main burners cut out/ no automatic vent damper?

Shevek · 11-13-2011, 07:08 PM

Hi,
I have a gas boiler from 1987. The main burners go out.
That is, with the electric and pilot on, I turn the thermostat up. The burners come on and are on for a couple of minutes and then shut off. To me this seems as if the automatic vent damper is not opening. The pilot stays if I do not turn on the mail burners but shuts off when the main burners suddenly turn off.
Here is the funny part, I cannot find an automatic vent damper on the flue in the furnace room (see attached photos). I don't hear a motor sound when I turn up the thermostat. Is it possible that there is no automatic vent damper? If that is the case, what else could cause my recently-working (last week) boiler to turn off?
thanks!

hvactech126 · 11-13-2011, 07:33 PM

if it is a standing pilot system then you need to check that the pilot does not need cleaned. The pilot flame could be pulling away from the t-couple with the main burners on thus extinguishing the pilot and the main burners.

how · 11-13-2011, 07:59 PM

If you had a motorized vent damper ( which would be pretty visable) the main burners would not come on unless the damper was fully open.
It is probably a thermocouple / pilot assembly issue. The therocouple needs to have its tip enveloped by flame to allow the boiler main gas to operate or for the pilot to stay on. . A really dirty thermocouple, a thermocouple with a weak dcmv output, a thermocouple that has slipped too low to be fully in the pilot flame, or a partially obstructed pilot orifice that is producing too soft a flame are the most common causes for pilot outage occuring after the main burners go out.

Shevek · 11-13-2011, 08:50 PM

Quote:

Originally Posted by how

If you had a motorized vent damper ( which would be pretty visable) the main burners would not come on unless the damper was fully open.
It is probably a thermocouple / pilot assembly issue. The therocouple needs to have its tip enveloped by flame to allow the boiler main gas to operate or for the pilot to stay on. . A really dirty thermocouple, a thermocouple with a weak dcmv output, a thermocouple that has slipped too low to be fully in the pilot flame, or a partially obstructed pilot orifice that is producing too soft a flame are the most common causes for pilot outage occuring after the main burners go out.

Thanks for the first comment!
I read up on thermocouples and I could see why a malfunctioning thermocouple would keep the main burner from coming on, but why would it keep it on for a few minutes and then turn it off? The pilot looks OK: steady blue flame with occasional yellow in the flame tip.
Should I just replace the thermocouple since it is cheap rather than trying to troubleshoot it?

REP · 11-13-2011, 10:11 PM

There are ways to check a thermocouple but they can take longer then replacing it and if there is any question then just replace it.Don't buy a cheap thermocouple,get a Penn or W/R,or at least a honeywell thermocouple.
I can think of a varity of possible problems from not the right gas pressure to a limit opening to a boiler that hasn't been torn down and properly cleaned in a while or a venting problem and/or the thermocouple is weak.

how · 11-13-2011, 11:02 PM

The gas valve safety is powered by the current provided by the thermocouple.
Below a particular threashold everything shuts down. That current output varies depending on the strength of the thermocouple and the difference in temp between the hot & cold junction of the thermocouple. The reason you'd want to watch the pilot flame right after the main burner goes off is to see if the draft through the exchanger pulls the pilot flame away from the thermocouple or not.

biggles · 11-14-2011, 01:33 AM

if you had a vent damper it would of been on that first piece of flue pipe above the furnace TC sounds weak..if you have a meter and remove the old one you could heat it up and actually read the contact end of it were it screws into the top of the valve to see "millivolts" generated....