Juggling Disparate Heating Load Calculations for sizing heat pumps new Maine home
Thanks for your great help with questions re our concrete-free slab and HPWH. Now I am looking for advice on sizing ASHP heating system given widely disparate heat load estimates: An energy model done using WUFI Passive calculates 20,500 BTU and Manual J room-by-room calculates 38,150 BTU. We will be using Mitsubishi HyperHeat heat pumps. HVAC contractor had a Mitsibishi engineer come on site who did the Manual J calculation and is recommending one large 48K with 4 inside heads—one of which is then ducted to 4 rooms and hall—seems too complex and large to me.
We are in Maine zone 6A, total 2700 sq ft, 2 level, walk-out basement, 3 pitch roof; vaulted ceiling 3/4 of upper level—remainder dropped; basement is open floor plan (all conditioned, about 1/3 not actively heated), central open stairwell between levels, tight, well insulated, R values roof 61, walls 41, basement floor 28.
I have much more confidence in the WUFI Passive energy model, except it was based on coastal Portland; our average winter lows are 10 degrees colder and summer highs are 2 degrees warmer. I was not involved in Manual J input, and am sure they did not have access to all needed input info, it also included a mudroom that is outside of envelope.
I am hoping we can heat well with total of 24K. Is that reasonable? One 12K mini-split for lower level and 2 x 6K mini-splits for upper level. (I think the GBA experts here would advise against a 12K 2-head multi-split, right?). I expect some lower level heat to come upstairs. Cooling needs are minimal. We do plan to install electric resistance heaters for back up and super cold spells. I do have a very good and long-term working relationship with plumbing/HVAC contractor, but based on what I read here and elsewhere, I am very concerned that the proposal is too complex for daily use/adjustment, oversized, pricey, and that the minimum outputs for both heating and cooling are not low enough.
Many thanks for your thoughts, recommendations.
Dorothy
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Replies
Most people that get miny splits are happy with them.
Please take some time read the stories of the unhappy people I think you will find the generally have two things in common.
One is the unhappy people have mutable indoor heads connected to the same compressor.
Two they got oversized equipment.
Both factors increase the minimum possible speed for the system and force the unit to cycle on an off.
Walta
I am currently building a house in VT (Zone 6) with similar size and specs to yours: 3700 sf including walk-out basement. R10 subslab, R20 foundation walls, R40 above grade walls, R60+ ceilings. We used Manual J and Wrightsoft for heating load, and both gave us close to 35000 Btu/hr +/- 10%. Because of sizing issues, we will probably end up with three minisplits delivering a max of 48kBtu.
Our house is about 30% larger than yours, so your heat loads are probably not too far off, though I think the 20kBtu from WUFI might be low. I haven't used WUFI enough to know its foibles but I know that it's pretty tough to use well. All of these modelling systems are sensitive to inputs and it is often hard to tell where/if errors are made. Manual J has factors of safety built in so it is generally considered to be "conservative" in that it gives you the maximum expected loads in somewhat worst-case scenarios. I would expect that your actual loads turn out to be somewhere between the two. Remember also that your lowest temperatures only occur for a few hours at a time and for only a few days of the year. Well insulated houses take a long time to cool down, so you may not even notice if the system is technically too small for the load but functions just fine just the same.
You will probably be OK with 24kBtu, especially with resistance heaters for backup and also if you don't mind being a little bit cooler when there's super cold weather. The resistance heaters COP is 1.0, so they're expensive to run, but at only a few hours a year, not too bad. Some very careful review of the models might help you zero in on whether to go with 24kBtu or 30kBtu for best efficiency but it might take years to notice the difference in costs.
thank you peter; that is helpful; also given that system will mostly be used for heating (and high ceilings) do you recommend using floor or low-wall interior units?
I guess that low wall units would theoretically be best, but very few people use them because they block furniture and are arguably even more ugly than the wall units. I'm not sure it matters much. The nearly constant low velocity movement seems to mix the room air pretty well. Also, very well insulated houses see less air stratification than leaky ones.
What's the rationale for ductless vs. ducted heat pump?
Ducted minisplits are often specified for areas that are cut up by small rooms with doors, like the bedroom area. Each room takes only a little bit of conditioning but they wouldn't get any when the doors are closed. Ductless units are better (simpler) for large open spaces like a typical modern open-plan design for the main living spaces. It sounds like this is the OP's plan.
And yes, on this list there is a clear preference for one-to-one compressor unit to interior head. This because the one-to-one units have a much better turndown ratio and better individual control of the space. Turndown ratio is important in mild weather when the system is under only part load. Large turndown ratios allow the unit to modulate down to a very low (and efficient) speed but continue operating. A unit with low turndown will be stopping and starting more frequently and that kills efficiency. Smaller one-to-one units can also be specified when differential solar loading is expected. It is entirely possible in sunny locations that rooms on one side of the house require cooling when the other side requires heating. You can't do that with a multi head, single compressor system.
Thank you. This has given me courage to stick with my general approach and has been a very helpful conversation for me. We will use 3 hyperheat mitsibushi mini-splits for total 27kBtu. Lower level 12kBtu. Upper level 9kBtu and 6kBtu. We have incorporated access to easily add a 4th unit if needed. Back up electric radiant heaters. My contractor is hoping he is wrong and that I will be warm.