Programable Thermostat for radiant heat

Discussion in 'Boiler Forum' started by Kai Winters, Nov 10, 2009.

  1. Kai Winters

    Kai Winters New Member

    Messages:
    3
    Location:
    Brownville NY
    Hi,
    I'm new to this forum. Found it using google.
    I look forward to some fun and interesting browsing/learning.

    My question is using a programable thermostat with my radiant heating/boiler system. Is it more economical to use a programable thermostat or a non-pro/therm? My HVAC installer recommended using a non stating that it is more economical and better all around to keep the system at a steady temp opposed to the ups and downs when a prog/therm controls the system.

    My boiler is a Weil Mclain Gold CGI Series 2, gas fired controlled by a Hunter electronic controller with 3 zones. Most of the house is covered in bamboo flooring on the main floor while the basement/finished family room is concrete floors. The house is 5 years old and my wife and I designed and had it built.

    I've always used a prog/therm in our last house which used gas fired forced hot air and it was a nice method of controlling our heating and costs.

    Thanks for taking the time to address my question and providing this forum.
    Kai
  2. nhmaster3015

    nhmaster3015 Master Plumber

    Messages:
    836
    Location:
    The granite state
    Because of the long recovery time you do not want to use a set back thremostat for radiant heat applications. In fact, ideally you would have no room thermostat at all, just a floor temperature sensor.
  3. Dana

    Dana In the trades

    Messages:
    2,812
    Location:
    01609
    I'm assuming the concrete floors are radiant slabs, and that the bamboo is over a wood subfloor(?).

    If the radiant on the bamboo flooring has the radiant tubing above the subfloor you can get reasonable results with a standard setback thermostats. If the tubing is below the subfloor between the joists it'll do better using something more sohpisicated to handle the delays (if this is a staple-up that doesn't use heat transfer plates, even MORE so.)

    But in high-mass radiation slabs (gypcrete,concrete, etc) the best results will be using PID-algorithm thermostats. PID (proportional-integral-differential) has a better shot of anticipating the heat load and keeping the temperature constant than other methods. Concrete will store a lot of heat, setting back the thermostat don't cut the heat loss out of the building until the room temperature actually drops, which takes a long time when you're bleeding down the stored heat in the concrete. Then, it takes some time to pump the heat back into the concrete to bring the room temp up, so you'd have to start stepping it up early (typically hours earlier than you would have to with forced hot air.)

    Some people have had moderate success using multiple step-ups on the setback, but by the time it's all said & done the savings are a fraction of what they might be using low-mass heating like hot air furnaces, and the comfort levels lower than if PID control is used to maintain a constant temp. Tekmar sells several PID models. Wirsbo uses a related proportional-integral approach that modulates pump on/off times. Both work pretty well.

    I hope your heating designer made the plumbing changes necessary to run the high-temp boiler with the low-temp radiation(?) without damaging the boiler? The water entering the boiler needs to be 130F+ to avoid corrosive condensation on the heat exchangers, but the water temp requirements of radiant heat are typically lower than that (particularly in slabs). There are several methods of addressing both requirements, but the longevity of the boiler is severely compromised with return-water constantly under 130F. (Depending on the vent materials it could be an issue for the exhaust venting even at 140F return water.)

    Also, since it's multi-zoned, the separate heat loads are likely a small fraction of the boiler's output each. For slabs this isn't too much of a problem since you have a lot of thermal mass to work with and the burn cycles will be long enough, but for the lower mass wood floor it may need a buffer tank to keep the boiler from efficiency-robbing short-cycles. You might want to monitor the burn cycles under various conditions. Anything under ~10 minutes per burn is affecting the as-used efficiency on a mid-mass boiler, and any burns under 5 minutes in duration means you're unlikely to achieve anything near the AFUE on the nameplate. (Maybe the Hunter controller manages that, maybe it doesn't. Got a model name/number?)
  4. Kai Winters

    Kai Winters New Member

    Messages:
    3
    Location:
    Brownville NY
    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
  5. Dana

    Dana In the trades

    Messages:
    2,812
    Location:
    01609
    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:

    [​IMG]

    (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. :)
  6. Kai Winters

    Kai Winters New Member

    Messages:
    3
    Location:
    Brownville NY
    Wow amazing replies...though a bit...make it more than a bit lol...over my head. My hvac installer said basically the same thing but a bit dumbed down for me.

    I was just looking for additional advice and commentary, which I certainly have received here.
    My last gas bill for the month of October is $78.00. This is for heat/hot water/cooking and laundry/dryer. There are 3 of us. This amount could be further reduced...I'm betting by at least 1/3...once my son leaves for the Navy. He lives downstairs requiring that heating zone to be higher...65f at this time and quite comfy...where I would have it at 50f if no one was living there. Add to that his electricity use, extra laundry, etc. and it adds up.

    I'm not complaining at all. In fact quite the opposite. I've never had such a wonderful heating experience as I have with the radiant. There is no sound and no sensation of heat...other than the warm floors when walking on them...Everything is just "warm" and cozy.

    Thanks for the advice and suggestions. They are much appreciated.
    Kai
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