Mini Splits: One head per floor with multiple rooms?

Users who are viewing this thread

Mark_P

New Member
Messages
3
Reaction score
0
Points
1
Location
New Jersey, USA
Hey guys, new user here. I'm in a similar situation as this individual was a few years ago on this thread.

My wife and I are looking to get mini splits installed in our cape cod house which is in central NJ by the shore, is 1300 sq ft (on paper), is on slab, has 2 (100 and 150 sq ft) bedrooms upstairs, and has 2 more (100 & 200 sq ft w/ low ceilings) bedrooms downstairs along with the living room / kitchen area (350 sq ft). The house is a bit drafty, probably because the insulation in the attic / kneewalls is garbage, and there is an unfinished utility closet that essentially acts as a bridge between the kneewall/attic unconditioned zone and the upstairs hallway conditioned zone.

We were about to go with 5 indoor units, which was going to be pretty over sized, but most contractors were pushing us that route. The best plan was a Mitsubishi set-up with (4) 6,000 BTU units for the bedrooms and (1) 12,000 BTU unit for the main living area. Even that would have been quite oversized, but we were hoping that the inverter technology would have allowed them to module down enough for our small zones. Quotes were around $15k-$20k which is a bit of a shock, but you get what you pay for, right?

Now, one company proposed to us an alternative solution, which would be one 18K head downstairs (in the main area, pointed in the direction of the 2 rooms) and one 12K head upstairs (in one room, pointed at the door and right at the other room), with room on the outdoor unit for a 3rd head to be added on if need be. The contractor says it's not ideal, but is enough capacity to cool the whole house, the heads can modulate down to 4.5k and 4k BTU respectively, and the cooling will hopefully circulate through all the rooms.

We're leaning to go with this contractor because a) the contractor is one of the most reputable in the area, b) they seem to be very knowledgeable and were the easiest to converse with, c) they'll redo our insulation in the kneewalls / attic and seal off / insulate the utility closet from the unconditioned zone, d) they'll get us $4,000 back in rebates, guaranteed in the contract (bringing it down to a $8,500 job including the insulation), and finally e) my wife doesn't like the thought of having a big indoor head in each room, aesthetically.

Side-note: they suggest going with the Carrier mini-split, which is a cold climate model. I'm sure they're biased, but they told me they prefer it over Mitsubishi for parts availability, warranty, support, etc. I imagine it brought the cost down, too.

Back to the 1-zone-per-floor set-up; I figure we'll need to leave our doors open most of the time and run the ceiling fans (we've got 4 downstairs and are going to add 2 upstairs) to help the air move around, but could that even be enough to normalize the temperature across all rooms?

I'm also wondering if added a return vent above each bedrooms door frame and/or at the bottom of the door would help as well when doors ever need to be closed.

Any other feedback would be appreciated!
 
Last edited:

Dana

In the trades
Messages
7,889
Reaction score
502
Points
113
Location
01609
If there's some fluff in the walls& attic and glass in the windows and doors that close the whole-house heating load of a 1300' two story in NJ is going to be under 25,000 BTU/hr and the cooling load is going to be under 15,000 BTU/hr (barring a west facing wall of windows), unless this is the leakiest house in the state.

A 6K Mitsubishi head is going to be sub-optimally oversized for the heating and cooling loads of those bedrooms.

If you have kneewall attics to work with, right-sizing a ducted unit for the upstairs (you can probably do the entire upstairs with an SEZ KD09 with a or the Carrier Performance 40MBDQ09---3 ducted cassette, a short plenum, balancing vanes at the take 0ffs from the plenum and some flex duct. Going with a 1 ton or 1.25 ton high-wall ductless head down stairs will probably work, assuming the floor plan is fairly open and not all doored-off, but the first floor bedrooms would likely be an issue in winter. A small ducted unit for the first floor bedrooms might be in order if you intend to heat with the mini-splits, or if the are on the west side with unshaded west facing windows picking up a lot of late in the day heat gain.

The cold-climate Carrier minisplits are re-labeled Midea units with Toshiba's compressor technology. Midea is a large Chinese manufacturer with high quality standards, and has been in bed with Carrier for over a decade (and with the Japanese company Toshiba for over two decades.) Midea makes everything from coffee makers to dishwashers to large industrial chillers- I'm not sure if they make electric toothbrushes too, but I wouldn't be surprised. :) The Carrier/Midea ducted minisplits have some advantages over the Mistubishi equivalents, in that they can be mounted vertically as upflow units as well as horizontal, and the cold climate versions all have pan heaters under the outdoor units to prevent defrost ice build up from damaging the exterior compressor (unlike Fujitsu's ducted units.) With Mitsubishi compressors that end with "-NAHZ " also have pan heaters, but the KD series ducted cassettes are horizonal mount only.

