Honeywell Aquastat L8124A Won't Shut Burner on High Limit on Call for Heat

N0cisc

Hi all,

A little bit of background first. I am running a Honeywell Aquastat L8124A on a gas burner attached to an Utica Starfire boiler. The boiler feeds three zones in the house: basement, where the boiler also resides; main floor; and top floor. The main floor zone thermostat is connected to the L8124A while the basement and top floor zone thermostats simply control their respective pumps - based on my very little knowledge I believe the burner only turns on when it reaches the low limit on a call for heat from basement or top floor zones.....please correct me if I'm wrong.
Settings:
Hi - 180
Low - 150
Diff - 10

The issue lies with the main zone, which is connected to the aquastat. When the thermostat asks for heat for too long, the High limit doesn't seem to ever kick in and the burner keeps firing, overheating the water and causing the pressure release valve to operate when water pressure is exceeded. Additionally, the circulator is cut while the burner is firing, even though water temperature is around 200 deg. and the thermostat is asking for heat. The high limit setting seems to function as expected when the thermostat calls for reasonable amounts of heat like 71-75 deg. i.e. the thermostat occasionally turns off.

I tried two different thermostats with the same results. I have also tried the "thermal paste on the temp sensor" trick, with no success.

I hope someone here has run into this before and has some good ideas.

Thanks for the help!

Jamie Hall

Wiring diagram? But if the thermostat is connected in such a way that it bypasses the aquastat -- and it's quite possible that it is, though it shouldn't be -- it will act as you say. Which is wrong... the thermostats should control a puma[ or pumps and /or zone valves, and while they may turn the boiler on or enable it, the aquastat should control the boiler temperature and firing.

STEVEusaPA

Does your boiler also do domestic hot water?
You have a triple aquastat. The other 2 zones must have a way to signal the aquastat that they are calling for heat.
Leaving the differentials out for a second...
-Your burners fire whenever there is a call for heat and the water temperature is below the high limit.
-Your circulator runs whenever there is a call for heat and the water temperature is above the low limit.
-Your burner also fires when the temperature is below the low limit, even if no zones are calling for heat (domestic how water usage).
If your aquastat isn't sensing temperature, either you have to replace the sensor, or replace the aquastat.

A runaway boiler is a very dangerous thing. Better get a pro in there where you can confirm your aquastat issue, and test/check/replace the pressure relief valve, for starters.

The other issues could be, in no particular order:
-not enough water in the boiler
-circulators aren't working
-air locked, system needs purging
-aquastat settings are drifting on the control and your high/lo limits are too close together, not accurate.

N0cisc

@Jamie Hall Sorry, no wiring diagram available but thank you for confirming my suspicions that the two thermostats not directly connected to the aquastat are incorrectly wired. I will probably need to a pro to untangle that mess.

@STEVEusaPA You are correct. The boiler used to provide domestic hot water until we installed a separate wall mounted, gas fired device just for that. However, the hot water coil and in/out pipes are still connected, but with the water turned off. Good suggestion about calling a pro; the more I look at my boiler the more I realize I need one. There is a brand new pressure release valve on there though

EBEBRATT-Ed

@NOcisc

Agree you better get it looked at.