How many minisplits for an old house?

| Posted in Mechanicals on

I have a three story brick duplex built in 1925. There is currently no ductwork (steam heat) and I doubt that it’s feasible to install. Windows are wooden double hung with storm glass, they are in fantastic shape and I’m not particularly interested in replacing them.

Unfinished basement, 1st and 2nd floors are all 1380 sq ft, but I’m not concerned about conditioning the basement. 1st and 2nd floors have 3 bedrooms, living room open to dining room standard doorway to kitchen. 3rd floor is 690 sq ft split into 2 rooms.

Given the steam heat, I only need splits that provide A/C, I assume these are cheaper? If not, can you totally shut down the systems through the winter when not calling for heat? I’ve seen there there’s some standby power draw, but can that be avoided?

I need at least two different systems since the 1st floor is an independent rental unit.

Special considerations:

We’re in a zone 6 climate (used to be 5, but “they” recently changed it)
The south west room on each floor is particularly susceptible to solar gain due to a lack of shade.
My wife likes to bake, so the oven is frequently in use.
For my family the all the bedroom doors are usually closed because the children like their privacy and the 3 year old gets into things.
Given all the non-optimal factors does it make sense to put a heat exchanger in each bedroom? Assuming the kitchen should be cooled that would mean a 5 head system on the 1st and 2nd floors, and a 3rd 2 head on third floor. That seems like overkill, but I’m not sure how I’d cool all the rooms with the doors shut.

9,000 Btu/h is the smallest head available on the system I’m primarily looking at. So it seems like I’d be looking at 9-9-9-12-12 = 51,000 Btu in potential total draw, but in reality the max load is more like 39,000. The condenser with this system is actually 42,000 btu so you can’t run every head full out, but I’m not planning to.

tl;dr – Here’s the actual questions:

If I don’t need the heat is it worth looking for a mini split without the heat feature? These seem harder to find
If not using the head feature can you avoid standby power consumption?
Is it totally crazy to have 5 heads and 51,000 btu of cooling (backed by a single 42,000 btu condenser) in six rooms totaling 1,380 sq ft.
Is there some better solution I’ve somehow overlooked? The house came with an assortment of window units, but there don’t seem to be appropriate (grounded) outlets near the windows they would make sense in.

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Replies

1. GBA Editor
| | #1

Aaron,
General guidance:

2. You'll probably want a ducted system -- for example, a ducted minisplit. But it's hard to say without knowing more about your load and seeing a plan. Ducts can be disguised in soffits or can be boxed in.

3. If all this seems complicated, you can always install a few window-mounted air conditioners.

2. Expert Member
| | #2

All multi-splits I've ever seen are heat pumps that both heat & cool, but there are many single head mini-splits that are cooling-only, but you'd have quite an array of condensers all over the yard to put a single head in every room.

Mitsubishi's cold climate multi-splits can all use the FH06 head, which is somewhat cheaper than the FH09s, and has the same minimum modulation outputs. With the room by room load numbers you'd be able to figure out which rooms need the 3/4 tonners and which can take the half-tonners.

Fujistu's multi splits can all use the 7RLS heads too, but SFAIK they don't have multi-split compressors with capacity tables below -5F, which may be a code-compliance issue if your 99% outside design temp is cooler than that. (A city name or ZIP code would be needed to take a WAG at what your actual design temperature is.) That's not a problem if your primary purpose is cooling though.

I'm not clear- is that 2 floors of 1380 square feet each, for 2760 square feet of fully conditioned space, or is that 1380' total, for floors 1 & 2, plus another 690' for the top floor, makes either 3450' or 2070'. Either way a cooling load of 39,000 BTU/hr seems on the high side for a house with that configuration (ridiculously high side if it's only 2070 square feet.) You're probably looking at no more than 3 tons of cooling load, possibly only two. It's likely that zoning by floor makes the most sense, and you may be able to make use of mini-duct cassettes to cool adjacent rooms within a zone, depending on where the room by room load numbers fall.

