We just finished building a SIPS house in Alaska. 1300.SF plus 500SF loft. In floor radiant in the polished slab. Fuel oil boiler. 2 zones on the main floor and a baseboard in the loft (which has yet to come on.)I'm interested in setback thermostats mainly to make sure the boiler doesn't fire up during the night. I know with radiant heat setbacks are normally not used for the typical work/weekend scheduling, but could I in essence turn my heat off for 7-8 hours at night? Ditto the HRV. Mostly a noise issue. Or would this whip saw my system too drastically?

Is there some reason the boiler shouldn't fire up during the night? Too noisy?

Go ahead - if you have enough thermal mass and a well insulated house, the temp won't drop much.

The boiler is noisy....no door on the boiler room right now....might be a moot point once we get that on there

How many inches of concrete?

The thermal mass advantage of the slab works in both directions. While it might coast you comfortably through the night, recovery will be slow, meaning you may have a lag that lasts well into the morning or if you are near capacity much longer. While the boiler might have enough capacity to catch up, the capacity of your delivery system is finite. Whether it is enough is known only to the designer. Personally I'd start with the door.

Try ir. If you can tolerate the temperature swing and the boiler can handle a reasonably expeditious recovery, you will probably save energy. Nighttime setback works by lowering the temperature differential between the house and the outside over some number of hours, with recovery just before the alarm sounds. Obviously thermal mass will mute that effect by limiting how much the inside temperature declines. And, as Joe points out, the number of btus to rewarm a high mass home won't be any less than the number needed for a low mass home even if the former starts from a higher temperature. Except. As recovery drags into mid-morning or later and the ambient air warms, you will be once again working against a lower temperature difference than you would have at 5 a.m. The difference would be significant with a heat pump, and worth waking up in a house that's 68 rather than 70. Given a modest decline, comfort is a judgment call, The folks who insist on 70 perhaps forget that commodes and tile floors experience considerable thermal lag even if the house is low mass overall.

I have a small home (SIPS)with radiant floors( concrete). My heat only comes on a night unless we hit 10 degrees or less. The only way you can realistically coast through the night is to have it ramped up before you go to sleep or have it cold when you get up in the morning.

You would be better off building a temporary cover for the door.

Setback thermostats should never be used with a concrete in floor radiant system.Setting back the temperature defeats the purpose of the benefit the concrete mass offers.The idea is to maintain a steady state. You may find that you use more energy to recover back to a steady state and definately a lag period as the mass attempts to recover.

Acwizard is giving you the conventional wisdom, based on the notion that setback doesn't work with mass. Because CW steers design, it is quite possible that recovery will be a struggle. But try it. If the house only cools a few degrees, be happy that the boiler is still running at noon, as long as it isn't short cycling. Thermal lag is your friend.