Normally, you don’t want to use PV generated electric for heating as the ROI isn’t there. However, this varies by location and this is quickly changing too. Our experience is with HR heating (new construction) and not with ER heating. However, we understand that ER heating can be a good choice for bathroom remodels, etc. A well designed/installed/protected HR heating system should last 100+ years. Hopefully we have some folks with ER heating system experience that can provide you some good guidance and suggestions.

Alton,
1. With any radiant slab it is important to insulate under the slab- Far more important than with other types of heat. I have installed electric mat type radiant under tile in a bathroom- never in a slab. It has been in place for 6 years and if it was a problem- I would have heard about it. It is on a timer so it provides a warm floor am only when the occupants are using the bathroom. The product I used was plastic coated, so presumably you wouldn’t have a corrosion problem.
2. If you want to generate electricity via solar PV and then use it for resistance heat- that seems like a poor investment. The installed costs of PV in my area (Connecticut) are running around 3.30$ per watt for residential stuff. Given the average insolation- you get about 1200 KWH per year for each installed KW. This nets you 4 million btu’s per year. given that the average house in Ct consumes north of 100 million btu’s for space heating ,this would be a 25 KW PV system just to handle space heating via resistance . This nets out to 75K$ for the solar system. ( somewhat cheaper than average due to size). If you can generate those btu’s via Heat pump, you can reduce that array size to 9kwh and cost to ~ 30 K for the PV. (Assume a COP of 2.7) It might even fit on your roof. So the question is can you install an air to water heat pump or geothermal (and maintain it) for less than 45K$.
3. If you use a water based system- you preserve your options for later- even if you just go with a electric boiler in the beginning. With electric radiant- you have already rolled the dice.

This also hammers home the point of how important insulation and air sealing are. Spend an extra 20K on the shell, knock the annual heat load down an order of magnitude- and electric resistance heat is not so bad an idea. Now you are at 8K$ for the solar array and resistance heat does not seem so bad. Pair it with a good minisplit and just use electric in key areas and you are in good shape.

We would fully concur with Eric's excellent guidance and well written response. If you can get the heating footprint small enough as Eric suggests, then electric boilers start looking real good. Electric boilers are very efficient and don't have the mechanical failure or maintenance issues that you would have with a gas-fired boiler. Electric boilers are also a much easier install.

At the very low heating loads of better-than-code houses in climate zones 1 & 2 there's very little additional comfort to be had from radiant floor. With R5 under the slab and a bit more at the slab edge, the comfort levels of heating with right-sized high efficiency variable speed air source heat pumps (ducted or ductless) is pretty good. gulf-coast AL/MI/LA it would have a heating season coefficient of performance of about 4.0.

The wintertime mean temperature in Auburn AL is about 45F, a temperature at which an HSPF 12 air source heat pump is hitting a COP of about 3.5 at full speed, and about 4.5 when modulating at part load. The COP of electric radiant floors is 1.0- it uses about 4x the amount of total power of high efficiency air source heat pumps. In a sprawling house with very low individual loads it might be worth springing for a Carrier GreenSpeed, but for open floor plans ductless is probably going to be the better value, and will be more efficient due to zero air-handler driven infiltration or duct losses, even if rated at the same HSPF.

True Dana, but let’s not forget that any heat loss or inefficiency associated with an electric boiler occurs inside the building. So it all comes done to cost, i.e., a ROI analysis that considers both heating system acquisition/maintenance cost and the operational cost.

In a climate zone 3 & lower locations the peak cooling loads are usually higher than the peak heating load, and if you're going to be installing air conditioning the up-charge for making that a heating/cooling heat pump (that uses less than 1/3 the power of a separate electric boiler or resistance radiant) is pretty small- much smaller than the cost of an electric boiler & pump + radiation or mesh-resistance radiant floors.

