Daikin Quaternity

Users who are viewing this thread

Charles2

Member
Messages
112
Reaction score
0
Points
16
Location
Georgia
With tapered hard duct feeding in & out of a 14.5" wide stud bay penetration a short section of higher-velocity duct won't add a lot of static pressure.
I did not know that sort of construction was permissible.

there isn't a theoretical limit on motor efficiency
Surely 100% must be the theoretical limit to motor efficiency?

current equipment isn't even half way to theoretical limitations imposed by the physics for using R410A to heat a 70F house pulling heat from 0F outdoor air. In Japan there are home air conditioners that would test well into the 40s or even low 50s on an AHRI SEER test
So theoretically 100 SEER is possible? Why aren't the Japanese low 50s SEER air conditioners available in the US?

Carrier/Midea is currently selling an R410A refrigerant mini-split in the US that tests in the low 40s.
I'm aware of that one. Have you seen/heard anything about its reliability?

any reasonable SHR will still handle it
How do you define reasonable in this context?
 

Charles2

Member
Messages
112
Reaction score
0
Points
16
Location
Georgia
Make a drawing of your house floor plans with the room cooling & heating loads marked within each room.
How should hallways and stairwells be treated?

When it asks where the ducts are located, do I answer where they are now, or where we want them to be?

Is a wall between a house and unconditioned garage considered an exterior wall?

In the Windows section, is the tool asking for area?

Should french doors be regarded as doors or windows?
 
Last edited:

wwhitney

In the Trades
Messages
6,532
Reaction score
1,822
Points
113
Location
Berkeley, CA
How should hallways and stairwells be treated?
If they have no exterior surfaces, then their load is zero. If you condition the adjacent spaces, they should end up at the average temperature of those spaces (for the appropriate sense of the word average).

Otherwise, assign them a load as usual, but then if they end up having very small loads so that you don't directly condition them, then I think a standard procedure is to divide up the load among the adjacent directly conditioned spaces. Perhaps in accordance to their joint surface area with those spaces.

Cheers, Wayne
 

Dana

In the trades
Messages
7,889
Reaction score
509
Points
113
Location
01609
I did not know that sort of construction was permissible.

As long as the transitions are tapered the effective "equivalent length" of a short section of smaller duct doesn't have a lot of impact, especially when the overall lengths is short, and all ells & tees & take-offs are radiused rather than sharp-throated. (Sharp turns create a lot of turbulence resulting in a higher static pressure drop.) There can be noise issues due to the higher velocity, but that's about the only drawback. When possible I usually target ~300-350 fpm duct velocity at the cassette's max blower speed for ducts fully inside of conditioned space then taper down to achieve a higher exit velocity at the registers for better mixing with the room air (YMMV), which means at the blower's lowest cfm the exit velocity can be too low to get adequate mixing unless it's tapered down.

If you want to play "junior duct designer" and sketch out a ducted cassette solution yourself you might find this bit o' bloggery from the folks in Decatur useful. The BetterBuiltNW HVAC tool (that uses appropriate Manual-J U-factor defaults for sizing mini-splits) also has a duct design widget.

Surely 100% must be the theoretical limit to motor efficiency?

Yes- and the load (friction through the coils) has a theroretical limit of zero, so divide by zero= not defined. ( If the surface area of the coil was the size of the Pacific Ocean it wouldn't need a blower, eh? :)) With real world blowers & coils it's always in the realm of practical engineering & cost limitations.

So theoretically 100 SEER is possible?

Maybe- I'd have to do the hard-math on that one. But it doesn't really matter. The practical economic & engineering limits are more important than theoretical limits.

Why aren't the Japanese low 50s SEER air conditioners available in the US?

That's a marketing question for the vendors.

I'm aware of that one. Have you seen/heard anything about its reliability?

It's been out there for about 4-5 years now- haven't heard about any problems with it. There is no theoretical reason it would be any worse than other Midea mini-splits.

How do you define reasonable in this context?