Bottom line, I would have no problem specifying Carrier mini-splits, and would even prefer the mini-ducted units over the Mitsubishi & Fujitsu competition if the installer supports them. I'm also a fan of the Fujitsu ducted cassettes despite the lack of a pan heater, which would be just fine on the Jersey shore. It doesn't stay sub-zero F for days on end in your location, and probably gets above freezing in the afternoons even during cold snaps. (There are people heating entire homes in MN and ME with ducted Fujitsus, but they have to keep an eye on the pan drain to be sure it doesn't get ice plugged during weeks of high temps that never get out of the teens.)

Do yourself a favor and run a room by room Manual-J load calculation using freebie online Manual-J-ish tools like LoadCalc or CoolCalc, using AGGRESSIVE rather than conservative assumptions about R-values and air tightness. Those tools will oversize reality by a double-digit percentage no matter what, but if you're conservative the oversize factor will be just plain silly. Most HVAC contractors COULD run real load calculations, but when they do they tend toward the super conservative, but an oversize factor greater than 1.2x on the cooling or 1.4x on heating can take a toll on comfort, and any bigger than 1.5x will make them run less efficiently.

Better than LoadCalc or CoolCalc, BuildBetterNW developed a design tool targeted at HVAC contractors specifically for sizing heat pumps using a simplified Manual-J underneath it all. Unlike most load tools the default values for air leakage & U-factors don't trend toward the conservative, giving more realistic load numbers without a lot a massaging. There are other parts to the BuildBetterNW tool for designing ducts (you won't need that) or specifying the capacity curves of the heat pumps (you won't need that either), but with a tape measure and some reasonable guesses on R-values and wall construction any newbie can do it. Unlike pro tools there aren't adjustments for shading factors for the windows to tweak, but it is possible to override the default U-factors for walls/windows/ attic (with some limitations) if you know what you're doing. I've only recently started playing around with that tool, but it's pretty good for a first-cut estimate, and WAY better than a HVAC contractor's WAG, (or even most HVAC contractors' Manual-Js! :( ) You need an email and a name to open an account to use the BuildBetterNW tool, but it's free. I understand there are some online video tutorials too (on the big video hosting site), but I haven't looked for them.

Regarding return vents- they're essential for ducted systems, but don't do a thing for ductless. Even a wide open door doesn't do a huge amount of convective heat transfer, but a transfer grille over the door or other jump ducts need to rely on air handler driven pressures to move that air.
 
Last edited:

Mark_P

New Member
Messages
3
Reaction score
0
Points
1
Location
New Jersey, USA
If there's some fluff in the walls& attic and glass in the windows and doors that close the whole-house heating load of a 1300' two story in NJ is going to be under 25,000 BTU/hr and the cooling load is going to be under 15,000 BTU/hr (barring a west facing wall of windows), unless this is the leakiest house in the state.

A 6K Mitsubishi head is going to be sub-optimally oversized for the heating and cooling loads of those bedrooms.

If you have kneewall attics to work with, right-sizing a ducted unit for the upstairs (you can probably do the entire upstairs with an SEZ KD09 with a or the Carrier Performance 40MBDQ09---3 ducted cassette, a short plenum, balancing vanes at the take 0ffs from the plenum and some flex duct. Going with a 1 ton or 1.25 ton high-wall ductless head down stairs will probably work, assuming the floor plan is fairly open and not all doored-off, but the first floor bedrooms would likely be an issue in winter. A small ducted unit for the first floor bedrooms might be in order if you intend to heat with the mini-splits, or if the are on the west side with unshaded west facing windows picking up a lot of late in the day heat gain.

The cold-climate Carrier minisplits are re-labeled Midea units with Toshiba's compressor technology. Midea is a large Chinese manufacturer with high quality standards, and has been in bed with Carrier for over a decade (and with the Japanese company Toshiba for over two decades.) Midea makes everything from coffee makers to dishwashers to large industrial chillers- I'm not sure if they make electric toothbrushes too, but I wouldn't be surprised. :) The Carrier/Midea ducted minisplits have some advantages over the Mistubishi equivalents, in that they can be mounted vertically as upflow units as well as horizontal, and the cold climate versions all have pan heaters under the outdoor units to prevent defrost ice build up from damaging the exterior compressor (unlike Fujitsu's ducted units.) With Mitsubishi compressors that end with "-NAHZ " also have pan heaters, but the KD series ducted cassettes are horizonal mount only.

