERV 8 x 1" filter can bring a low static system to it's knees if allowed to get dirty. He did test both a 2" & 4" filter along with the many 1" filters, as well as an unrated 1" filter that would block particles bigger than a cat (but not necessarily cat hair, and definitely not cat dander.)
 

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That would be on the small side for a low-static air handler system, but common enough in high-static high velocity systems with moderate to low room loads.
Years ago I did a full manual D calculation for a well-insulated house on a slab with a 1.5 ton standard central heat pump. The room with the lowest load had a duct size of 7", so my calculations must have been WAY off.

With a design heating load of 8345 BTU/hr @ +25F an -FH06 or -FS06 is a decent choice due to it's very low (but still COP 4+ efficiency) 1600 BTU/hr minimum modulation level @ +47F, with plenty of spare capacity at 25F.
It's a decent choice for heating, but is it a bad choice for humidity control and efficiency in the summer?

And I'm still wondering whether the inappropriately-dressed garage bedroom occupant could be satisfied simply by embiggening the duct diameter to that bedroom?

this guy is a bit hard to listen to
Wow, that's a lot of info, but I actually followed OK. I'm good about changing filters regularly, so I'm comfortable sticking with 1" MERV 8. I'm curious - he only used 16 x 20 filters in his test box. What if he had compared 16 x 20 to 25 x 25 of the same construction? Would he have seen half as much static pressure?

The ground floor loads can be handled comfortably by the (narrower) -9RLFCD, which is only about 30" wide (+ 4" for connections).
Since I'm not restricted to a tiny closet for the ground floor system, is there a wider Fujitsu or Carrier/Midea you would prefer there?
 
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It's a decent choice for heating, but is it a bad choice for humidity control and efficiency in the summer?

The half-ton Mitsubishi is still fine for humidity control. On normally hot humid days it will be running at more than half it's maximum output for at least part of the day, which in the absence of big humidity sources such as excessive outdoor air leakage or an aerobic class with half dozen sweaty participants should be enough to keep up with the latent load. If the humidity starts to creep up or days when it's not hot- just humid, running it in "DRY" mode for 2-5 of hours will bring the humidity way down without overcooling the place.

And I'm still wondering whether the inappropriately-dressed garage bedroom occupant could be satisfied simply by embiggening the duct diameter to that bedroom?

Possibly. With a right sized modulating system that's running very high duty cycles room to room temperature differences shrink, and making it possible to fine-tune it with balancing vanes.

Wow, that's a lot of info, but I actually followed OK. I'm good about changing filters regularly, so I'm comfortable sticking with 1" MERV 8. I'm curious - he only used 16 x 20 filters in his test box. What if he had compared 16 x 20 to 25 x 25 of the same construction? Would he have seen half as much static pressure?

It's not a linear function- the static pressure is roughly a function of the velocity squared. Doubling the cross sectional area cuts the velocity in half, reducing induced static pressure by more than half, bringing it to ~1/4 the static pressure. This is a typical velocity vs. static pressure curve for a filter:

epb2JEDu38m8Osor4njktixbBOWZG4Ke1peC9KVIRaCjQUzlHAgmvsm-4raS8nw1KgZK0oSJ8-_GounxZ3FIiUWOKGUSv4lcBNWi29lxexHy


It's not exactly a V**2 function either (ignored in the captioning that came with that image.) At 520 fpm the static pressure is ~0.35 w.c., but at (520/2=) 260 fpm it's ~0.08" w.c., which is slightly less than 1/4.

Since I'm not restricted to a tiny closet for the ground floor system, is there a wider Fujitsu or Carrier/Midea you would prefer there?

The Fujitsu low-static 9RLFCD or 12RLFCD have some of the lowest min-modulated output in the industry, but some of the 3/4 ton low static Mideas go even lower minimums, with a COP >3.5 @ +47 at that minimum. The Fujitsu may be slightly more efficient, but the Midea/Carrier will run pretty much 100% of the time. Sometimes at very low modulation the exit temperature in heating mode will be extremely tepid at minimum output, inducing wind-chill if it's blowing directly on humans. With any of these it's important to consider the that aspect when placing the registers/diffusers.
 