Three tons of cold climate ductless can typically deliver 38-40,000 BTU/ hr or so at -5F, and might be a viable heating solution- it's worth looking at a full Manual-J for both cooling and heating, since upsizing slightly to cover more of the heating load would be unlikely to adversely affect cooling performance & efficiency. It's unlikely that you need fully 4 tons of compressor for either cooling or heating. Be aggressive rather than conservative on the assumptions in the Manual-J, or the oversizing factor may end up sub-optimally high.

3. | | #3

Yes, 1,380 x2 + 690 = 3,450' of conditioned space.

Like I said, I'll be using the hydronic heat system, so I don't particularly care if the split provided heat. If I'd save money and/or reduce maintenance by doing cool only I'd be all for it.

Zip code is 44107, I don't know of any code requirements for design temps.

The 1,380 on the first floor is a rental unit so must be separate from the rest.

I don't want window units. Building and routing ducts seems harder than just hanging more heads, I don't mind the look of them. I could maybe give up the top of a closet and build some soffit to duct 2 bedrooms together.

For the 1st floor, I suppose it would be much easier to hang a 1.5 ton in the dining room and just be done. I don't know how the distribution to the bedrooms would go with the doors closed most of the time is all.

For the 2nd and 3rd, could do the same 1.5 ton in the dining and .5 ton in each of the 3rd floor rooms.

Certainly simpler/cheaper with only 4 heads and two condensers. Going that route I just worried about being undersized and distribution to the bedrooms. Better zoning if I can cool just the bedrooms at night and just the living space during the day.

4. Expert Member
| | #4

Your 99% outside design temp is about +5F give or take (Cleveland's is +6F), with a 1% design temp of about 85F (Cleveland's is 86F). That's neither a very severe cooling or heating design temp to deal with.

Mini-split cooling output is rated at 95F outdoors, 80F indoors, a 15F delta, and you're looking at about 10F temperature difference at the 1% outside design temp (75F in, 85F out) so it's primarily about direct solar gain (mostly windows), and latent loads. At that lower delta-T the mini-split capacity is well above the rated-tested number. A typical 3/4 ton unit would have a max capacity of about 1-ton at a temperature delta of only 10F, which would probably cover the first floor load, maybe even the second floor.

A typical first-floor cooling load will be about a ton per 1200' of living space, could be as much as a ton per 1500' with better shading factors. The only the 1380 first floor unit would need more than 1 ton of mini-split would be if it had a lot of unshaded west facing window, taking large solar gains late in the day after the brick is already heated up by the sun.

Only the top floor is likely to have a load to square foot ratio less than a ton per 1000', and at 680' and even there 1-ton mini-split would probably be overkill, but probably not super-extreme overkill, depending on how much attic insulation there is.

These are all rule-of-thumb type WAGs and not a substituted for a Manual-J. If your shading factors are pretty good a ton of compressor per floor might be extreme overkill for the cooling load.

A 1920s brick veneer with insulated 2x4 interior and clear glass storms over wood sash single panes will typically come in around 13-17 BTU/hr per square foot of conditioned space, depending on air leakage and the shape of the house. If it's a pretty rectangular footprint and reasonably air tight it's likely that you'll be even lower. Order of magnitude you're probably looking at about 50,000 BTU/hr for total heat load @ +5F probably less, but could be more. If the basement walls aren't insulated it'll add quite a bit to the first floor's heat load.

As previously stated, I don't know of ANY multi-zone options that are cooling-only, but there are MANY cooling only 1-head single zone wall coil types. If the floor plans are at all suitable for point-source cooling, cooling-only mini-splits are pretty cheap- on the order of \$2-2.5 K/ton installed for high SEER (>20) name-brand units. This pretty-good name-brand 1-ton runs about \$1200-1300 for the basic unit, and can deliver ~13,600 BTU/hr of cooling at max speed at a delta-T of 15F.

There are cheaper, lower efficiency units out there, some of which have reliability issues, but the up-charge for a heat pump version that still has a rating at +5F is only a couple hundred per ton, and depending on outdoor temp, local utility rates and load conditions, will be cheaper than running gas-fired steam, often a lot cheaper during the shoulder seasons.

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