Any SHR <0.9 at low to mid-modulation would be good enough a the seasonal averages for most reasonably tight houses that don't have a dozen tropical fish tanks, fung shui water features, or tropical indoor gardens, or otherwise need the full ASHRAE 62.2 ventilation rate 24/365. Using DRY mode isn't a crime, and is generally efficient than bringing down the indoor humidity with a separate dehumidifier converting a the latent load to a sensible load, (with the exception of a whole house dehumidifier that dumps the latent heat of vaporization to an outdoor coil.)

A Fujitsu DRY mode may not always keep up the latent load when there is low or no sensible load, so some other dehumidifier may be necessary during high outdoor dew point weeks. It looks like this week qualifies as high enough humidity to matter in Atlanta, with the dew points hovering around 70F (the top green line in the graph).

But without high ventilation rates or lots of indoor humidity sources (cooking tubs of gumbo in a pot without lids? Are 5 people taking 30 minute showers?) it may not need to run in DRY mode all the time to get there, since the forecasted daily high temps and sunshine levels will be generating a significant sensible load causing the equipment to run a higher modulation level (=colder coils, better SHR) than when the sensible load is small.

How should hallways and stairwells be treated?

When you have completed the Manual-J load calculations the answer would be pretty obvious. Unless the hallway/stairwell has a large skylight the cooling and heating loads are de minimis. If using a central common return with jump-duct returns to the common area from doored-off rooms the return air is good enough for conditioning ultra-low load spaces. Typically the loads of hallways and stairwells are "within the noise" error of the rest of the calculation, and don't affect system sizing.

When it asks where the ducts are located, do I answer where they are now, or where we want them to be?

Run it both ways, compare the results.

Is a wall between a house and unconditioned garage considered an exterior wall?

You may have to run a fudge-factor here, but yes. If the garage doors are normally left open it's absolutely an exterior wall. If the garage door is normally closed treat it as a somewhat higher-R wall (and higher still if the true exterior walls of the garage are insulated.) Eg: if it's a 2x4/R13 partition wall and the exterior walls are not insulated, treat it as a 2x6/R20 wall. If the garage walls are insulated (R13 or better) treat it as an 2x6/R20 + R5 c.i. wall. It will be close enough.

In the Windows section, is the tool asking for area?

Some tools ask for square feet, others want you to enter the width x height (of the entire area, not just the glass) in inches.

Should french doors be regarded as doors or windows?

Windows. Most manufacturers of exterior patio doors etc. publish the U-factors, which can be entered manually with some load tools.
 
Last edited:

Charles2

Member
Messages
112
Reaction score
0
Points
16
Location
Georgia
at the blower's lowest cfm the exit velocity can be too low to get adequate mixing unless it's tapered down.
I was wondering about that. I was thinking about having the new registers for each bedroom just a foot or so from the tops of their doors, the undercuts of which serve as the return air paths. Would the throws always be sufficient? I'll need at most 16 feet of throw.

The BetterBuiltNW HVAC tool (that uses appropriate Manual-J U-factor defaults for sizing mini-splits) also has a duct design widget.
That tool is so marvelously comprehensive! I can see why so many contractors just size everything with rules of thumb.

the exception of a whole house dehumidifier that dumps the latent heat of vaporization to an outdoor coil.
Do all whole house dehumidifiers dump heat outside?

A Fujitsu DRY mode may not always keep up the latent load when there is low or no sensible load, so some other dehumidifier may be necessary during high outdoor dew point weeks.
Do you have any idea why portable dehumidifiers are so incredibly unreliable? Why can't they be as reliable as refrigerators?

It looks like this week qualifies as high enough humidity to matter in Atlanta
I'll say. Today feels like the hottest this year.

Run it both ways, compare the results.
I did and the difference was almost half a ton! I wish I knew what the true R-value of the attic duct insulation is. I'm just having to guess at so much.

and higher still if the true exterior walls of the garage are insulated.
Again, I'm having to guess since I don't know for sure which walls are insulated. Nor do I know whether the garage ceiling is insulated. Years ago I found 64 sf of the attic completely uninsulated, which I corrected.

Some tools ask for square feet, others want you to enter the width x height (of the entire area, not just the glass) in inches.
I'm not asking about some tools, I'm asking about the BetterBuiltNW HVAC tool.:)

Most manufacturers of exterior patio doors etc. publish the U-factors, which can be entered manually with some load tools.
I doubt I'll be able to identify the make and model, and a substantial part of the 3 french doors is insulated steel.
 