Bottom line, I would have no problem specifying Carrier mini-splits, and would even prefer the mini-ducted units over the Mitsubishi & Fujitsu competition if the installer supports them. I'm also a fan of the Fujitsu ducted cassettes despite the lack of a pan heater, which would be just fine on the Jersey shore. It doesn't stay sub-zero F for days on end in your location, and probably gets above freezing in the afternoons even during cold snaps. (There are people heating entire homes in MN and ME with ducted Fujitsus, but they have to keep an eye on the pan drain to be sure it doesn't get ice plugged during weeks of high temps that never get out of the teens.)

Do yourself a favor and run a room by room Manual-J load calculation using freebie online Manual-J-ish tools like LoadCalc or CoolCalc, using AGGRESSIVE rather than conservative assumptions about R-values and air tightness. Those tools will oversize reality by a double-digit percentage no matter what, but if you're conservative the oversize factor will be just plain silly. Most HVAC contractors COULD run real load calculations, but when they do they tend toward the super conservative, but an oversize factor greater than 1.2x on the cooling or 1.4x on heating can take a toll on comfort, and any bigger than 1.5x will make them run less efficiently.

Better than LoadCalc or CoolCalc, BuildBetterNW developed a design tool targeted at HVAC contractors specifically for sizing heat pumps using a simplified Manual-J underneath it all. Unlike most load tools the default values for air leakage & U-factors don't trend toward the conservative, giving more realistic load numbers without a lot a massaging. There are other parts to the BuildBetterNW tool for designing ducts (you won't need that) or specifying the capacity curves of the heat pumps (you won't need that either), but with a tape measure and some reasonable guesses on R-values and wall construction any newbie can do it. Unlike pro tools there aren't adjustments for shading factors for the windows to tweak, but it is possible to override the default U-factors for walls/windows/ attic (with some limitations) if you know what you're doing. I've only recently started playing around with that tool, but it's pretty good for a first-cut estimate, and WAY better than a HVAC contractor's WAG, (or even most HVAC contractors' Manual-Js! :( ) You need an email and a name to open an account to use the BuildBetterNW tool, but it's free. I understand there are some online video tutorials too (on the big video hosting site), but I haven't looked for them.

Regarding return vents- they're essential for ducted systems, but don't do a thing for ductless. Even a wide open door doesn't do a huge amount of convective heat transfer, but a transfer grille over the door or other jump ducts need to rely on air handler driven pressures to move that air.

Dana, thank you for the super detailed reply.

I will certainly check out one of the free Manual J calculators you linked and see how it jives with the proposed set-up.

That’s good news to hear for the Carrier units!

We’re using hydronic baseboard heating, so the mini-split would only serve as backup heat. We’re going for cold climate models because the rebates from NJ make them cheaper than the standard models.

With that said about our heating situation, would you say that the single unit per floor set-up still stands a chance in an AC-only situation, or should we still strongly consider ducted units?
 

Mark_P

New Member
Messages
3
Reaction score
0
Points
1
Location
New Jersey, USA
@Dana So I went ahead and did my best to run the CoolCalc Manual J calculation and did two scenarios: aggressive & conservative. Aggressive got me to roughly 10,000 BTUs for cooling load and conservative got me to 14,500 BTUs cooling load. The biggest difference was what I specified for the attic's R-value (R11 vs R38, since I'm getting my attic insulation redone and I *believe* it'll get me up to R-38) and the house's envelope tightness (loose vs semi-loose).

Unless I made some errors along the way... this is insanely lower than what anyone has proposed.

I've legitimately had contractors come here and propose a 60,000 BTU set-up!!!!

So, I'm definitely going to share this with the contractor we wanted to hire, but how do I got about doing this without coming across as arrogant? I'm sure contractors don't like dealing with know-it-all customers.

Also, considering the ducted units... we've got closet space in the adjoining wall between the bedrooms downstairs that also touches the external wall that might be a perfect fit. Additionally, our sealed off utility closet will also give access to the two rooms upstairs as well.

I think I'll give this contractor a call later, if you guys don't mind advising me as to how to go about it.
 

Dana

In the trades
Messages
7,889
Reaction score
502
Points
113
Location
01609
Typical 1% cooling design load square feet per ton ratios tend to run about 1300-1500 square feet per ton in reasonably tight reasonably insulated homes that don't have a gazillion square feet of west facing window area. This is true even in the steamy southeastern US. As a general rule smaller homes will have fewer square feet per ton than bigger homes, but it's really all over the place when there is a lot of window area, where shading factors make huge difference.

10,000 BTU/hr is (/12,000 BTU/hr per ton =) 0.83 tons, so for 1300' of conditioned space that would be a ratio of 1300/0.83= 1566 square feet per ton. That feels about right for a tight 1.5 story 1300' cape style house.

