Which heat pump set up? (Cold climate)
somedaynow
| Posted in General Questions on
We’ve had a few quotes for heat pumps for our 1947 cape cod (ish) house and are trying to figure out which route to go. The primary issue is that 2nd floor, which has 2 beds and a full bath, has no AC. And it’s uncomfortably warm all year long. We’re hoping to eventually use any system we install for heat too. Right now we have radient baseboard heaters throughout the house for heat. Central AC in the main floor. And two “whisper quiet” window ACs that sound like a large trucks idling in each bedroom on the 2nd floor. The 2nd floor finished area is only about 500sq ft. Because of the awkward layout, every contractor is recommending a ductless head in each of the small bedrooms.
One contractor noted we could do a ducted unit with supply vents in the hallway. The system and ducts would be in our large knee wall/ storage dormer, which is insulated at the roof line, albeit imperfectly. But they felt we wouldn’t be happy with that system because it wouldn’t cool/heat the bedrooms well. There is one NW gable wall in the hallway that could fit a ductless head. But every contactor has raised concerns about the cool air falling down the stairwell before getting to bedroom 1. And having to make a 90 degree turn at the end of a long hallway to get to bedroom 2. Nobody is willing to put a ducted system in the top attic to reach the bedrooms, and obviously ducts in an unconditioned space like that isn’t great anyways. And all the contractors agree there’s no extending the vents from the current central AC to get upstairs unless we plan to do a gut remodel.
I’ve consider stealing a bit of closet that has a small shared wall with the two beds and the hallway to put a mini ducted system in. But the closet only has interior walls, it’s in the middle of the upstairs. So we’d need to use the built in pump for the condense line and I’ve read a lot of negative stories about noise and leaking issues drain pumps can cause. So I’m hesitant about this set up. And we would need to look at other brands of heat pumps/potentially more contactors. The minimum output on the concealed duct units are very bad on the Mitsubishi and not much better than two single Mitsubishi ductless unit for a ducted Fujitsu. We’ve mostly looked at Mitsubishi hyper heat units because it seems the most common that contractors use here. But several also do carrier/Bryant, which seem to have lower minimums. But it also seems like carrier/Bryant isn’t as highly regarded quality wise.
I’ve also considered trying to put one ductless unit in one bedroom and then adding air transfer vents between the two rooms but I worry about noise transfer. The bonus of the wonky layout is that the bedroom doors are far apart and that helps minimize noise transfer. Right now my husband and I use one of the rooms. But within a couple years, we’ll have our 2 kids each in an upstairs room, with us in the main floor bedroom. So maybe we shouldn’t care as much about noise transfer for kids rooms.
The two leading proposals we have (all Mitsubishi hyper heats) are to either 1) put in a 6k btuh single zone head/condenser for each 2nd floor bedroom or 2) replace the downstairs central AC with a 30k btuh multi zone unit. The 30k btuh multi zone unit would hook up to an 18k btuh air handler for the first floor and the two 6k btuh heads for the second floor bedrooms.
The current AC unit is only 5 years old (installed by prior owner) but our boiler is 22+ years old. We’d love to test out the heat pumps for heat while the boiler still works for backup. But I don’t feel great about replacing a 5 year old AC unit that’s working fine.
The 30k btuh unit qualifies us for $8k in state energy rebates (funded by federal IRA funds). So the 30k btuh unit actually comes out the same or slightly cheaper than the two single units for upstairs, because those won’t qualify for the rebate. And if we do the two single units we will need to bring in an electrician to upgrade our panel or add a sub panel. The 30k btuh unit wouldn’t need any electrical upgrades because it would just take the place of the current central AC breaker. So less expense and hassle electric wise.
But I’m worried about the higher minimum output for the 30k btuh unit. The 18k btuh air handler has a minimum cooling 4300 btuh and heating 6200 btuh. The 6k btuh heads have a minimum 1700 btuh cooling and 1600 btuh heating. But the 30k btuh condenser has a minimum 12,600 btuh cooling and 11,400 btuh heating. Even the minimum output for two single zone 6k btuh units seems high for upstairs because the rooms are so small and we like to keep the doors shut at night.