Last edited:

Fitter30

Well-Known Member
Messages
4,292
Reaction score
778
Points
113
Location
Peace valley missouri
Never seen a whole house dehumidifier dump air to the outside because your dehumidifier isn't made for over cooling the space. The colder the air the harder it is to pull RH from. Latant is easy relative is hard.
 

Dana

In the trades
Messages
7,889
Reaction score
509
Points
113
Location
01609
I was wondering about that. I was thinking about having the new registers for each bedroom just a foot or so from the tops of their doors, the undercuts of which serve as the return air paths. Would the throws always be sufficient? I'll need at most 16 feet of throw.

The type of register grille can either help or hinder throw. For <16' of throw you probably won't have to resort to anything special such a high throw commercial diffuser. Installing the register as close to the ceiling as possible helps extend the throw of any register grill by maintaining a laminar flow next to the ceiling rather than slowing and spreading it via turbulence. A straight-fin adjustable register such as a Hart & Cooley A618MS mounted near the ceiling allows you to tweak and fine-tune the amount of spread by the angle(s) of the fins. With the fins all pointed straight the throw is pretty long, but for better mixing you may want to adjust some of it to deflect at a bit of angle.

22452-2.jpg



But with very few exceptions (like a super-insulated low load high performance house) door cuts are (almost) NEVER going to provide an adequate return path, and that's even more true for low or mid-static air handlers/cassettes, unless your door cut is big enough to toss a cat through. Using a single 16" o.c. 2x4 partition wall stud bay as a jump duct provides about 50 square inches of cross sectional return "duct" area, and a pair of them puts it in the 100" range, inducing very low static pressures. To get the same return path impedances out of a door cut takes 2" or more of door cut, bad for light & sound privacy, bad for keeping the cat out of you room.

The Tamarack Industries Perfect Balance door vent series might work, if you don't mind reworking your doors that heavily. Unlike a simple door cut it cuts down sound and light transmission, and provides a decent amount of return path.


Do all whole house dehumidifiers dump heat outside?

Almost none do, but if your sensible load is small enough a Quaternity pretty much does.

Do you have any idea why portable dehumidifiers are so incredibly unreliable? Why can't they be as reliable as refrigerators?

That's one of those enduring mysteries...

They're made do be cheap, which often means "unreliable". The compressors are about 3-4x as big as that found in a typical refrigerator, and ron an odd duty cycle.

If you are heating your domestic hot water with electricity, DO consider a heat pump/hybrid water heater. They run longer more efficient cycles than a typical dehumidifier, have about half the compressor size, and rather than dumping the heat of vaporization into the room air as sensible heat, it dumps it into water inside an insulated tank. In some homes they cover pretty much the entire latent load.


I did and the difference was almost half a ton! I wish I knew what the true R-value of the attic duct insulation is. I'm just having to guess at so much.

The difference in additional load between R4 ducts and R8 ducts is small compared to R0 ducts and R8 ducts. A half ton to 1 ton is pretty typical.

What did the whole-house cooling & heating load numbers come in at?

If the replacement gets perfectly sized or even slightly undersized for the "before house upgrades" picture it will still usually keep up except during days when the temps are 10F or more under the 99% design temp or 10F hotter than the 1% design temp, and won't be ridiculously oversized for the "after upgrades" version of the house.



I doubt I'll be able to identify the make and model, and a substantial part of the 3 french doors is insulated steel.

If it's glazed with clear-glass double-panes call it U0.4. If it's low-E double panes call it U0.3. If single pane, U0.8. It'll be close enough.

If it's double pane glass but you don't know if there's a low-E coating, use an LED flashlight, lighter or candle to view the reflections. There will be two pairs of reflections, one for each surface of glass. If they're all the same color there is no low-E coating. A low-E coating will impart a color distortion (the color varies by coating type) to one of the inner-surface reflections. See the pink outlier in this row of yellows:

mx0382phvsj51.jpg
 

Charles2

Member
Messages
112
Reaction score
0
Points
16
Location
Georgia
But with very few exceptions (like a super-insulated low load high performance house) door cuts are (almost) NEVER going to provide an adequate return path, and that's even more true for low or mid-static air handlers/cassettes
Is that because those air handlers move fewer CFM than conventional air handlers, or some other reason?

