Heating from basement
I have a build I am planning and am going back and forth on how to heat it. For the purpose of this discussion let’s set aside any cooling needs.
I want to heat with radiant, not because I want warm floors I understand in a super insulated air tight home I will not have warm floors, but I like them for other reasons, they are quiet, and I like not seeing any evidence of heat registers and I like no blowing of air, etc.
I plan to use ICF basement walls so I will have R-22 minimum below grade, I could be swayed to add another inch of foam internally if makes a huge difference. Likely R-10 under slab. Above grade walls will be atleast r-40 and roof r-60.
In my current design there is an open stairwell to the basement with only a railing style half wall. Main floor is an open floor plan with vaulted ceilings and a half loft open to below.
I have read articles both here and elsewhere that in an airtight super insulated home the entire envelope can often be heated from the lowest level with a small temp differential maybe 2 degrees or so being seen with each level.
I am trying to decide if I heat just the basement slab or if I should also put radiant in the main floor, or even if I should not heat the basement slab at all and only heat the main floor. But the basement is getting poured either way and adding pex is easy enough. I have read a couple posts of people claiming they did both levels and the thermostat on the main floor never calls for heat if the basement is on.
For context this build is for an efficient home for my aging mother. All the main living space is on the main floor, the basement will likely have a play room for grandkids and other storage but there will be no bedrooms or any reason for her to need to go down there. The open loft will be mostly a library for her large collection of books, there might be a guest bed up there but it will be very infrequently used. I’m am basically unconcerned with the temp in the loft (and only minimally concerned with the basement temp), but if I could heat the basement to say 69-70 to keep the main floor 67 without adding the expense of a second zone that seems like maybe the way to go?
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Replies
I would start off with a room-by-room heat loss calculation. From that, you'll see how much heat has to flow from the basement to the rest of the house. There's two ways it can flow, either through convection as air rises, or through conduction through the basement ceiling/ground floor floor. For either way you'll need a temperature differential in order for heat to flow -- the basement has to be warmer than the ground floor, the ground floor has to be warmer than the top floor.
Do the math, but I think you'll find the levels of temperature differential and air flow you'll need will be higher than you'll find acceptable.
A further thought:
The whole point of a heating system is to provide comfort. Comfort comes from matching the heating output to the heating load precisely enough to keep the temperature within a desired range.
The heating load in houses can change quickly, this is particularly true in houses that have solar gain through the windows. In general, to maximize comfort you want to have a heating system that can respond as rapidly as possible to changes in heating load. The system you've described is pretty much the opposite of that.
You wrote, "For the purpose of this discussion let’s set aside any cooling needs." But if you have AC, you can use that delivery system to deliver heat when and where you want it, and quickly.
My position on in-floor heat now is size your ductwork for cooling, use in-floor to add whatever else is needed to meet your heating loads.
I don’t really plan to add any central air cooling. I have never had AC in my own house and have done fine with that my mother who I am building this for also has never run AC and I can’t foresee her starting now at this point in her life. I do think I might add a couple of mini split units to the build though. This will allow for some cooling if desired. But the main reason I was thinking about them is actually heat. While I think the in floor heating can and will handle the load fine if designed correctly, the recovery time is quite slow. If for example a party is being hosted and the front door is constantly opening and closing I thought it might be wise to have an additional heat source with a much faster recovery time.
The house will obviously need and have an HRV system can this be designed in a manner and possibly oversized or changed in some way to be used to help move around the warm air rather than just for the ventilation purposes needed in such a tight home?
Again, do the room-by-room heat loss. I think you'll be disappointed to see how much heat the HRV can move around the house compared to the heating load.
Depending on how picky your code and insurance people are they could simply say NO based on the requirement that every room must have a heat source provided.
If the thermostat is on the main floor the system will adjust itself and keep the thermostat at its set point. What the basement and loft temps will happen to be will depend on the current outdoor temp.
Walta