14,500 BTU/hr is (/12,000= ) 1.2 tons, which would be a ratio of 1300/1.2= ~1100 square feet per ton, which is still a realistic number.

As a sanity check, this competent HVAC design company in Decatur GA that does dozens of Manual-Js every year compiled a graphic plotting square feet per ton against house size for a few dozen Manual-Js they did for clients (most of them from the Gulf states):

square-feet-per-ton-air-conditioner-sizing.png


Of the homes under 2000 square feet about half of them came in under 1000' per ton, and that was in a region with higher latent loads and somewhat higher 1% design temps than you have on the Jersey shore (= mid to high 80s not mid-90s). My bet would be the lower square feet per ton homes had single pane windows and no wall insulation (still surprisingly common in the southeast). (The graphic was plucked from this blog piece written by one of the owners of that company.)

So you have about one ton of cooling, and it wouldn't be insane to bump that to 1.25 or 1.5 tons of equipment, but anything more than that would run less efficiently and would likely provide lower comfort. Note that with mini-splits you'd need to run them in "DRY" or "DEHUMIDIFY" mode (or whatever the particular vendor calls it) to achieve good latent cooling when the immediate load is on the low side (or even sometimes on the high side). In their normal highest SEER "normal cooling" mode only about 5% is latent cooling- it'll cool the place down, but it can you clammy since it doesn't dry enough.

Cold climate SINGLE zone (or ducted) mini-splits are preferable even in cooling-only applications. This is because cold climate minisplits all use enhanced vapor injection compressors, which in addition to better low-temp heating capacity, improves efficiency AND a higher turn-down ratio in both heating and cooling modes. The higher turn-down allows you to oversize them by as much as 50% (depending on model) and still deliver high duty cycles that deliver decent comfort & efficiency rather than cycling on/off all the time. All Fujitsu units (including the ducted versions) use the right type of compressor, but don't have options for integrated automated pan heaters for managing defrost water re-freezing in the outdoor unit.

The cold climate Mitsubishi MXZ xxxxxx NAHZ multi-zone units (as well as most other multi-zone units, including Fujisu) have fairly hefty minimum modulation levels- half or more the level of your 1% design load(!) , which means the heads will almost never modulate, and the compressor will lose efficiency to excessive cycling. eg: A 2-ton 3 zone MXZ -3C24NAHZ can only throttle back to 12,600 BTU/hr of cooling @ 85F (1% design temps at the shore are in the mid to high 80s) no matter what size heads you hang on the zones. That means in your case the MINIMUM cooling output of the compressor is roughly your 1% design load, and it would modulate only about 1% of the time(!), which means it can't hit it's efficiency numbers.

With an oversized mini-split when it's not removing enough moisture leaving it humid indoors condensation forms on the inside of the unit between cycles when the blower isn't running between active cycles, and they spit water into the room when vanes open up at the beginning of an active cycle. This can happen even when operating in DRY mode if the oversize factor is high enough, which is why you absolutely DON'T want to go with half ton heads in tiny rooms.
Against my advice someone did a "ductless head per room" on a house I consulted on in Vineyard Haven, Marthas Vineyard MA. In a ~120' north facing bedroom in a walk-out bedroom with an extremely low sensible cooling load the room never dries out. Worse still, the Fujitsu 7RLF head tied to the 2 ton multi-zone compressor spits water onto the bed in that room every time it cycles on in summer, even when the outdoor humidity isn't torrid. It's bad enough when it's really humid outside they have to turn off that zone to sleep, or get rained on for a few seconds every hour or two. (The installed a noisy room dehumidifier to fix the splash in the face problem, but it's not really optimal.) You don't want to become a star in that movie series!

Even the 3/4 ton ducted Fujitsu can deliver 12,283 BTU/hr at 95F outside (well above the 9000BTU/hr capacity at which it's efficiency was rated), yet throttle back to as low as 3,431 BTU/hr @ 85F outside. So one of those would still do OK serving a couple of low-load bedrooms, even though it has enough at max capacity to cool the whole house. The 3/4 ton Carrier (or Bryant or Midea unit with the same model numbers) does slightly better, with even more capacity at 95F, and, throttling back to an even lower 2600 BTU/hr @ 85F. If there is any reasonable way to duct it, a separate 3/4 tonner ducted unit per floor is probably pretty close to the "right" solution for both comfort and "as used" efficiency.

What is the ceiling height on the first floor?

Can you sketch out the floor plan of each floor, with the room dimensions (to the nearest foot is fine), and the ceiling heights, with the CoolCalc load numbers on each room?
 
Last edited:
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