If you are heating your domestic hot water with electricity, DO consider a heat pump/hybrid water heater.
Definitely would buy a hybrid if my current heater leaks and I can find a cooperative plumber. IIRC there are only 2 brands available now, right? Which do you prefer?

In some homes they cover pretty much the entire latent load.
By "some" I imagine you mean very tight. How many pints of condensate per day can they extract?

What did the whole-house cooling & heating load numbers come in at?
Your grammar reminds me of a joke I heard from a political science professor at UGA in the 80's. The Georgia Bulldogs were named after the Yale Bulldogs, and the football teams used to play each other, mostly at Yale. One year when the game was held in New Haven, a Georgia fan got lost and asked a Yale student, "Can you tell me where the stadium's at?" To which the student replied, "Sir, at Yale we are taught never to end a sentence with a preposition." The Georgia fan scratched his head for a minute, then asked, "Can you tell me where the stadium's at, asshole?"

I'm amused to see Southern grammar penetrating the northeast, but I'm not confident enough to give you an answer yet. How do you input significant external window shading? I have 6 windows that face west, but they have 8 foot deep covered porches shading them, so they don't admit sun until early evening.

If it's double pane glass but you don't know if there's a low-E coating, use an LED flashlight, lighter or candle to view the reflections.
Does it matter on which side of the door the light is held?
 

Charles2

Member
Messages
112
Reaction score
0
Points
16
Location
Georgia
I found this label on the top of one of the doors. Does it give any clue about make and model?
upload_2021-7-30_20-17-0.jpeg

Here's another:
upload_2021-8-1_18-51-13.jpeg
 
Last edited:

Dana

In the trades
Messages
7,889
Reaction score
509
Points
113
Location
01609
Is that because those air handlers move fewer CFM than conventional air handlers, or some other reason?

Even with conventional air handlers it's too restrictive to rely on door cuts as a return path. The notion that a door cut is adequate is a commonly held misconception (by people who don't do math... Arithmetic is just a theory after all, right? :) Let's make life easier and just call Pi 3.000, eh?)

Definitely would buy a hybrid if my current heater leaks and I can find a cooperative plumber. IIRC there are only 2 brands available now, right? Which do you prefer?

By "some" I imagine you mean very tight. How many pints of condensate per day can they extract?

The Rheem Prestige series are pretty decent- some have more digital bells & whistles than others, but even the most basic are solid units.

The amount of humidity a water heater extracts per day depends on how much hot water you're using, and whether or not it's in "HEAT PUMP ONLY" mode.


I'm amused to see Southern grammar penetrating the northeast, but I'm not confident enough to give you an answer yet. How do you input significant external window shading? I have 6 windows that face west, but they have 8 foot deep covered porches shading them, so they don't admit sun until early evening.

Does it matter on which side of the door the light is held?

8 foot porch roofs cut most of the PM solar gain, so you can call it mostly-shaded.

Light goes both ways (if it didn't you wouldn't see the reflections). Any side will do- the color distortion is obvious either way.
 

Charles2

Member
Messages
112
Reaction score
0
Points
16
Location
Georgia
When the BBNW sizing tool asks what percent of the area under the slab is unconditioned, how should I answer? In the Overrides section there is also an option to adjust the slab F-value, but I have no idea what that is.

Previously I asked how a wall between a house and an unconditioned garage should be treated, and you (Dana) provided guidance. But I see no way to enter variable wall R-value for the room adjacent to the garage.

If it's glazed with clear-glass double-panes call it U0.4. If it's low-E double panes call it U0.3.
Surprisingly, I have found that the externally shaded windows are not low-e, but the exposed windows are. I would not have guessed that this cheap tract house would have tuned windows. But I don't see how to enter the 2 different U values in the BBNW sizing tool.