One contractor used an iPad lidar software to do a manual J and came up with the 30k btuh unit recommendation. But I actually think the proposed equipment is based more on them thinking that each upstairs bedroom needs a unit to effectively cool/heat them and thinking that our current ductwork can’t support more than a 18k btuh air handler system. The multizone condenser when connected to an air handler can’t be less rated btus than the sum of all the connected units. Thus a 30k btuh unit. (If they were all ductless heads the condenser could be less btus than the sum of the ductless heads.)
I’ve used cool calc to do my own manual J and I’ve calculated current heating usage using the article on comparing therms used to heating degree days for the same period. I put in cool calc some of the upgrades we’re making in the next 6 weeks to 2 years, but I didn’t change the air infiltration rate, even though we hope that improves too. On Cool calc I get:
2nd Floor
4900 btuh heating
4650 btuh cooling btuh
Main floor
14,500 btuh heating
17,000 btuh cooling
Basement
11,900 btuh heating
Total cooling: 21,650 btuh (main, 2nd floor)
Total heating: 31,900 btuh (main, 2nd floor, basement)
The basement has radiant heat in the unfinished and some of the finished areas. It doesn’t currently have any ducts from the AC system.
I live in Wisconsin and so the cool calc loads don’t make complete sense to me in that the cooling load is higher than the heating load. But when I compare to our current heating usage, the two worst months (late Dec to Jan, and late Jan to Feb) I get 32,000 and 32,600 btuh at the 1% design temp of -3. That’s not far off from cool calcs 31,900 btuh total for all three floors, especially since cool calc assumes more insulation in the top attic and new windows vs our current ones. Although, the current heating usage has the basement heated at a lower temp than the rest of the house and cool calc is set at the same temp for all floors.
I’m going to try to include a picture of a not-to-scale but hopefully close enough floor plan of the 2nd Floor. The ceilings are 7ft high, except the hallway, which is 7ft high for about two ft of width and then slope down to 4ft high for the remaining 4 ft of width. The top of the doors are only a few inches from the ceiling so soffits over the doorways isn’t an option.
I guess my questions are what would you do to get AC/heat to the top floor? (That is not a portable AC unit or window units. I can’t stand the noise of even the “quiet” ones.) Will we regret a multizone for both floors in terms of the higher minimum output even if it’s looking like the most economical option? Any other creative solutions to get right sized equipment for such a small, closed off space that will keep each bedroom comfortable? Any ideas on brands/models that would better suit our multiple low-ish capacity needs?
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Replies
If it is the same cost I'd just go for the larger multisplit.
Is there no way to have one ducted unit for the entire house and route duct to the upstairs rooms through the first floor ceiling or walls? You might need a chase for that.
What size is your current central AC that serves just the downstairs? If it is possible to route duct to the upstairs bedrooms could you possibly install that duct and just use the existing unit until it is time to replace the boiler and install one central unit to serve the whole house?
You say you have a boiler, what type of heat do you currently have, hydronic with radiators? For the whole house? Could you estimate your heating demand from the fuel source for this boiler and its efficiency?
I don't think we're going to get sufficient ducts from the basement to upstairs for one ducted system. There actually is one duct chase that goes from the basement to bedroom 2. It blows cold air but doesn't do anything temp wise. We still had to add a window AC (and so did the prior owners). It's flex duct and too small. And there's no return air. There's not really enough return air even for the first floor. One problem trying to get ducts up is the web of pipes for the hydronic heat they have to fitted around. The undersized duct to bedroom 2 can't be made bigger, even if we were willing to make the chase on the first floor larger, because in the basement it's wedged tight in-between a bunch of pipes. 3 contractors all agreed we're not getting ducts up there without significant remodeling. We could seek more opinions.
The current AC unit for the main floor is 18k btuh. One contractor mentioned that the current set up couldn't handle a larger sized system. Maybe that changes if we add ducts. I suspect redoing the duct work might cost more than replacing the equipment/adding ductless heads upstairs.
Boiler is hydronic base board radiators. This is for the whole house, including most of the basement (partially finished, heat in both finished and unfinished areas). We don't use the basement for more than storage right now, so I'm not too worried about heating it. But we didn't mess with the zone knob this year so it was heated this year and reflected in the heating demand estimate I did. I did estimate the heating demand from the current boiler at 32,000 btuh (Jan numbers) to 32,600 btuh (Feb numbers). I used the method in this article: https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler