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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 8687
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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When my boiler reaches the high temp limit and shuts down the

Resolved Question:

When my boiler reaches the high temp limit and shuts down the fflame percent sometimes stays at 100% instead of dropping to 38% so when it goes to start at 100% i get ignition lockout. Why?
Submitted: 5 years ago.
Category: HVAC
Expert:  Phil replied 5 years ago.

Phil : All work to be done by licensed professional. WE KEEP GOING UNTIL YOU ARE HAPPY. . I come and go. Be patient. This is step ONE.
Phil : Hello. The electronic controller is described by clicking the link in the left margin of this page.
Phil :
Phil : .
Phil : These are prone as all electronics are to malfunction, primarily in this case due to sensor input errors. The thermistor temperature sensors involved then to fail by first becoming irratic. That will produce the symptoms you are having.
Phil : .
Phil : When the boiler shuts down while firing 100% the residual heat build up in the boiler, over heats the water in the heat exchange when the pump and burner shut down.
Phil : That trips the high limit safety cut out.
Phil : .
Phil : Let me know if that comports with what you see on your end.
Phil : If thats accurate, please click the big green accept button, that will allow me to go beyond this 'just an answer' level to some extended trouble shooting and coaching with you.
Phil : .
Phil : Thanks!
Phil : Phil
Phil : .

the water temp does exceed my boiler high limit of 175f. I have the boiler max at 200f should I lower that to equal my high limit

Phil : Hello again.
Phil : .
Phil : These thermostat controls are never 100% accurate, they can be off 5 or 10 degrees, so that if one is reading too high, and the other is reading too low, their functioning can over lap causing the sorts of problems you are having
Phil : .
Phil : Set the boiler high limit to 200F... thats the usual range for those.

I will make adjustments in the morning. If i accept now can I still contact you?

Phil : To be clear though it has to be the 'high temperature LiMIT' you can set to 200F.
Phil : then the other thermostat that cycles the boiler, must be set well below that limit... most often in the 160 to 170F range.
Phil : that avoids this over lap issue.
Phil : Yes you can accept now, and to get back to me, just log back onto your account and select the question and start typing.
Phil : Thanks!
Phil : Phil

so even though I set the high limit at 175f the water gets 195f is this normal. The boiler heat percentage does lower as low as 25% and runs at this level. But sometimes when it shuts down it shows 100% on the screen with the heater off and if I power down the heater it goes to 38% and then I push the small reset on the circuit board and it shows 0% then it starts.

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Expert:  Phil replied 5 years ago.
Hello again.... if you set either of the thermostats, the high limit safety cut out... or the operating temperature control thermostat at 175F the hot water temperature you see in the pipe loop and radiators etc should not go above 175F

***** however****
On boiler shut down, when the pump and gas valve go off... residual heat in the boiler will drive the temperature in the boiler itself up 20 degrees or more... sometimes tripping the high temperature limit.

a proper control systems arrangement, lets the water pump run for a few minutes after the gas valve shuts down.

You should set the high** limit** at 200F for now, when I am on the job with instruments I often set them as high as 210F..

Let me know how this looks on your end.

we can go from there.