Radiant and Programmable Thermostats

mtnxtreme

I have a 2100 sq. ft home with an, in floor radiant system, sandwich style, the tubes were installed in 1-1/2" of cement poured over a sub floor with sleepers and 1/2" fifnshed plywood over the cement and 1-1/2" sleepers @ 16" oc. My burner is a propane fired Baxi burner, no storage, it fires on demand for domestic hot water and/or radiant flooring as needed. We did the tubes and pour ourself, and had an HVAC guy do all the manifold, Baxi instal, and hook ups. He told me to use regular thermostats vs. programmable as the system takes too long to warm up, so we would see more saving just leaving them on 68 all winter. Can anyone shed any light on this, should I go programmable, and if so any tips on programming for radiant systems. ?

Alan (California Radiant) Forbes

Programmable preference

I prefer a programmable thermostat and install them for my radiant customers on any kind of radiant floor, be it 1-3/8" GypCrete to a 6" slab.  Typically, it is programmed to set back the temperature during unoccupied or sleeping hours (say 60F) and then brought up to 68F when the space is occupied.  With radiant heating, it takes awhile to bring the house up to temperature which is a function of the mass of the floor and the heatability of the space.

Jean-David Beyer

it takes awhile to bring the house up to temperature

"With radiant heating, it takes awhile to bring the house up to

temperature which is a function of the mass of the floor and the

heatability of the space."



While I agree with this, there is another factor that should not be ignored. If you use aggressive outdoor reset, it will take a long time to recover from a setback, especially an extreme one. During Sandy, my power (and, therefore, my heat) was off for about 6 1/2 days, That resulted in a lot of setback. IIRC, my temperature inside dropped about 10F. I do have radiant slab at grade, so that would take 8 to 24 hous to recover, perhaps. But it would have taken much longer because the outdoor temperature was not even below freezing. So the reset was running about 80F through the slab -- just barely enough to keep the house to temperature, but not enough to recover from the "setback". To speed things up, I raised the minimum supply temperature from 76F to 96F. It got more comfortable in about 8 hours and the system stabilized in about two days. That is one of the effects of having the reset set so aggressively: you cannot recover quickly from a setback, radiant slab or not.

Gordy

Set it

And forget it unless your going out of town for extended periods, or unless you want a slightly cooler sleeping temp. Learning thermostats with smart recovery can bring the house up to temp before you get out of bed in the morning.

Jean-David Beyer

Set it And forget it

Absolutely.



But with my slab, if I want to cool down the bedroom by 9PM, I gotta start the setback by noon at the latest. And if I want it warm again by 7AM, I better have the heat resume as I am going to bed. I have a fancy setback thermostat that allegedly learns from experience, but I have never seen one even start to recover from setback more than an hour or two before the desired new temperature. And it takes my slab 8 to 12 hours to recover and another 12 hours to stabilize. So I do not do setbacks except when I am going to be out of town for a week or more.