Well that would explain why the lights are enough to cause the radiant not to run. If I was to turn on all the lights ((20) 65 watt bulbs) that would be about 4000 BTU/hr. My load is about 6000 BTU/hr.

Yeah- basically in order for the human to feel a warmed floor the floor temp needs to be up over 76F-80F depending on thier circulation and foot coverings. If the whole floor was being used as strictly a heat loss replacement to maintain room comfort, a well insulated low load room may only need 75F-76F floor surface temps to make it. So reducing the emitter area and raising the tempertaure slightly should give you the warm floor "feel" wihtout overheating the space, and you still get the all-ove radiant effect since radiant is a line of sight heat exchange medium.

Ok, but at what load condition do you target this? under design conditions? I could buy that but it would be a modest benefit in that case.

If not, then your water temperature characteristics *have* to be significantly impacted, if you're trying to hit 76+ under, say, half load conditions when you'd only need that "naturally" under load conditions. You'd, in effect, be doubling your BTU/sq ft requirement.

Very curious geoff!

I'm not sure what you are asking. Basically the same issue you describe would occur for a "full floor system" vs a "limited area floor system". If the room heat loss is 2,000 btuh at peak, then you can do that with 500 SF of warm floor at 4 btuh/SF output, or just use 200 SF of warmed floor at 10 Btuh/SF in that 500 SF room.

Basically the water temp wouldn't be that much different to do it, and then at part load conditions you are just ramping down, but at an offset from the same reset as if the whole floor was being used as the heat emitter. The exact control and water temperature strategy depends on the emitter type- high mass radiant, mid-mass radiant (under sub floor staple-ups) and low mass radiant (thin-set tile or sheet goods with tube on top of the sub-floor in a panel system). I think you'd get much more heating fluid variations based on the exact floor emitter design and floor types. Ultimately it comes down to surface area "A" at Temperature "T" to create heat output in a room for a given heat loss.

Let's take a room with a 10BTU/sq ft heat loss under design conditions. 75 degree max surface temp.. barely noticeable, if at all.

Pick any installation type you like.. let's start with a 12" o.c. slab, full coverage, with a floating wood floor. That's about a 90 degree supply water temperature under design conditions, if you pipe the whole floor.

Let's say you want it to be noticeably warm when you are at half load (half design conditions). So you pipe about half of the room, so you're at 75 degree surface temp at half load, starting to look at getting warm, and under design conditions you're at an 80 degree surface temp for the areas that are piped.

Your supply temp requirement just went up to about 110 degrees. Not the end of the world, but you did just add about 20 degrees you didn't have to.

so either I'm not looking at that right, or you're saying it's ok. I would personally rather run the mod/con boiler cooler all heating season long and maintain a better condensation benefit... though I can see the arguement against it, I think overall comfort would be superior with total coverage and you'll generally be more efficient while you're at it.

You just won't live up to the marketing hype

Not sure how you calculated water temps but by my estimation the water temp required to get the 80F floor would only be +/- 100F, all other things being equal. Again- there are so many variables like tighter tube spacing in the smaller radiant emitter zone to get water temps down where you like them. And don't forget peak loads only happen maybe 10% of the time, and part loads are about 75% of the time, so any radiant system is going to be operating at part load most of the time anyway. So the "limited area" radiant floor will be operating down near your favorite temperatures for the majority of time anyway, and would be a little bit warmer to the touch than the "whole floor" approach. It's not the instantaneous load that sucks energy, it's the yearly load profile at the majority of the operating hours.

Those are, of course, very good points as always Geoff. I suppose the difference in water temps between the two approaches, under diminishing loads, would likewise diminish... hmm.

Thanks for the wisdom Geoff! I'll have to keep this in mind the next time it comes up.

Not a radiant heat response but an energy use response. Go to HomeDepot and check out the can lights they have that use around 15 to 19 watts each. They will put out less heat versus your 40 to 100 watt bulbs and you can use less electric and not have to change the bulbs as ofter. They work well and you cannot really tell the difference with the newer bulbs. Sometimes you get one that humms and will need switched out but I have several in my house with no objections. If you want to dim the bulbs these will NOT work.