Are minisplits enough, or do we need floor heating too?
user-5996667
| Posted in Energy Efficiency and Durability on
We are retrofitting an old bungalow in Toronto. It will be air-tight with above-code R-values. We are going all-electric with ductless cooling and heating. For heating, would mini-splits be sufficient? Or do we need backup heating like hydronic floors?
The finished house will have two floors, about 3000 sq. ft. plus basement. Ventilation will be HRV.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Thomas,
Ductless minisplits or ducted minisplits should work fine.
I'm not sure what you mean by "backup heating." If you are worried about staying warm during a power outage, installing a hydronic heating system won't help -- because a hydronic heating system requires electricity to operate.
If you want a backup heating system that operates during power outages, you should install a wood stove.
There are many mini-splits with capacity specifications at Toronto's +1F/-17C 99% outside design temp and lower. As long as you consult the extended temperature capacity tables and pick something that has a bit of margin at that temperature no backup is needed, even for those rare days where it hit's -20C or even -25C and beyond. The Fujitsu -__RLS3H series or Mitsubishi -FH__NA series both have published capacity at -25C. For ducted units, the Fujitsu -__RLFCD has ratings a -20C. The Mitsubishi MVZ series air handlers do too, when married to a cold climate multi-split compressor. They all still operate at lower temperatures, than the lowest temperatures indicated in the extended temperature capacity tables.
So, you still have to do your homework and calculate the loads to get it right. Oversizing by too much can lead to lower efficiency, a common problem for those insisting on a ductless head in every room.
To Martin Holladay,
We're mostly not worried about power outages, we're worried about days when it's so cold that the minisplits wouldn't be powerful enough to heat the house sufficiently. (Although the wood stove idea has a great deal of charm.)
Also, there will be a bedroom and a rec room in the basement and we want to make sure they'll be warm on very cold nights.
Thomas,
In Toronto, you should be able to design a minisplit system that doesn't need any backup equipment for cold nights.
But if you are worried -- some users of minisplits are -- you can buy a portable electric space heater for $50 or $100. Almost everyone who buys this type of space heater to supplement a minisplit never uses it. But sometimes people need to buy something not because it's necessary, but to relieve their anxiety.
A better-than-code-min retrofit project I was involved with a handful of years ago is heated by three Mitsubishis. Outdoor temperatures dropped to -27C earlier this year, and didn't rise above -15C (the local 99% outside design temperature) for a few days. That house had no problem keeping up with the heat load at -27C, despite the fact that the lowest temperature at which those particular minisplits have a specified output is -25C. This is not unusual.
If you size the mini-split correctly for the 99%outside design temperature load with a bit of margin and you'll have no need for backup even during cold snaps.
Thank you for all the helpful answers. One more question:
Would the flue from a wood stove affect the house envelope? Could cold air leak back into the house through it? And some people express concerns about risk of fire in airtight homes, is that a real worry?
Thomas,
Q. "Would the flue from a wood stove affect the house envelope?"
A. Of course. (Any feature that penetrates the envelope affects the envelope.) A chimney -- whether brick or stainless steel -- represents a thermal bridge through the envelope, and that thermal bridge degrades the performance of the roof insulation. Moreover, a wood stove chimney can contribute to ice dam formation. If the chimney is installed properly, however, air leakage near the chimney can be almost eliminated. (However, there will always be some air leakage up the flue.)
Q. "Could cold air leak back into the house through it?"
A. Yes, this can happen when a house is depressurized (for example, when a powerful range hood fan is operating.) If you have a wood stove, it's usually a good idea to open a kitchen window before you turn on a range hood fan.
Q. "Some people express concerns about risk of fire in airtight homes. Is that a real worry?"
A. I've never heard of that. I can't think of a reason why the fire risk would be higher in an airtight home.