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What is the highest temperature the gauge should read on a boiler?

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8ttam2
8ttam2 Member Posts: 4
When my boiler burner shuts off, the temperature reading on the gauge continues increasing for about 20 seconds afterward to about 30 degrees higher than when the burner shut off.

What is the highest temperature I should allow the water to reach in the boiler? The service manual specifies a minimum low temperature of 135 degrees but does not specify a high temp. I know that the recommended high setting on the aquastat is 200 degrees but that would put the actual water high temperature guage reading in my boiler around 230 degrees which seems too high.

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  • Jamie Hall
    Jamie Hall Member Posts: 23,426
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    It'll do that, depending on exactly where the temperature gauge probe is in relation to the hot boiler metal.

    However... Assuming this is hot water hear, is it really necessary to run up to 200 degrees to get enough heat in the house? You might find you could be comfortable -- and save some fuel -- with the aquastat set lower -- perhaps try 160 or 170. You still might want a high limit shutdown switch at 200.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    Canucker
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,505
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    You may want to consider a control strategy that purges the remaining heat in the boiler to the zones, or an indirect tank, rather than leave it in the boiler.
    But it's pretty normal on a boiler with a lot of mass to do that.
    I'd turn it down to 180°, and make sure it's reading correctly-both the thermometer and the aquastat. Older mechanical ones tend not to be so accurate.

    There was an error rendering this rich post.

  • 8ttam2
    8ttam2 Member Posts: 4
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    This is in a house I just bought that has a Mini-Gas mg-75 boiler which only heats the house. It has a Honeywell L4081A aquastat I have a separate water heater for the hot water. Problem is that the boiler is having a hard time keeping up heating the house.

    When I first inspected the boiler, the high setting on the aquastat was set at 200 degrees. The boiler would stay lit until the temp gauge read about 220 degrees before it shut off, then the temperature gauge showed the temp increasing to almost 250.

    Then, when the temp would hit 190 degrees, the ignition control would spark for a few seconds but not fire up. Then the 5 minute reset would initiate and then try again and would fire up when the water was below 130 degrees. I'm not certain, but I think the gas valve might have a lockout that will not allow the burner to start if the temp exceeds 200 degrees.

    When I adjust the high setting downward on the aquastat until the gauge reaches 200 degrees on the temperature gauge at its highest point, the aquastat is at its lowest setting, which is 130 degrees. With the aquastat set at 130 degrees, the burner shuts off when the temp gauge reads about 170 degrees and then the temperature increases up to 200 degrees before it starts going down again. This also seems to have helped with the ignition control as it will kick back on now at about 145 degrees.



  • STEVEusaPA
    STEVEusaPA Member Posts: 6,505
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    Your water temp is dangerously too high. Going to steam would be bad, and shocking the boiler with cool water will destroy it.
    I would recommend you have a competent hydronic tech come out and figure it out ASAP.
    Could be boiler doesn’t have enough water in it, an air problem, bad/wrong circulator and/or bad/problem with aquastat.

    There was an error rendering this rich post.

    1MatthiasGordy
  • Brewbeer
    Brewbeer Member Posts: 616
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    What kind of radiators do you have? Do they all get equally hot?
    Hydronics inspired homeowner with self-designed high efficiency low temperature baseboard system and professionally installed mod-con boiler with indirect DHW. My system design thread: http://forum.heatinghelp.com/discussion/154385
    System Photo: https://us.v-cdn.net/5021738/uploads/FileUpload/79/451e1f19a1e5b345e0951fbe1ff6ca.jpg
  • 8ttam2
    8ttam2 Member Posts: 4
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    I've now got the boiler aquastat adjusted so that the water temp does not exceed 200 degrees.
    It has three zones with finned baseboards. Zone 1 has 37lf of baseboard, zone 2 has 41lf and zone 3 has 26lf. The zones don't have temp guages on them but when I point my infrared camera at the baseboards all of them are reading the same temperature.
    I bled the all water from the return side of the boiler and the water came out really dark with very few air bubbles.