[GBlist] re: setback thermostats

[Chris Koehn <chris@koehn.com>]

Steve,

I'm certainly not an expert on these, but I've owned and operated a few, and
spec'd quite a few more for homes we've built. The answer is: it depends.
Mostly it depends on your HVAC plant- is it forced hot air? Set back is much
more efficient with an FHA system than, for instance, our geo thermal hybrid
radiant/FHA system. Such a system- or any radiant system- can't efficiently
accommodate set back for short or even moderate periods, as it takes more
energy to bring it back up to occupied set point than that which is saved by
lowering the set point when unoccupied (in our case, the geo system can't
keep up with a drastic increase in set point, and so electric resistance
heat kicks in to supplement- about as inefficient as it gets, but necessary
in case of system failure or extreme conditions) . My HVAC contractor
advised that we not set back unless we were going to be gone for an
"extended period"- more than a week.

We did a little experimenting in our old house- a timber frame with SIP
enclosure (tight and really efficient, FHA natural gas heat) and night time
set back didn't save a bit of net energy, but setting back for 48 hours
would begin to become advantageous. This is partly due to the greater
thermal mass needed to be warmed back up in a timber frame home- so again-
it depends..

Hope this helps,

Chris 

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