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The half-ton Mitsubishi is still fine for humidity control.
Alright, suppose I go with the 1/2 ton Mitsubishi for the garage bedroom. That leaves 8,620 btuh of heating and 7,709 btuh of cooling for the other 2 bedrooms to be covered by a Fujitsu 9RLFCD or a 3/4 ton Carrier/Midea. Which do you prefer?

BTW, the Fujitsu NEEP page you linked describes the equipment as "single-room". Why would they do that?

Possibly. With a right sized modulating system that's running very high duty cycles room to room temperature differences shrink
Temperature differences have certainly been a problem, but I think the culprit there might be bad duct sizing. I'm guessing you have no good way to answer this question, but I'll ask anyway: Suppose the inappropriately-dressed garage bedroom occupant desires the temperature to be 5 degrees warmer in winter and cooler in summer than the other rooms served by the upstairs system. By what % would you increase the Manual D or BBNW duct size to satisfy that inappropriately-dressed occupant?

This is a typical velocity vs. static pressure curve for a filter:
Sorry, the image did not display for me. Let me ask a related question: Is it more cost effective to double the filter area by doubling its thickness or by doubling one of its other dimensions?

The Fujitsu may be slightly more efficient, but the Midea/Carrier will run pretty much 100% of the time.
I guess you tend to focus more on heating up there in MA, but I tend to focus more on cooling down here in GA. I see that when the Midea/Carrier is at minimum cooling output, its COP is a horrible 0.41, and even when running at its rated capacity, the COP is only 1.74. For comparison, the Fujitsu has a COP of 6.43 at minimum cooling capacity, and 4.25 at rated capacity. It seems that Fujitsu is the clear cooling choice, no? Or do you prefer the Midea/Carrier for better humidity control? I don't see SHR for either choice.
 

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Alright, suppose I go with the 1/2 ton Mitsubishi for the garage bedroom. That leaves 8,620 btuh of heating and 7,709 btuh of cooling for the other 2 bedrooms to be covered by a Fujitsu 9RLFCD or a 3/4 ton Carrier/Midea. Which do you prefer?

The vendor with the best local technical support and distributor (for spare parts & warranty issues) is usually the right choice. There are lots of Carrier installers, but not all of them understand their mini-split lineup, so be picky when vetting them.

BTW, the Fujitsu NEEP page you linked describes the equipment as "single-room". Why would they do that?

Sloppiness/laziness/imprecision by folks at NEEP. What they really meant by that description is "single zone", differentiating it from multi-split systems. While ducted mini splits can be installed in single room applications, that is by no means a requirement.


Temperature differences have certainly been a problem, but I think the culprit there might be bad duct sizing. I'm guessing you have no good way to answer this question, but I'll ask anyway: Suppose the inappropriately-dressed garage bedroom occupant desires the temperature to be 5 degrees warmer in winter and cooler in summer than the other rooms served by the upstairs system. By what % would you increase the Manual D or BBNW duct size to satisfy that inappropriately-dressed occupant?

Install balancing vanes at the take-0ff from the plenum for every duct run, tweak as-needed or desired.

Sorry, the image did not display for me. Let me ask a related question: Is it more cost effective to double the filter area by doubling its thickness or by doubling one of its other dimensions?

Adding thickness & pleating is usually easier to implement than fattening the section of duct, but in new duct construction all options are on the table.

I guess you tend to focus more on heating up there in MA, but I tend to focus more on cooling down here in GA. I see that when the Midea/Carrier is at minimum cooling output, its COP is a horrible 0.41, and even when running at its rated capacity, the COP is only 1.74. For comparison, the Fujitsu has a COP of 6.43 at minimum cooling capacity, and 4.25 at rated capacity. It seems that Fujitsu is the clear cooling choice, no? Or do you prefer the Midea/Carrier for better humidity control? I don't see SHR for either choice.

You are correct that heating season efficiency is a bigger focus than cooling efficiency in MA, but both are important. In most of GA the cooling degree-days add up to roughly the same number as heating degree-days but often the seasonal net cooling load is much higher (due to direct solar gains from glazing) than the seasonal heating load. So it's appropriate to pay attention to cooling season efficiency in your case, especially when your 1% cooling load is roughly the same or higher than your 99% heating load.

