The house was constructed around 2000. I can't find any kind of identifying markings, and it's in the back of the house above the boiler (not near any incoming utilities which are in the front). It's roughly the size of a faceplate and about an inch deep. The bottom has a cable going into the house that seems to match some of the cables used by the boiler like the ones for the thermostat, maybe 18 gauge or smaller. It may be connected to the boiler somehow, there is insulation stapled in place and I haven't detached it to see where the cable goes but it's difficult to get to being above the boiler.

I tried to unscrew it to open it, but I ran into resistance turning counter clockwise and eventually broke the plastic inside, now it just spins freely.

When trying to pry off the cover, it has some movement around the edges, maybe a few millimeters, so it's not glued shut. However it is still on pretty tight

Any idea what this could be? Box Box

  • 2
    Some utility companies have put up remote read equipment over the years. It could be someway to read your water meter.
    – DaveM
    Commented Mar 25, 2022 at 17:35
  • 3
    Maybe a temperature sensor for reading outdoor temps. Is the bottom open so maybe see up inside it with a mirror?
    – crip659
    Commented Mar 25, 2022 at 17:35
  • There's a screw in the middle. Remove it and the cover should come off. If you don't recognize what's inside, take another photo and add it to your question using the "edit" button, and then the "moon and mountains" icon above the composition area. Commented Mar 25, 2022 at 17:43
  • @DaveM I don't think it's related to the water meter as it's on the back. I do have a Sensus device on the front for reading the water meter.
    – rtaft
    Commented Mar 25, 2022 at 17:44
  • 1
    @crip659 After some research, it uses the outside temp to lower the operating temperature or even shut the heat off if the outside temp is high. So if it's 65 out and the heat set to 70, inside is 69, the boiler could be set to operate at 130F instead of 180F to conserve energy and to prevent the 'boiler room' from getting too hot.
    – rtaft
    Commented Mar 25, 2022 at 18:52

1 Answer 1


Thanks to @crip659 for the tip that helped solve this. It is a Boiler Sensor - Tekmar Outdoor Temp Sensor - Tekmar 70. Combined with the Tekmar 256 Boiler Control. It can be used to adjust the operating temperature of the boiler. An example would be if it were below freezing, it could be configured to operate at 180F, while if it were warmer outside, it could be configured to run at 130F. It is supposed to save energy and also help prevent the room the boiler is in from getting too hot when it is warm outside. I do have this controller inside.


  • Interesting. I hope it is not in direct sunlight, or that will dramatically affect its reading. Smart 'stats like the Nest do that a different way - they use the Internet to get the temperature from the nearest airport or weather station. Commented Mar 25, 2022 at 20:39
  • It's called an "outdoor reset" (odd name, but it is what it is). It adjusts your boiler set point according to out door air temp. I have one in my house and it was frustrating because in our mild Pacific NW weather, it would turn the boiler down so much the house wouldn't get/stay warm. Since my heat is in-floor hydronic, I just bypassed the outdoor reset and did a perm set point of 130. You didn't mention what radiation your system is: In floor? Air handler with fan coil? hydronic baseboard, radiators? It makes a difference as to what the ideal set point is. Commented Mar 25, 2022 at 21:30
  • ...ran out of space: depending upon the sophistication of your boiler the outdoor reset curve can be adjusted. It sounds like you might have a condensing boiler, they lose efficiency at high set point because the flue gases don't condense so they don't release the energy of a gas turning to a liquid. Give us many more details about your system and you might get more accurate advice. Commented Mar 25, 2022 at 21:56
  • @GeorgeAnderson boiler is used for radient heat and hot water tank. Not sure what more advice I need, I just wanted to know what it was.
    – rtaft
    Commented Mar 25, 2022 at 22:19
  • @Harper-ReinstateUkraine it's on the north wall so it's not in the sun. Old tech so no network capabilites.
    – rtaft
    Commented Mar 25, 2022 at 22:24

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