floor heat

[OurTown]

The best way to keep your setup from "short cycling" is to adjust the output of the boiler.....not use the Delta T spread on the return stat. Most boilers have an adjustment on the main gas valve (Mod valve or modulation valve) that allows you to adjust the heat output.

Stated another way, why would you want to waste gas, and overheat your slab an extra 10 degrees, just to keep the boiler from short cycling? Ideally, you would have your slab temperature, just above freezing, on a near constant basis.

I'm using tankless water heaters and they are set for 110F. I just checked my slab surface temp in one area just before they kicked on and was about 32F and when they kicked off were about 35F. I'm sure it's not optimized yet. The heaters only ran for 4 minutes with it being 26F outside. Maybe that's okay but I think it's not scrubbing the heat off probably so that's what makes me believe the pump has too much flow. @2Biz has a 15 degree delta I believe and IIRC has a consistent slab temp. The way it is setup now, of I changed the delta to 5F I bet it would only run for a minute or so. I'll play around with the temps a bit.

 

[OurTown]

Well, I guess I hijacked this thread. I changed the output to 105F, the return temp on at 65F, and the off at 75F. It's now 23F outside and the heaters ran for about 12 minutes, then dropped to 70 in about 2 minutes, then back on at 65 in four minutes. So even at a 5 degree delta they will only be off for two minutes at 23F outside. I'm thinking they will stay running when gets really cold outside. What are your cycle times?

 

[gee gee]

Yes, you absolutely can and you should. You need to wire a simple low-voltage "outdoor temperature lock-out" sensor in series with the floor sensor. Set the outdoor sensor to something like 38. If the air temp is above that, it just keeps the floor heat from kicking on, saving energy, regardless of the slab temp. Very common setup in commercial applications.