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Outdoor temperature sensor

Hey guys,
what is the exact purpose of the outdoor temperature sensor? Just to shut the boiler off when the sensor reaches the set temp right? If you have a solo 110 with an indirect water heater, when the outdoor sensor is satisfied will that prevent the boiler from answering a call for heat from the water heater, or does it override? Thanks


  • Gordy
    Gordy Member Posts: 9,514
    The outdoor sensor works in conjunction with the boilers outdoor reset. The ODR has a curve that is setup based on radiation type, and water temps needed at a given outdoor temperature to heat the structure. Very important piece of hardware in getting the most performance, and comfort out of the boiler. Location is also important.

    A properly tuned outdoor reset program actually turns the thermostat into a redundant high limit switch.

    You don't need the same temp water to heat the structure through all outdoor temps. Typically the hottest AWT will only be 2% of the heating season the rest of the time cooler AWT can be used to get the boiler in a hard line condensing range.

    You do understand that a mod/con run at high temps does not do much better in efficiency than a CI boiler. The key is getting the return temps into the sub 130 range. The lower the better.

    This is a two fold equation that involves sizing the radiation to allow for those cooler return temps. This gets to be more critical with base board, rads, and even poorly designed radiant.

    chris87SWEIRobGJean-David Beyer
  • unclejohn
    unclejohn Member Posts: 1,749
    the out door tstat is never satisfied. It is for informational purposes only. If your boiler is set up right a call for domestic will override the outdoor set point and fire the solo 110 to 196*f.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    What outdoor thermostat? On a typical mod-con, there is an outdoor temperature sensor, not a thermostat. The sensor may shut off the domestic heating (but not domestic hot water) if it gets too warm outside. But what it mainly does is controls the hot water temperature supplied to the radiators, baseboards, or radiant slab or panels so as to provide only as much heat as needed. Any more is a waste of money. Furthermore, if you have a radiant slab as I do, supplying hot water at too high a temperature results in high overshoot that is uncomfortable.

    The outdoor temperature sensor is ignored if the boiler is satisfying a call for heat from an indirect hot water heater.

    It is the indoor thermostat that, ideally, is never satisfied. I cannot set mine up that way or my house does not heat properly. It can work at a wide range of outdoor temperatures, but it depends on the sunlight available, and the outdoor wind speed (leakage), so my reset is adjusted so the indoor thermostats are satisfied 6 to 12 hours a day most of the heating season, and most of the time in September and May. There is no heat demand June through August, and almost none in September or May.