supplementing electric underfloor heat

John@WattsRadiant

will generate about 12 Watts/sq ft (about 40 Btu/hr/sq ft).
Certainly an uninsulated slab beneath will be a major heat sink, but in a 7 x 18 area you're pulling about 1.4 KW when the mats are on.
John

David_7

Our 1930s DC house is heated by hot water radiators with a 22 year old A. O. Smith copper boiler which we now know is not ideal with the large galvanised pipes that are part of the system but that is another story and we have decided to keep the boiler until something goes wrong.

This winter we would like not to close off our glassed-in (9' wide by 18' long) side porch. It is on the north side of the house and separated from the living room by a french door. This typical side porch has one side which is the formerly outside brick wall of the house with the other 3 walls consisting of almost floor-to-ceiling thermopane windows. The floor has ceramic tile on top of a hefty concrete slab which is also the ceiling of the unheated garage below. The current heating is by two 6' electric baseboard heaters installed in 1981 along the long wall of windows. We are now considering installing underfloor electric heating over the existing ceramic tile and topping it with new tile. We are pretty certain this would make a big difference to the comfort of the room but unsure about whether also to have some other form of heating such as the existing baseboard heaters (or a more modern replacement for them) or a radiator tied into the rest of the house system which we could easily locate on the brick house side of the room. We do plan on contacting professionals for some of the work (we would probably only DIY the floor heating) but would like to find out as much as we can before doing this. Sorry this is so long and thanks.

Dale

9x18

That's a pretty big porch for a north side to be heated wirh elect. mat. I guess I would see what wattage you have now and ask yourself how it feels now. The porch may never need to be kept at house temp but I would still do a heat loss at whatever temp you like, for the DC area. What I don't like is the lack of insulation under the current floor slab. Unless you have stock in the local elect. company I would put some sort of insulation between the current slab and the elect. mat. With a boiler I would have insulation and quick trac and a radiant zone priced, This would add about 2 inches to your current floor height. The baseboard or panel rads next to the house wall would be well worth doing unless you want to make the room freeze damage proof, I don't know if that's a big deal where you live. Bottom line I don't like the elect mat idea over and uninsulated slab, not enough heat output alot of which would go down not up

Rick_11

Elec Heat

I have installed a couple of bathroom tile jobs with radiant electric, and while the comfort is great, I haven't found any that are designed to "heat" the space. The manufacturers of the indoor electric radiant state "for comfort only". I think even hot water radiant would be a challenge with 3 walls of glass and an unheated space below. If anyone does know of a supplier of electric radiant designed for actual interior space heating, I would like to have the source. Rick

David_7

Thanks a lot for this information everyone. It just shows that it is not sensible to rush into these things - it started with our just redone kitchen contractor dropping in and saying that he was just installing underfloor electric radiant heat (we went to see it and it was a small powder room) and such a system would be great for our porch! The hydronic system tied into our boiler as suggested by Dale seems to make the most sense perhaps supplemented by a radiator or the existing electric baseboard heaters. We certainly have the depth needed as there is a 4" step down to the porch. As a stopgap/increase comfort measure we thought of putting down a wall to wall carpet over the cold tile (there are area rugs now)with a thick insulating pad. By the way what type of insulation would be recommended for use under the trac hydronic system? Thanks again.

Dale

Room

Reading your reply it looks like you have plenty of room for a concrete overpour, the good sunrooms in my area have radiant tube in the floor usually from a water heater just for that room since FA is common here. If you go to the wirsbo site you will see insulation details ( the edges too of course) , if you still want tile and since the floor can be raised you can put in regular tube and have a nice insulated non energy wasting floor, If it were me I would also put in the rads or panels rads on a trv in case you wanted a higher temp than the floor was rated at. Even if you're not planning to use it right away I would put in a couple of temp sensors with the concrete and bring the wires into the basement, if you ever want to do IO reset you'll be happy you did. Also I would go with a larger and closer tubing spacing than minimal, you might want a condensing boiler some day and the low temp zone will already be in place. Good luck