Electric Boiler for radiant heat in a small 600sqft home.
Hey folks, I know this topic has been discussed before, but would like to personalize it towards my specific needs and get some insight. I have built a small, very well insulated cabin in So. Vermont and am leaning towards an electric boiler for my hydronic radiant in my slab. The slab is insulated with 3″XPS foam, and the timber frame is monopoly framed with 4″ continuous exterior insulation on walls and 6″ continuous on the roof.
I have a small hearthstone soapstone wood stove, as well as a 12k hyperheat minisplit in the main living area. My intention with the radiant is to keep it relatively low, but just warm enough to have comfortable toes on the floor in the winter.
I have read most posts regarding electric boilers. And I am fully aware that there are better options like HWHP etc. However, I want to explore electric since it seems like a low initial cost, as well as smaller overall unit size (since I only have 600sqft, I am very limited on space). Would love to hear some insight and if anyone has experience with small electric boilers, since I believe I would only need around a 4-6kw unit.
**Also, it is worth noting that here in VT the electricity rates are pretty comparable to propane. Thanks all!
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The usual tradeoff is low initial install cost, but high operating cost over time. If you are using propane, and you cost per BTU is similar to electricity (which seems odd to me, but maybe if you have very high electric rates), then the operating cost is of minimal concern, since your input energy costs are about the same. Electric heaters can be assumed to be 100% efficient too, since essentially all of the input energy goes into heat, there are no losses with hot flue gasses or anything like that.
Note that keeping floors feeling warm will usually result in overheating the indoor space, so keep that in mind.
Bill
Thanks for the reply, Bill
User...102,
That's a lot of different, and you could argue redundant, heat sources for a very small space. If either the wood stove or heat pump are on you will never get the slab hot enough to feel warm. It's doubtful whether it will feel warm even if you only use the radiant heat and not the others. It's probably too late, but the best way to use radiant heat if what you want is warm feet is in specific small areas, not over the whole slab.
I do agree with you that there are some redundancies to the heat sources. The wood stove is mostly because I love making fires, and not because I think I will NEED it. As far as the radiant goes, I'm dont think im looking for "warm" feet, but more so not the bitter cold that the concrete can feel like as-is. Not exactly sure what temperature that would be. Maybe 60's
User ...102,
Any material below the temperature of the surface of your feet (around 85F) will feel cold. But it isn't so much the temperature of the concrete as its conductivity that makes it feel particularly cold to the touch. A well insulated slab such as you have, with no radiant heat, will track the indoor air temperature to within a couple of degrees. The problem is that to heat the cabin with radiant heat, the slab only has to go a few degrees above that.
Apologies - at this stage in your build all this is probably not useful. Hopefully someone can give you good advice about which boiler to use.
No worries, it is helpful either way! Not sure why everybody I speak to up here in VT raves about radiant in their slab then. Seems like most responses here on GBA lean towards radiant (hydronic) being more of an inconvenience than anything else.
User ...102,
It's straight physics. The larger and less efficient the house, the hotter the slab, the happier the occupants.
The worse-insulated a house is, the more impressive radiant heat is.
A lot of people in the GBA community have tight houses where radiant doesn't work that well.
If you build a house the way everyone did in the 1950s with drafty single pain windows. Intentionally leaving gap so the house can breathe. Zero insulation in the walls and maybe R10 on the attic floor. You will need to put a lot of BTUs into the slab to keep the room at 70°. That means the slab is down right hot likely 105° maybe more. It is absolutely wonderful unless you are the one paying the fuel bills. If you build anywhere close to todays codes on the coldest day of the year the slab might get up to 85° and it will not feel warm under foot.
Walta
Thanks Walta, that explains it nicely.
If you're planning on using a resistance boiler, there's nothing to be gained by making the system hydronic instead of just running resistance wires under the floor. The resistance wires are cheaper and easier to install, easier to control and don't require as much space.
I find even a couple of degrees above room temperature makes a big difference on comfort.
I have an overhang and without floor heat that would be about ~3F bellow room and noticeably cold so, just setting the floor heat to maintain 4F above room temp makes it significantly warmer without adding much heat to the space.
With 600sqft, the most you can get out of floor heat at 20btu/sqft is 12000BTU. That works out to 3.5kW which is a standard water heater element. You could even use a small DHW tank with a big element to heat the place. If you have TOU pricing, you could even bump that up to a really big tank and pre-heat the tank at night time and coasting through the day. Cheapest energy storage option out there.