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Recommended Temperature

kinglerch Member Posts: 14
My radiant system is pretty simple. It is a natural gas fired condensing water heater (boiler-like), electronic ignition. When heat is necessary, a pump takes the hot water to the central air duct where a radiator releases the heat. Then the somewhat colder water returns to the condensing water heater.

I haven't really experimented with different temperature setpoints, but I noted that if I set it to higher temperatures (160-180) the water heater fires up less often than lower temperatures (120-140). Is it most efficient to run the water heater at higher temperatures in the winter and have it cycle less often, or at lower temperatures where it cycles more often to keep up with the ambient temperature? Is there an idea temperature to set radiant heat to? I live in snowy oHIo, BTW.


  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666

    Need details to answer. Also note I am not a professional.

    In general, the colder the return water to a condensing boiler, the more efficient it can be. And the easiest way to get colder return temperatures is to supply the hot water at lower temperatures. But you need to know the heat loss of your building because if the water is not hot enough to heat the building on your design day, you are clearly in trouble. So the heat loss of your building, and the ability of your radiators (I will call them) to deliver the heat will determine the minimum temperature you must able to supply.

    If you have outdoor reset (you did not say), you could supply lower temperature water on warm days and go up to maximum temperature on the coldest days.

    As far as cycling rate is concerned, the faster the worse, but how fast is too fast? One of the professionals here could advise you. On my old boiler, with some tuning (one thing I changed was I increased the difference between the hottest and coolest the boiler got to 10 degrees F; the former owner had them as tight as the aquastat would go; I also lowered the average temperature from about 160F to 135F since the boiler was so oversized), I got it to something like 90 seconds on and 120 seconds off. I consider this much too fast. It was even worse when I started.
  • kinglerch
    kinglerch Member Posts: 14
    more details...

    This is just for my house, so the temperature control is via a regular thermostat. When the house gets below the desired setpoint, the thermostat turns on the pump, which cycles the hot water through the radiator in the vent system.

    The house is very well insulated, above below and sides. I currently have the water heater temperature set to 140, with a differential of about 20 degrees (it will kick back on when the temperature inside the tank gets down to 120). Since the ambient air inside the house would normally only be set in the low 70s, maybe I don't even need the water set that high....maybe even 100 would be the most efficient?

    I haven't timed the on-off times, but if I could get a recommendation, maybe I can set my water temp based on that too.
  • NRT_Rob
    NRT_Rob Member Posts: 1,013
    i'm not sure your observation is correct

    I can only think of a single mechanism that would allow a hotter water heater to run less often unless you are also changing your differential. your air handler would run less frequently, because it would satisfy the thermostat faster. however when it does fire you are likely to be using "more energy" per unit time because it's dumping faster into the air stream.

    you don't get to cheat physics this way, and you'd be best off turning down your water temperature, UNLESS you are short cycling, which might happen if you are spoofing a sensor in the tank with your return water temp.

    If you're doing domestic, the tank should be 140 with a tempering valve out to prevent scalding.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
  • CMadatMe
    CMadatMe Member Posts: 3,086
    Ideal Radiant Temp

    The ideal temp is the temp you need to overcome the heatloss at your design temp. For instance, if you design for 0 degrees outside as we do here and I need 120 degree water to get the btu output of the floor to overcome my loss then I need 120 degrees at 0. I don't need 120 at 5 or 10 or 15 or 20 or 30 degrees outside. I may need anywhere from 115 to 90 degree water to overcome the heat loss on that temps day.

    My question to you would be how is the radiant controlled? In other words what are you using to get the water temp you need for the radiant, ie, mixing vlv, injection pump? If your using a water heater and just changing the temps without some type of outdoor compensation the system will never know what it needs on that day's heat loss. You'll be running up and down the stairs all the time to find your comfort range for each day.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
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