Cold climate heating strategy
Hey everyone,
I would like some feedback on my proposed heating systems for our new house build. Here are some details about our house;
-Climate zone 7A
-Heating load of 30,000btu
-Design temp of -41c! (Yukon)
-Quad pane windows
-R56 wall insulation
-R90 roof insulation
-HRV
-Shooting for a .75ACH/50
I am thinking about using a Mitsubishi Hyper Heat 30,000 heat pump, and two 18,000btu indoor ceiling cassettes, one on each floor in a main area. Our front entrance, and 2.5 bathrooms will have electric in-floor heat, and the 3 bedrooms will have electric baseboard heat as a backup. The main living area on level 1 will also have a wood stove.
I’m curious as to opinions on the ceiling cassette air handlers. Do you think that will be sufficient for supply the whole house with the majority of the heat? I have added baseboard heat in the bedrooms, but am hoping to only have to use them when it is very cold out.
I appreciate the feedback. Any articles or case studies I can read about heating a house with 1-2 air handlers?
Matt
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Replies
Normally what you do is calculate room-by-room heat loss, and then make sure that each room has enough heat to match its heat loss.
Are those ceiling cassettes ducted? What you would do is run ducts to each room on the floor, with the ducts sized so that each room gets the right amount of air to provide the amount of heat you need. Generally the airflow between rooms is so low that you don't get a meaningful flow of heat, so you design with the assumption that there is no flow between rooms, each room has its own supply. This is particularly true with bedrooms, which often have the doors closed when they are occupied.
Originally I wasn’t thinking of a ducted system for the bedrooms. After more reaching on GBA and other sites, and your response I’m thinking that relying on a 18k btu unit outside of the bedrooms may not be enough to heat the bedrooms.
I do have a heat loss calculation that I can reference for each room in the house.
As I see it the problems with ceiling cassettes
1 They are mostly in the attic and outside the ideal thermal envelope.
2 Often they have ductwork in the attic that is likely to leak air outside of the thermal envelope.
3 They are on the ceiling so they collect return air near the ceiling. Since heat rises the return air entering the cassette is the warmest air in the building making the HP work harder and operate less efficiency.
Consider the rarely used floor mounted cassettes. That will suck up the coldest air and heat it.
Consider a ducted mini split in the 200 rooms closet feeding ductwork in a dropped ceiling the corridor 103 and parts of 106
Great plan.
As cold as it is will the 201 bathroom sink and kitchen sink doors need be open to prevent freezing?
Walta
I appreciate you looking at the plans, your routing suggestions are appreciated. I don’t think the sink cabinet door will need to be opened to prevent freezing. I live in a horribly insulated house (r8 walls) and we don’t have plumbing issues.
Do you need an HVAC design done and signed off by an HVAC engineer to get a building permit? That would probably be the best place to start to discuss options. They would be the ones doing the Manual J (heat loss) calculation too.
I love the plan.
I do not need a HVAC design for a building permit. I do have a heat loss calculation that I had done for the build. I also love a good plan!
-41c/f? Don’t. Even with vapor injection pumps (ie hyperheat)…
Besides resistive heat- what are you other fuel options?
My 2 cents
The most common is home heating fuel, some folks have propane, wood heat is common and like everywhere there is a big shift to all electric.
If you have only a few cold snaps then the wood burner and electric aux. heat might do it. If you have weeks and weeks below like -20°C then you might have to look at a dual fuel system. Granted that this makes the system more complicated. Reminds me of the BrHEAThe system:
http://cchrc.org/media/Feasibility_Study_of_Integrating_ASHP_in_Cold_Climates_for_NRCan.pdf
What you are suggesting is possible but you'll have to do a bit of a deep dive into the engineering data books.
Most hyper heat units have ratings down to -25C to -30C. Some units continue to run bellow this at reduced capcity and some shut down. The ones rated for -30C do have an above 1COP so you do want something that will continue to run. In cold weather you'll loose a fair bit of capcity so you'll have to oversize a bit.
Ceiling units are not the easiest to work with. The 4 way units are very large so you'll have to watch your rafter plans to make space for it. The 2 way units are smaller but as pointed out earlier, they are open to your attic, so you'll have to build an insulated and air sealed doghouse for it in the attic.