Also, I don't see how to enter which windows are externally shaded and which are not. Should I try to fudge it with the Internal Shade Coefficient on the Options tab?

These window questions seem really important, because according to the tool, the windows are by far the biggest contributor to the cooling load.

BTW, previously I asked whether french doors should be regarded as doors or windows, and you said windows. But I read in the BBNW sizing tool that "For opaque doors with more than 25% glass, enter the glass area in the window columns and opaque area in the door column." So at least I'm confident with how the tool wants me to treat french doors.
 
Last edited:

Dana

In the trades
Messages
7,889
Reaction score
509
Points
113
Location
01609
When the BBNW sizing tool asks what percent of the area under the slab is unconditioned, how should I answer? In the Overrides section there is also an option to adjust the slab F-value, but I have no idea what that is.

Previously I asked how a wall between a house and an unconditioned garage should be treated, and you (Dana) provided guidance. But I see no way to enter variable wall R-value for the room adjacent to the garage.


Surprisingly, I have found that the externally shaded windows are not low-e, but the exposed windows are. I would not have guessed that this cheap tract house would have tuned windows. But I don't see how to enter the 2 different U values in the BBNW sizing tool.

Also, I don't see how to enter which windows are externally shaded and which are not. Should I try to fudge it with the Internal Shade Coefficient on the Options tab?

These window questions seem really important, because according to the tool, the windows are by far the biggest contributor to the cooling load.

BTW, previously I asked whether french doors should be regarded as doors or windows, and you said windows. But I read in the BBNW sizing tool that "For opaque doors with more than 25% glass, enter the glass area in the window columns and opaque area in the door column." So at least I'm confident with how the tool wants me to treat french doors.

To be sure the BBNW tool is a very scaled down simplified tool with far less flexibilty (and far fewer ways to screw up ) than a pro tool. For cooling load go ahead and fudge the shading factors of the windows, and/or assume they're all low-E. How big is the difference in cooling load numbers if one assumes all low-E vs. all clear glass?

For the heating load go ahead and treat the partition wall to the garage as an exterior wall. Since it's all just blank wall (except for the door) the difference won't be huge, unlikely a to affect the equipment sizing or duct sizing to the adjacent rooms.

"F-value " or "F-factor" should probably stand for "Fudge factor" :cool:, but it's really an ASHRAE defined parameter all about the heat loss/gain at the slab edges. This article offers some hints. IIRC the F-factor for an uninsulated (including no slab edge insulation) slab on grade is ~0.75 BTU per hr per foot of perimeter length per degree F or a bit less. (A quick search came up with this F-factor table confirming the WAG in the previous sentence.) It's equivalent to a U-factor, except that it's loss per unit length (of the slab edge), not square feet (of wall, roof, window,etc.) area.
 

Charles2

Member
Messages
112
Reaction score
0
Points
16
Location
Georgia
For cooling load..., For the heating load...
Is looks to me that the tool calculates heating and cooling loads simultaneously. Or are you telling me to do 2 separate runs with 2 different inputs, then taking the cooling load of the first calculation and the heating load of the second calculation?

How big is the difference in cooling load numbers if one assumes all low-E vs. all clear glass?
Here it seems clear that you are asking me to do 2 separate runs with different inputs.

"F-value " or "F-factor" should probably stand for "Fudge factor" :cool:, but it's really an ASHRAE defined parameter all about the heat loss/gain at the slab edges.
Interesting. R-10 slab edge insulation would be part of the possible deep energy retrofit I mentioned earlier. Incidentally, I once did a manual J calculation for a well-insulated single-story house I designed, and found that about 1/3 of its heating load came from the slab edge. That's commensurate with the contribution of the windows to the cooling load, so slab edges should be regarded as very long, narrow thermal windows.

IIRC the F-factor for an uninsulated (including no slab edge insulation) slab on grade is ~0.75 BTU per hr per foot of perimeter length per degree F or a bit less. (A quick search came up with this F-factor table confirming the WAG in the previous sentence.)
Nothing close to 0.75 is a selectable option. What to do?
 
Last edited:

Dana

In the trades
Messages
7,889
Reaction score
509
Points
113
Location
01609
Is looks to me that the tool calculates heating and cooling loads simultaneously. Or are you telling me to do 2 separate runs with 2 different inputs, then taking the cooling load of the first calculation and the heating load of the second calculation?

Yes run the calculations twice, one with tweaks to the inputs, the other without, and compare.


Interesting. R-10 slab edge insulation would be part of the possible deep energy retrofit I mentioned earlier. Incidentally, I once did a manual J calculation for a well-insulated single-story house I designed, and found that about 1/3 of its heating load came from the slab edge. That's commensurate with the contribution of the windows to the cooling load, so slab edges should be regarded as very long, narrow thermal windows.


Nothing close to 0.75 is a selectable option. What to do?

I don't currently have access to the tool from this computer- what are the F-value options? (Are they using different units?) Per the table for an R-0 uninsulated slab on grade the F-factor is 0.73, with R10 vertical slab edge that drops to 0.48 ( for insulation extending to 48" below grade)- 0.54 ( 24" depth below grade). Whatever the options are in the tool, pick the biggest when running the "before upgrades" loads.

Under IRC 2021 prescriptive insulation requirements per TABLE N1102.1.3 zone 3 (in brown, below) call for R10 down to 2' below grade, but in zone 4 (in yellow) that becomes 4'. (No slab edge insulation required for zone 2, in red.)
cz-map_georgia.png
 

Charles2

Member
Messages
112
Reaction score
0
Points
16
Location
Georgia
Are they using different units?
The tool states, "This value is different from the U-Value in that it heat loss per linear foot not square foot."

I don't currently have access to the tool from this computer- what are the F-value options?
The options are 1.18, 0.449, 0.355, 0.314, 1.135, 1.108, 1.091, 0.442, 0.343 and 0.298.

with R10 vertical slab edge that drops to 0.48 ( for insulation extending to 48" below grade)- 0.54 ( 24" depth below grade)
Unfortunately, because of the way this slab was formed, and because there are patio and driveway slabs adjacent to the house slab, I would only be able to insulate down to grade.
 

Dana

In the trades
Messages
7,889
Reaction score
509
Points
113
Location
01609
The tool states, "This value is different from the U-Value in that it heat loss per linear foot not square foot."


The options are 1.18, 0.449, 0.355, 0.314, 1.135, 1.108, 1.091, 0.442, 0.343 and 0.298.


Unfortunately, because of the way this slab was formed, and because there are patio and driveway slabs adjacent to the house slab, I would only be able to insulate down to grade.

Running the calculation using 1.091 first, then again with 0.449 and splitting the difference (recording it on spreadsheet or piece of paper), woul get you close enough to the 0.72 number. A concrete driveway slab is about twice as thermally conductive as most soil types, so even 1.18 might be closer to reality in your case. Since you aren't planning to insulate below grade usin 0.449 as an aggressive estimate of the "after" picture wouldn't necessarily be too far off. Running the calculations multiple times with different F-factors will give you a good idea as to the size of any error.

With a <$50 circular saw blade and a ~$100-$ 125 hammer drill with a chisel bit it's pretty easy to cut a 2"-3" channel in the slab next to the foundation and dig down a foot (or even two_. In less-rocky soil much of the slot trenching can even be done with a garden hose if you're willing to deal with the muddy mess. The more thermally conductive driveway slab will largely defeat the slab edge insulation if you don't at least insulate down to a few inches below slab level, creating at least SOME thermal break between the driveway slab and the house slab edge & stemwall.
 

Charles2

Member
Messages
112
Reaction score
0
Points
16
Location
Georgia
Running the calculation using 1.091 first, then again with 0.449 and splitting the difference (recording it on spreadsheet or piece of paper), woul get you close enough to the 0.72 number.
OK, so splitting the difference and assuming non-low-e windows that are mostly shaded, I get 29,000 btuh for the heating load; 22,200 btuh for the cooling load; and 5100 btuh for the latent load. None of the individual room cooling loads exceeds half a ton, but the garage bedroom heating load at 9,561 btuh is about twice as high as any other room.

If I only move the ducts from the attic into conditioned space, I get Heating: 25,700 BTU/hr; Cooling: 18,500 BTU/hr; and Latent: 3,600 BTU/hr.

