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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 8700
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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I have a Dunkirk/lennox boiler (2001) GWB9 with a UT

Customer Question

I have a Dunkirk/lennox boiler (2001) GWB9 with a UT electronic controls board. Whats happening is my gas valve opens, I have flame for about 5 seconds and then it shuts off and tries again. Ive replaced the ignitor. Ive also replaced the pressure switch. The aquastat seems to be fine. Any thoughts?
Submitted: 1 year ago.
Category: HVAC
Expert:  Phil replied 1 year ago.


So far it sounds like a failed flame sensor circuit.

Please attach a well focused, close up photo of the wiring diagram glued inside the service panel,you can use the paper clip icon at the right end of the toolbar at the top of this dialog box,Include a picture of the thermostat wire connections to the printed circuit board in the same area.

If there is no paper clip icon, look for the '+attach files' link at left of your send button.

I can mark it up with trouble shooting instructions. You will need a multi meter ($30 at a hardware store) and some experience working with appliances, let me know a lot about your skill sets in those regards.

If none of this in your skill sets, tell me which direction you are from the center of the nearest city and I will look for a few well rated service outfits in your area.

we can go from there.

Customer: replied 1 year ago.
Hi Phil, thank you for your response. This is my first time using this service. Just a little background on myself, I am an electrician and I actually specialize in wiring heating an AC systems (that's all I do), so I quite familiar with the control aspect of these systems. Although, I typically do not service/troubleshoot them when there is an issue. I, too, thought it was a flame sensor. Its my understanding that the flame sensor is part of the glow plug ignitor (this is the first thing I replaced). Then I replaced the pressure switch (still doing the same thing). The aquastat is mechanical and seems to be working fine. The only other thing in the loop is the UT controller. So basically I'm thinking its a bad controller. But i didnt want to by one if its something simpler than that. Last year i had the burner and screen cleaned so I dont think it is that. When it fires of for the few seconds it does the flame looks very good. Any thoughts? I can by a new controller but it just seems to be an odd sympton of a bad controller that it would allow it to run for a few seconds and then turn off. Unless it is not seeing the signal from the flame sensor that there is fire. Any thoughts?
Expert:  Phil replied 1 year ago.

Hello again,

are these or any other indicator lights on the PC board or display ON?

So far it sounds like a flame sensing problem this can happen if the furnace is not properly grounded or in older water heaters where the ground connections might be corroded.

You can do a heat stress test on the control board.

Use a hair drier to slowly and gently warm the card to 120F(warm to the touch but not hot and not over140F)... if you get any change in behavior that means the card is bad...

A micro crack has opened in the printed circuit at a solder joint, or inside its CPU in that case .... or a short circuit has developed as the card expanded when you heated it.

Constant expansion and contraction as these electronics heat and during their duty cycles over the years is what wears these cards out.

It is also a good idea to replug all of the connections in the unit,that burnishes any oxidized terminal connections and will sometimes solve the problem, especially at the printed circuit card and in any sensor molex connectors. Do that with the power off to the unit of course.

Let me know what you can discover, we can go from there.