So yes, the Fujitsu compact duct units are likely going to work better for you than the Midea/Carrier units I linked to. But there are a number of different Midea compact duct series, some with better cooling efficiency than that one. Unfortunately the search functions on the NEEP search page are heating-biased (being a northeastern organization), which means pulling the options one by one to check cooling efficiency. They don't all suck at minimum-modulation in cooling mode eg: An 40MBDQ12---3 married to an 38MAQB12R--3 compressor is in the COP 3.5 range across temperature & modulation range in cooling but it's hard to beat the Fujitsus xxRLFCD series for cooling efficiency in a compact duct. Some of the Mitsubishis are up there on cooling efficiency too, but most of their low-static versions have pretty gutless blowers.

With modulating equipment there is no single SHR, since it depends on modulation levels. In general the harder it's working, the more favorable the SHR, which is why oversizing mini-splits can end up being a clammy-moldy non-solution in the southeastern US. Sizing them reasonably for the load can be pretty good, especially when you have "DRY" mode as the backup for days when sensible loads are too low for the mini-split to hit it's mid to high range, such as muggy-cloudy days when its only in the low 80s outside while dew points are hanging north of 70F. Those conditions don't happen every day, and most of the time the SHR is good enough to mange the latent mode in normal cooling mode. If indoor RH is creeping up in the afternoon, running in DRY mode (even overnight) can do a world of good.
 

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The vendor with the best local technical support and distributor (for spare parts & warranty issues) is usually the right choice.
As I stated before, my goal is to choose the equipment and installer so carefully that spare parts and warranty issues are irrelevant.:D

Install balancing vanes at the take-0ff from the plenum for every duct run, tweak as-needed or desired.
Why balancing vanes rather than dampers?

But there are a number of different Midea compact duct series, some with better cooling efficiency than that one.
All available in upflow?

Some of the Mitsubishis are up there on cooling efficiency too, but most of their low-static versions have pretty gutless blowers.
If I recall correctly, you said that none of the compact ducted Misubishis are available in upflow, so they cannot be considered.

Is it stupid to mix brands? For example, 1/2 ton Mitsubishi in the garage bedroom, 3/4 ton Fujitsu for the other 2 bedrooms, and 3/4 ton Carrier downstairs?
 

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2 questions about the SEER of VRF/inverter systems:

1. Why can the ductless systems achieve 50% better SEER than the ducted systems? Is it simply a matter of leaky ducts in unconditioned space, or is there another reason?

2. Why does SEER decrease as system size increases? I would have guessed the opposite.
 

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I'm pretty sure NONE of the Fujitsu model numbers are available under the Rheem nameplate. SFAIK Fujitsu hasn't OEM'ed any of their mini-splits to others
Dana, see https://s3.amazonaws.com/WebPartners/ProductDocuments/C8DBACD7-A753-4814-A902-F4E0277B4EC0.pdf . The Fujitsu 9RLFCD we've been discussing and the Rheem RIDH09AVFJ have identical dimensions. Rheem does say, however, that "Unit can be placed horizontally or vertically. However, internal drain pump will not operate when the unit is mounted in a vertical configuration." Since this won't be installed below grade, is there a problem? Also, the F in the above model # stands for "floor". Why would an indoor compact duct unit need to be installed on the floor?
 

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Dana, see https://s3.amazonaws.com/WebPartners/ProductDocuments/C8DBACD7-A753-4814-A902-F4E0277B4EC0.pdf . The Fujitsu 9RLFCD we've been discussing and the Rheem RIDH09AVFJ have identical dimensions. Rheem does say, however, that "Unit can be placed horizontally or vertically. However, internal drain pump will not operate when the unit is mounted in a vertical configuration." Since this won't be installed below grade, is there a problem? Also, the F in the above model # stands for "floor". Why would an indoor compact duct unit need to be installed on the floor?

The AHRI submittal sheet and technical manual for the Rheem also matches the Fujitsu xxRLFCD series in both style on content, so it would appear that you are correct, Rheem's ducted mini-splits really are relabeled Fujitsus (which was new to me- thanks!).

It doesn't need to be installed on the floor (indeed it can't be if installed vertically.)
 
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