Right Sizing a Ductless Minisplit HVAC System
Hello, I’m looking at switching my heating and AC from a gas boiler and window units to min splits.
I’m wanting to make sure I provide enough heating and cooling without going too big.
My contractor is recommending the two systems below, and I am attaching the Manual J and house drawings. I show the approximate locations of the units in the drawings, and rooms are label relative the the Manual J.
The house is divided into two apartments, my tenant lives on the top floor (right-most drawing), and I live on the main floor and basement. The spaces are separated by an entrance hall (room 4 in the drawing). Grey shading means I am not expecting to heat those areas.
I live in Baltimore, but am not planning on using ultra low heat models as it so rarely gets super cold here.
Main Level
- 7kbtu in each basement and front dining room
- 12kbtu Living room
- 3 ton regular heat outdoor heat pump
Second Level
- 7kbtu as requested in each of the rooms
- 2 ton regular heat outdoor heat pump
Any thoughts?? I have tried to look up exactly how to read the manual J but am having trouble understanding.
Thanks in advance!
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Could you confirm how your house is constructed? How old is it? Is it freestanding or attached to adjacent houses? (I ask because it looks narrow.)
Edit: Initial thoughts... assuming the Manual J is correct (can't tell if it is without knowing construction type), the contractor is proposing over 24,000 BTU/h (2 tons) of capacity beyond the Manual J maximum load (which is a 33,344 BTU/h total cooling load).
Thanks Patrick, yes Manual J is done knowing all the specifics of the house, lots of back and forth with the preparer on details so I think it's pretty good. It's a brick row home with houses on either side (the purple in the drawing indicates where the adjoining walls are). Windows are original, with some storm windows. I have had the attic insulated to about R40. I will replace the windows eventually.
RE the overage, this is exactly why I am posting the questions here, seems like too much, I already brought them down from even bigger units. This is the third contractor I've talked to and I had the Manual J done because all the bids seem over the needs.
So would you think two 2 ton outdoor units would be fine? I'd still be over but it seems like a 3 zone product doesn't come in less than 2 tons.
As far as the Manual J goes, does 2,034 square feet of exterior brick wall sound right to you? (It seems high from my envelope math, based on the drawings. Since the drawings aren't fully dimensioned, I'm doing some estimating.) And to be clear, I specifically mean exterior brick wall, not the party walls, and this would be with window square footage not included.
I'm wondering if they modeled the party walls as exterior, which I think is a mistake.
That does seem a bit big but don't think it's including the party walls. I get 14-1500 w quick calcs. I am on a sloping lot with a walk out basement, so the back of my house has 3 stories exposed. Also notice in the drawings that there is an area without a party wall where there is space between the two houses, kind of a notch, so more exposed surface there 20 feet plus 3 stories there.
Most mini-split systems are capable of a high degree of modulation.
This means that their lowest output levels are much lower than their highest. This is achieved by some combination of multi-speed or continuously-variable-speed compressors and fans, and also the potential for zoning of multi-head systems/multiple systems. This did not used to be true: old-school central heat pumps often had only one speed, and addressing part-load conditions (the typical operating condition) required cycling them on and off repeatedly. This was not optimal and is the basis on which many conversations about right-sizing heat pumps derives.
One result, then, of using mini-splits is that the entire conversation of right-sizing is simply not as important as it used to be. There are at least two other considerations that might reasonably come into play. One is cost: when you start talking with contractors about mini-splits, you are likely to get a wide range of quotes (not as wide as 15 years ago), with very little correlation to size of systems in my experience. That is, a bigger system may be significantly cheaper than a smaller system just b/c the contractor, even controlling for high-quality systems only (which is much more important than the right-sizing convo IMO).
There may also be conditions in which over-sizing makes sense: for a vacation home or other situation where quickly conditioning the space has particular utility, for example.
Warmly,
Keith
Thanks Keith, I am aware of the modularity, but my understanding is the units don't go all the way down to zero, so if the load needed in a room is lower than the unit will go, it will bottom out all the time, and I lose the benefit of the variability. Correct?
Manual J is a great tool but it is a computer program if you feed it garbage the output is useless.
Have you done a fuel use study for your building?
The fuel use study is based on hard numbers not guesses and feeling like many of the numbers input into the manual J
https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler
In terms of dollars and cents if you are on city gas a heat pump is likely to cost more to operate.
In terms of pure comfort, it is almost impossible to beat a gas furnace.
Walta
Heads in a bedroom running from a multi-zone outdoor unit are a disaster waiting to happen. The outdoor multi-zone units don't ramp down much, and you'll over-heat or over-cool the bedrooms.
Look at the minimum heating and cooling outputs in the shoulder season temperatures (40F in the winter, 70s-80s in the summer) and you'll see that often the min output is more than a single zone can take.
Search thru the archives here and you'll find lots of people that regret this design approach.
Thanks on this. Looks like the 2 ton multi zone they are specifying ramps down to 6800 heating and 6100 cooling. (Specs attached)
So to get minimum output for one room in a 3 zone system would I divide that by three? Meaning the heating goes down to 22oo, cooling to 2033?
Those are just around the smallest room total (for any of the rooms where I plan to put a wall unit) in my manual J.
Does this make sense, would that mean a 3 zone system similar to what I initially asked about could work OK?
Or am I better off with something very different? I'm reading some things on this site that seems to indicate I could have less indoor units. This "rules of thumb' article seems to say they only have one indoor unit for each floor. https://www.greenbuildingadvisor.com/article/rules-of-thumb-for-ductless-minisplits . I guess then I am relying on air moving through hallways and into other rooms. I imagine you don't get speedy change in temp in rooms without wall units, but once the home reaches desired temp then a system like this works fine? Would you all recommend this vs. heads in each of the main rooms?