It might be silly if one were springing for the heat pump JUST for heating a high-R house in those low-heating-load climates, but at this point air-conditioning has become the something that nobody wants to live without in that region (it's sure hard to SELL a house without AC any more!) Even high-R houses have cooling loads related to window gains and interior heat sources, so it's not as if many homes are going to do without.

Agreed, if you live someplace where AC is required, heat pumps start looking real good. Of course, like first reducing your electrical footprint requirements before you size and install PV, some first folks move to avoid having to live in places where AC is required.

Posted By sailawayrb on 20 Mar 2014 02:34 PM
Agreed, if you live someplace where AC is required, heat pumps start looking real good. Of course, like first reducing your electrical footprint requirements before you size and install PV, some first folks move to avoid having to live in places where AC is required.

If population trends in the US are any indication a lot more people move to avoid high space heating loads (and you don't have to shovel snow as often) than are moving to avoid the need for cooling. 

That said, I know at least one person who moved from sunny Orange County CA to foggy-dew King County WA to avoid the heat and  have the best of both- low to non-existent AC loads, and very modest heating loads and rarely snowfalls deep enough require shoveling.  Now that she's heating with a mini-split she also runs it in cooling mode sometimes, probably more hours (but lower output) than when cooling with a window-shaker.  But there are a lot more people moving to the sunny gulf coast & desert southwest than moving north.

That’s true, there are certainly a lot of folks (especially retiring baby boomers who don’t have to commute) moving to hot places and places that will be getting even hotter with global warming. Hopefully, they will be living well above current sea level too.  Following the herd mentality has hardly ever proven to be the right strategy... Once you buy-in to having a mini split, you might as well use it for both cooling and heating, and it will likely last longer to continuously use it too.

If you are going to do low load radiant heating for comfort reasons, put it only where the occupants will really notice it. For example, tiled bathrooms.

We design, install and service radiant floor heating systems both hydronic and electric.

Electric cable is used to heat everything from a small bathroom floors (we installed Therrmosoft mats last Monday) to snow melting. We used a matted cable with a low 1/8" profile that fits between tile and sub-floor with more output than you typically need, even here in chilly Minnesota. The cable spacing is typically 3" OC, our favorites Warmly Yours, Thermosoft and Danfoss LX. The material cost for these fiberglass mat-style systems is relatively high, even by radiant floor standards.

For larger systems such as a basement, will freehand a system. For these systems we use use a spool of wire rather than the mats, as the heat loads will not require the same amount of wire. As in snow melting the cable is set on top of the insulation and the concrete poured over all. Our master electrician sizes the service panel to accommodate larger systems.


In my own home NG not available so given remaining choices of propane or electric boiler I chose the latter at a third the cost of propane. Since the system is still hydronic based I can change my mind and install a condensing boiler later on. Though the bulk of our work is with hydronic radiant floors (HRF) the electric boiler driven HRF is typically installed for half the cost of a gas-fired system.

Electrical heating cable is PVC clad and will stand up to nearly every building material i.e. sand, concrete, XPS etc. so like PEX tubing, PVC clad heating cable is dependent on the quality of the slab and more important the quality of the substrate.

Electrical boilers are said to be 100% efficient but as Dana points out, this is conversion efficiency. Essentially all the electricity sent to a resistance cable or heating element in a boiler is converted to work/heat energy. No COP for your regular electric boiler, but maintenance is minimal if the boiler is properly installed and the heat transfer fluid is managed correctly. The same is true of a condensing boiler or water heater with the notable exception of significantly reduce cost of operation, in most locals.

Many houses here in the Minneapolis area were built air over water, using conventional forced air furnaces for upper levels an electric boiler with electric storage type water heater in the basement. Given the universally modest heating and cooling loads in most basements we do not cool and use combi-water heaters for to satisfy both space and DHW loads in one 24" diameter footprint and unlimited DHW performance. We use build our own combi water heating systems using condensing water heaters such as Polaris and Vertex and also use the newest version of Bradford White's Comb-Cor.