Hydronic Radiant Slab Sensor to Accompany Woodstove?
JonPiana
| Posted in General Questions on
Hello GBA Community,
I built a slab on grade house in Vermont with hydronic radiant heat and I’m trying to figure out best options in terms of thermostats. It is a hempcrete house with walls that are 12″ thick, so it has a lot of thermal mass and is well insulated.
We will be heating the space with a woodstove primarily, and have hydronic radiant heat in the slab which we will use as our backup heat, but also our hope is to use it as a way to keep the floor at a warm temperature all the time. If we relied on an ambient air thermostat then the radiant system will never come on because I am steadfast with burning wood. So I was hoping there would be a solution to use a slab sensor to ensure for my wife that the floor is warm and will kick on despite the ambient temps being higher. I would be most grateful for any insight into setting up an appropriate system with these goals in mind.
Gratefully,
Jon
Barnard, VT
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I use the TH115-AF-024T Aube/Honeywell units.
These can be set to run in air/floor mode (switch on the back) where you can program a set point for air temp but have limits on the floor temp. This will let you set a minimum temperature for your slab and the thermostat will keep it there even if there is no need for heat.
I have done this for kitchens where you want the warm toes feel even on warmer days.
It is very common for electric radiant heat systems for bathroom -- which function in much the same way -- to have sensors embedded in the floor. That could be used to control a circulator or zone valve.
Jon,
I may be wrong, but I don't see how this can work.
In a well insulated house continually heated by a wood stove, all the materials inside the thermal boundary should be very close in temperature to the air. If there is adequate insulation under the slab, it is simply another one of these materials. It will be losing a small amount of heat to the underside, but should probably stay in pretty close equilibrium with the rest of the interior of the house, meaning it will be within a degree or two of the air. That means there is no difference for a sensor to pick up to activate the in-slab heat.
The second problem is that for the floor to feel warmer than it would if there was no in slab heat, it would have to be heated until it was warmer than the air and other inside surfaces. Keeping a large thermal mass like the slab at that elevated temperature would mean it supplied much more heat than the house required without any input from the wood stove.
Spot on, there's no way to make this perform the way he's asking. Even without the wood stove, you probably couldn't maintain a floor that feels warm to the touch without overheating the space. Add heat from a wood stove and it's well beyond possible, short of leaving a bunch of windows open . Then you'd have uncomfortable drafts, not to mention the waste of energy.
I actually read somewhere about a house that ran most of the year with air conditioning and floor heat both active to achieve warm toes without overheating. Needless to say, that is an absurd waste of energy. Even if you did it with a heat pump moving heat from the air to the floor.
You can maintain the slab at or near house temperature without issues with the controls I suggested.
This would mean that some of the heat from the house will come from the slab even if the wood stove is running but that will be minimal.
The idea isn't to get the slab to warm toes feel (at that point the house will be heated or even overheated by only the slab) but just enough to take the edge off the cold slab. Around the stove the slab would be warmed by the fire but the perimeter would still be colder, depending on how well insulated, it will be a couple of F bellow room temperature. Doesn't sound like much but you do feel it.
It also means that the slab will have to heat up less when the fire goes out, so some of the lag of the high mass system will be reduced.
I recommend an experiment. Get two identical 12x12 floor tiles. Put one on a table in a conditioned space and the other on an insulated slab if you have one available, or maybe leaned against an exterior wall to get it to the temperature that the slab would be. Leave them for an hour or two to equilabrate. Maybe one is at 68 F and the other at 65 F. Now put them on the floor, side by side, and stand on them with bare feet. I don't think you'll feel much difference. Both will feel cool.
Comfort is of course a subjective thing, but there have been some attempts to quantify what various temperatures feel like:
“With shod feet, 90% of people report comfort with a floor that is between 66°F and 84°F, and with bare feet a floor between 79°F and 84°F. Your feet (and most of your skin) are typically 91.4°F, so nothing less than that temperature is going to add heat to your feet.”
"With bare feet, the conductivity and specific heat of the flooring material is more important to comfort than the temperature of the floor."
So if the aim is to use the radiant heat to even out the temperate in the house as the wood stove heat fluctuates over a 24 hour cycle, it might work. but if, as the OP seem to be saying, the idea is to consistently keep the floor warm enough that it can be felt, there simply isn't much chance of that when the floor is well insulated, and the primary heat is provided by some other source.
I haven't tried with tile, but I can tell you it makes a big difference with hardwood.
My bedroom is in an overhang. Since I like to keep it colder than the rest of the house, it needs very little heat. Even with 16" of insulation underneath, without any floor heat, the floor is always cold.
By setting the floor heat to maintain the slab at just above room temperature, the floor is much more comfortable without overheating the space. It is not warm, but definitely not cold, much easier on bare feet.
Floor heat puts 2BTU/sqft/degF into a space. Say you have a 1000sqft space, keeping the slab 2F above ambient only puts 4000BTU into the space. You would need a highly insulated space for that to be a large fraction of your space heat. Without heat the slab would probably be around 2F bellow ambient.
A 4F difference is something that you would definitely feel over hardwood, not sure about concrete or tile.
I decided to put my money where my mouth was, and I walked around with an IR thermometer and bare feet. Most of the house is a 68 degree hardwood floor. It's cool, but not cold, enough to make me want to put my socks back on.
I was surprised by how much better the 68 F pine floor in one room felt. No longer really cool feeling--pretty neutral.
On oak steps going down to a unheated entry area, each step was about a degree colder. 65 F was where I found it clearly uncomfortable.
I also had floor tile handy. At 68, it felt colder than the 65 F hardwood. At 80 F it felt luxurious, gently warm. At 77 it felt just barely detectably warm. At 73 F, it felt pretty similar to the 68 F wood floor.
My conclusion is that trying to make a tile or concrete floor comfortable to bare feet is going to lead to overheating in a well insulated space, but that, consistent with what Akos reports, comfortable bare feet are more achievable with wood, especially softwood. Maybe not the luxurious spa-like feel of an 80 F tile floor, but it could make the difference between feeling a little uncomfortably cool and feeling neutral.
Charlie,
That speaks to the second quote in my post #9 above, which came from our old friend Robert Riversong:
"With bare feet, the conductivity and specific heat of the flooring material is more important to comfort than the temperature of the floor."
While you can probably tell the difference in a side by side test between a 66F slab and a 68F slab, it's highly unlikely that anyone is going to regard the the 68F slab as anything but chilly. Especially someone who has expressed a desire for a toasty floor. They are going to be disappointed in anything below about 75F. If you want a floor that is at essentially room temperature, you're better off just insulating more under it than actively heating it. We have R-48 under our slab, and it tracks mean temperature to within a few tenths of a degree, which I would defy anyone to distinguish in a side by side test.
Oh yes, I agree. I'm just hoping that the people who don't understand that yet will be more convinced by trying it themselves.