After the possible deep energy retrofit, which includes partial slab edge insulation and adding R10 board to the walls, I get Heating: 20,400 BTU/hr; Cooling: 17,200 BTU/hr; Latent: 3,600 BTU/hr.
 
Last edited:

Dana

In the trades
Messages
7,889
Reaction score
509
Points
113
Location
01609
OK, so splitting the difference and assuming non-low-e windows that are mostly shaded, I get 29,000 btuh for the heating load; 22,200 btuh for the cooling load; and 5100 btuh for the latent load. None of the individual room cooling loads exceeds half a ton, but the garage bedroom heating load at 9,561 btuh is about twice as high as any other room.

If I only move the ducts from the attic into conditioned space, I get Heating: 25,700 BTU/hr; Cooling: 18,500 BTU/hr; and Latent: 3,600 BTU/hr.

After the possible deep energy retrofit, which includes partial slab edge insulation and adding R10 board to the walls, I get Heating: 20,400 BTU/hr; Cooling: 17,200 BTU/hr; Latent: 3,600 BTU/hr.

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 2.5 ton ARU30RGLX Fujitsu is good for over 30K @ +17F, and would clearly work without moving the ducts, but it's minimum output @ 47F is over 9K, which means it will do a lot of cycling on/off during the shoulder seasons. The 2 ton ARU24RGLX is on the edge of covering the load, just shy of 25K @ +17F, but has a much lower 5400 BTU/hr minimum output @ 47F, which would be more comfortable & efficient during the shoulder seasons due to the much longer cycles, at the cost of having to burn some strip heat in an after-burner "toaster" to cover the 99% load. If the eventual goal is a deep energy retrofit the 2 tonner is a more likely bet. 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.

The 1.5 ton ARU18RGLX is only good for 19,800 BTU/hr @ +17F, and might not cover the "after DER" load, but would with a toaster. The 1.5 ton ARU18RLF is more efficient and 21,400 BTU/hr of capacity @ +17F and a very low 3100 BTU/hr minimum @ +47F, albeit with somewhat wimpier air handler. With a new duct system it's easy to design for the 0.5" w.c. max of the -18RLF (down from 0.8" w.c for the ___RLGX series) , but the existing ducts would need to be analyzed to see if it would work without modification. (It should be do-able, unless they are already a bit undersized.) Simply mapping out the static pressure drops on the existing air handler & ducts with a manometer would tell you how likely that is. (Be careful not to drill in to the coil when making ports for static pressure testing.) At the usually lower max cfm of the 1.5 ton cassettes the static drops are also lower. If it turns out the filter is inserting too much drop (often is), it can be minimized by upgrading to a big fat pleated filter in an oversized return when swapping over to a ducted mini-split. The max speed of the -18RLF is 554 cfm @ 0.5" the max speed of the -18RGLX is 618cfm @ 0.8" w.s. (vs. ~800 cfm for the -24RGLX, or 1000cfm for the 30RGLX)

A typical old school 10-14 SEER 2.5 tonner would be putting out ~1000 cfm at an operating static pressure in the 0.5-0.8" w.c. range. If whole system drop on the air handler is <<0.8" w.c. (should be, if the ducts & filters were designed correctly) you won't have any problem making the existing ducts work with the lower cfm of the -18RLF, to be considered only when the whole house heat load is within striking range. (If the full DER isn't going to happen any time soon, the -24RGLX is a better bet.) No matter what you install for replacement equipment, a static pressure map is a good starting point for determining whether the ducts need to be upgraded. 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. High static pressures can burn out ECM motors pretty quickly, whereas PSC motors will take the abuse for years (while chewing through a lot of electricity just in blower power.)
 
Top
Hey, wait a minute.

This is awkward, but...

It looks like you're using an ad blocker. We get it, but (1) terrylove.com can't live without ads, and (2) ad blockers can cause issues with videos and comments. If you'd like to support the site, please allow ads.

If any particular ad is your REASON for blocking ads, please let us know. We might be able to do something about it. Thanks.
I've Disabled AdBlock    No Thanks