Am new to this forum, so hi everyone. I have a home with an in-floor hydronic heating system. My question is this: what are the optimum programmable thermostat settings to keep energy usage to a minimum (use oil) and what should the swing setting be. Have heard that it is best to keep temperatures near constant with a floor hydronic system. Thanks for your help, John

Set back is for scorched air.

You would be better served by a properly installed re-set control, which controls the water temperature delivered to your radiant floor while protecting a low efficiency boiler from thermal shock and corrosive condensate (all most all oil boilers). This will produce better comfort as the floor will be "on" more and increase system efficiency (lower fuel bills) more than any setback thermostat or setting thereof.

Set back thermostats can be used of course depending on the type of radiant floor you employ (not so good for high-mass concrete). You can try a few degrees allowing time for the system to catch up. I have used this stategy to assure a warm bathroom floor for the morning shower.

My flooring is high mass concrete. Needed a new boiler this past year and had a Biasi B-10 low mass boiler installed with outdoor reset control. Question ... what is the optimum temp setting on the boiler in terms of how hot I should let the water get on cold days? Currently my high setting is at 170 degrees F.

This is much too high though it will probably not reach this temperature before satisfying the thermostat is is not good for concrete and set too high to be efficient.

The "design" water temperature is determined by "design" conditions i.e. the ASHRAE standard low temperature for your area, the emitter i.e. the slab and the heat load of the stucture the radiant panel serves.

It is certainly not 170°F and I have some designs that operate at body temperature on the coldest days of the year (cooler on all others) which increases comfort, lowers heat loss for the building and makes a condense boiler rain like Seattle!

Your contractor should be familiar with the proper setting for your area OR a radiant floor heating designer can do a heat load, model the radiant floor installation and come up with a number.

The alternative is to lower the design water temperature until the the boiler will not satifisy the thermostat and move it back up.

WARNING: If your boiler is not a condensing boiler, messing around with water temperature controls may void the warranty and certainly destroy a heat exchanger in a remarkably short amount of time!

The Biasi B10 is not a condensing boiler, not even a low-mass boiler it's an oil-fired cast-iron beast:

http://www.qhtinc.com/biasib10.html

If oil fired, if it's not plumbed & controlled to guaranteed the water entering the boiler is 140F+ you'll have corrosive flue condensation issues. Even then you'll need a stainless liner in the flue. Return water temps lower than that result in condensation on the cast iron heat exchanger plates, and a very truncated boiler life.

The "right" output temperature for the boiler so that the water returning to the boiler reliably stays above the condensation hazard zone depends a lot on the system design and how it's plumbed & controlled. If this is plumbed with the boiler's output going directly into the slab , with the return from the slab going directly into the boiler it'll ruin the boiler in one heating season no matter what the high-temp limit is set to. Hopefully it has SOME protections designed in. There are several methods of achieving this end, some more efficient than others. But 170F out with a 140F return is a typical delta-T for a cast-iron boiler. Hopefully your aftermarket outdoor reset control is doing something more intelligent than just ramping the high-limit on the output temp (which can be appropriate for higher temp radiation, but never for slabs.)