When there is a call for heat from the house thermostat, the controller in the well of the boiler, which is set to 180F, shuts the boiler off when the boiler temp reaches 190F according to the pressure gauge (or sooner if the thermostat stops calling for heat). I'm told a 10-degree variation is acceptable.

Yesterday, I replaced the Amtrol expansion tank because it had dripped a few drops of water down at the schrader valve, and I had been told that this meant the bladder inside had failed.

The system was off for several hours, during which time several showers were taken by family members. So the domestic hot water tank was cool when I turned the system back on.

It was a fairly warm day, and I had pushed the thermostat in the house down below 50F, and so only the domestic hot-water tank's aquastat was calling for heat. The aquastat was wired (not by me, but by the installers some years ago) to the main controller via a transformer.

The boiler did not shut off until the pressure/temp gauge read 215F. Shouldn't the 180F controller setting trump the aquastat's continued call for heat?

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1 Answer 1


First of all "because I was told" is a very bad reason to replace something. Next, the house thermostat has nothing to do with this problem. Finally, the hot water tank has nothing more than its own thermostat, so it can't control the boiler temperature.

The high-cut temperature relay may broken or the boiler's thermostat is failing. It's also quite possible that after replacing the expansion tank, you failed to completely bleed the heating system, leading to large air bubbles and quite possibly a failure of the boiler thermostat sensor to be able to read the temperature of the water at the location of the external thermometer you're reading.

  • Is the Honeywell controller that sits atop the boiler supposed to trump the aquastat, or does the aquastat have its own high-temp cutoff setting? I did bleed the system but will bleed some more. I did not say that the house thermostat had anything to do with it. I said just the opposite, in fact, that it had nothing to do with it.
    – mr blint
    Nov 28, 2016 at 19:39
  • 1
    Without seeing your system I can't definitively answer your question in the comment. The way 'most' systems work (as in my house) is that the hot water tank appears to the boiler to be just another heating zone. The boiler does not set the temperature: it just supplies hot water until the thermostat for a given zone (house or water tank) reaches its set point and turns off the feed to that zone. I would hope (!!) that you can't set your water heater above roughly 140 F; the furnace as you noted should be providing 180F water as a heat source. Nov 28, 2016 at 19:45
  • Honeywell L6006A aquastat. Can be set to 240F !! (Or at least the dial has degree markings that go up to 240). Was set at 120. Changed it to 130. Boiler came on and stayed on until its gauge read ~20psi and 210F. Only the domestic water zone was calling for heat. Home heating pipes remained cool.
    – mr blint
    Nov 28, 2016 at 23:59
  • I don't see any numerical suffix after L6006A. At 210F, a few drops of water were released by the overpressure relief valve.
    – mr blint
    Nov 29, 2016 at 0:04

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