Wow thanks for such and in depth reply. I've had to read it several times to wrap my head around it hehehe.
Basement includes a family room and two bedrooms plus typical utility area, workshop and storage. The floor consists of concrete, solid slab, with the radiant tubing embedded. Living areas are then covered in wall to wall carpeting.
Main floor consists of bamboo over subfloor. Beneath subfloor between joists, manufactured joists, is the tubing which uses transfer plates and is covered by foil backed insulation throughout.
The system itself works wonderfully and warm feet in the winter is sweet. It is by far the nicest system I've encountered. My heating bills are also quite low compared to everyone I've spoken to in my area.
My HVAC designer/installer did take the lower temp into consideration when we discussed the components of the system.
I have 3 zones, one in the main living areas of the main floor, second is the master bedroom, dressing room...we like the bedroom much cooler for sleeping, third is in the basement area. Generally I keep the temp set at 67f during the winter months. I also have a Unico central air/secondary heating system and sometimes use the heating portion to add a quick blast of heat when it hits -20f and lower.
The house footprint is 1400 square feet per floor with roughly half used as living space on the basement floor.
Thank you very much for such a detailed reply,
Kai
With the lower-mass zone tubing below the subfloor as opposed to above there's a significant lag, and some anticipation function is required to keep temps steady. Setbacks using standard programmable T-stats will most likely result in overshoots when recovering from setback, which cuts into potential savings. Standard T-stats even at constant settings will also have some under/overshoot issues due to the thermal lag, more noticable when it's really cold out or at times when the load shifts quickly (sunny cold days at sunset, etc.) A proportional-integral approach will work better.
Serving the wood floor zone is where short cycling is most likely to occur, unless there is a buffer tank and/or controls to set a minimum burn length, that will sometimes/often extend well beyond the point where thermostat setpoint has already been satisfied. If it's set up as simply a primary/secondary loop and no extra mass the boilers high-limit will likely be tripped in 2-3 minutes every time (even on the coldest day of the year.) If the controls are smart enough to keep it burning and dump heat for a preprogrammed amount of time into the high mass zones (or an indirect HW heater tank) it'll be more efficient.
For under-floor staple-ups the temperature requirements are usually high enough (120F+) that buffering it with a "reverse indirect" hot water heater automatically sets a minimum burn due to the mass of the tank and the hysteresis of it's aquastat. Something like this:
(In your case change "To baseboard" to "To bamboo floor zone". ) The buffer/HW heater basically becomes the point of hydraulic separation between the boiler loop and secondary loops in the system.
If your system is already configured & controlled to produce sufficiently long burns there's little gain for you in this approach. But if it's short cycling this configuration would result in double-digit percentage savings. If you're currently running a separate hot water heater you'd likely get a ~50-80therms/year savings on your hot water fuel use as well- as much or more than going with a more expensive tankless system. There are several vendors of "reverse indirect" buffer/hot-water heaters, Turbomax, Everhot EA-series, Ergomax, to name a few, but you have to seek them out. They differ from standard indirects in that the bulk water stored is boiler water, and the potable is on the inside of the heat exchanger coils instead of the other way around. It takes more heat exchanger to do this, making them somewhat more expensive than standard indirects. "Tank-in-tank" indirects may be usable as buffer-HW heaters too, but I'd have to read the specs more closely before going there.
It's a hunk of change though- if your annual fuel use is already quite low, waiting for your current hot water heater to croak before diving in makes as much sense as anything. (Saving 15% of "not much" is can be pretty close to "nuthin'" ). But don't get sucked into buying a separate tankless HW system or boiler embedded/attached mini-coil. For most people this will be at least as efficient as a non-condensing tankless (and higher flow, more convenient), and significantly more efficient than a tankless coil in the boiler. If it's stopping the boiler from short-cycling the net efficiency gain of the heating system can sometimes exceed all fuel used for domestic hot water (!). The key to getting the best efficiency out of a boiler is longer-fewer burns while under part-load conditions (which is 100% the time, for multi-zoned systems with non-modulating boilers), and thermal mass is how to make that happen. The slab zones have it, the staple-up zones, not so much (unless you add some.)
OK this is WAY more than you asked for- I'll get off my soap box now.