Since you are not in a place that needs cooling, your plan of point source heat pump and some resistance heat should work.
My $0.02 is go for a single zone slim ducted units or a multi position air handler. This will let you run ducting to all the rooms so you'll rely less on the the baseboard heat. These ducted units can also have the option of a resistance strip heater, so you can skip the baseboards.
If ducting and air handler is outside the budget, I would go for wallmount units. Each should be on its own outdoor unit. This one to one configuration tends to be the most efficient from all manufacturers plus you'll get better turn down which is needed as you'll have to oversize for the very cold. Again, make sure to check the engineering data book, you want something that can supply heat most of your winter.
For technical information on Mitsubishi units you can go here:
https://mylinkdrive.com/USA/M_Series/R410A_Systems-1/Outdoor_Equipment/R410A_Outdoor
This is probably a decent starting point:
https://mylinkdrive.com/viewPdf?srcUrl=http://s3.amazonaws.com/enter.mehvac.com/DAMRoot/Original/10006/M_SUBMITTAL_PVA-A42AA7_PUZ-HA42NKA1_en.pdf
There's two questions here: one is sizing the heat pump, the second is heat distribution inside the house.
To the first, when we were talking about this a few months ago I made up this spreadsheet that models heat pump performance in your climate:
https://docs.google.com/spreadsheets/d/1sIpGCu2Rkb-SqlzY0vw-DUIAQGv8KMxMXG3p1QjuAaQ/edit?usp=sharing
When I made it you were talking about maybe doing air-to-water, so the performance curve was based on a Chiltrix and the column headings all say "AWHP." I've updated it to use the performance curve of this 4-ton Mitsubishi: https://ashp.neep.org/#!/product/112104/7/25000/95/7500/0///0
If you look at that page, they have a capacity vs temperature, it only goes down to -13F but it's a straight line from 17F to -13F. If I extrapolate it out as a straight line I get 64% of nominal capacity at -37 and a COP of 1.05. There's not guaranteeing it would actually run at that temperature, it might shut off, but in theory at least it can still put out heat. You'd only have to go to resistance heat when it's below -37, which I calculate as 88 hours a year and 6% of your electricity use.
The table is all in metric units. I put in 8.8 kW for the heating load which is the same as 30,000 BTU/hr.
Look it over.
Do you think it is really worth running the heat pump full speed and wear it out quickly when the cop is below 1.2 ? What would be the cop of the heat pump at these temperatures but reduced capacity, wouldn't it be better to run it half speed with assistance of heat strips ?
You are proposing that running a heat pump at half capacity would extend it's life? I don't know that there is data to support that. Capacity of a variable speed heat pump is typically controlled by a variable flow refrigerant valve, not necessarily speed of the compressor. For example, the AC compressor on one of my vehicles is belt driven and does NOT have a clutch. It's just connected to engine speed all the time. All control is done by a variable flow valve.
That may be what the engineers are getting at when they shut down the heat pump at a low temperature, but quite a bit above the point where COP gets to zero. With compressors ensuring the flow of lubrication is a big deal, it wouldn't surprise me if at very low temperatures the lubricant flow becomes an issue.
I can agree on that. At -40, you could almost do the Dairy Queen blizzard flip with a cup of oil. But if it is designed to run there, I wouldn't have concerns running it to the max that the system allows. Part of the cold temperature derating could be from the system controls.
Here is a link to a pilot program regarding heat pumps in the Yukon. Please take a look, I’d be interested in your feedback!
From my reading, it appears that oversizing a outdoor unit (42,000 unit for a 30,000btu heat load) is very inefficient. As they say, this is only a 3 month timeframe for this data, so it may change things a little.
https://yukon.ca/sites/yukon.ca/files/emr/emr-air-source-heat-pump-pilot-project-technical-report.pdf
The data only covers february to april. The coldest period is missing... but data around -10°C daily average is abundant.
Seeing ICop values under 2.0 at 0°C in this report is very disappointing and suspicious. I feel something is wrong with installation, sizing or usage I don't know, but I would not be happy with these results in my home.