Gas Furnace - pilot on but furnace not firing

pitman9

what makes you suggest a bad thermocouple if the pilot is lit?

kevin_34

Gas Furnace - pilot on but furnace not firing

I have a gas hot water Boiler with 3 zones. The thermostats are turned up, the pilot is on and the circulator pumps are on but the gas burner is not igniting. Any suggestions?

Big Ed

Top of my head

Gas valve set on pilot,bad end switch,bad or disconected transformer,bad or poped spill switch,gas valve shut off but you have seperate pilot valve,bad gas valve ,Bad electrical conection,bad aquastat,tripped low water cut off,racoon made a nest on the vent damper so its stuck closed or it could be bad.The vent damper that is, supply or return valve is closed and the boiler is sitting on high limit,manual reset high limit is tripped, ect,ect........

Dale

Likely

From what you say the most likely culprit is a bad thermocouple, If you have one that is. If you do have a thermocouple and the pilot flame looks blue and steady I would just put in a new thermocouple. All the other things suggested by the other post could be bad too, and if it were mine I would see if the gas valve was getting 24 volts at the appropriate terminals, but if you are a homeowner I'd try the thermocouple.

Unknown

Do you have a multimeter?

If so do you have 24 volts to the gas valve. If you do and the valve is in the on position you could have a bad gas valve or a broken wire to the valve. We know the power is on as the circulators are running.

A little more information would help, what are the make and model of the gas valve? From your post I assume you have three circulators. They must be controlled from a relay, one of the relays is probably attached to the boiler pull the cover off and see if you have 24 volts at B1 and B2 (this is of course with the thermostats calling). If not then you could have a faulty relay or a loose or broken wire somewhere.

This will give you a start, get back to us and let us know more so we can help you.

Darin

Not enough information

Here's an intersting twist. Are you dealing with a millivolt system? If you are, the normal relays won't work. You need relay's with gold plating to maintain low resistance through the contacts. 24 volts will pass through fine(and energized the pump) but the burner won't light or will drop out intermittently. Look for B1, B2, and B3 terminals on the aquastat. If there is wires on B1 and B2 and B3 is blank then look for a voltage drop through the aquastat. I'm sure your dealing with something more modern but I thought I'd give my 2 cents(hey, I'm bored).

Mad Dog_2

YES!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Get a Pro in there to diagnose the problem and fix it. Kevin, is this your home? Mad Dog

To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"

kevin_34

Thanks for your feedback - I have a pro scheduled for this afternoon (my wife is too cold to wait for me to figure it out). The system has 3 relays and they all seem to be engaging which should call for fire. I don't want to mess too much with a system I'm not familiar enough with and may cause more problems. I'll post back to let you know what the fix was....Kevin

kevin_34

Home owner update

OK, the repair guy not here yet - I vacuumed all dust and webs etc around the pilot and burners. Had to relight the pilot. I took the cover off to check B1 B2 - it is a milivolt system by the way, as I was checking for voltage the burner kicked on!! Still going to have the system checked out.

Dale

My guess

This sounded like an old non 100% shutoff system, I recently looked at an old A cock B cock boiler, with the external basso safety. I should say we have alot of old water and steam where I am and not really alot of the 70s - 90s product. And now we are getting the really modern stuff, Munchkin ect. I wasn't (but should have been) thinking of powerpile but of course the old powerpile would look like this too, lot of HW vs 8187s out there still. If Timmie had known a millivolt system he would have alot more to say. Timmie has a great class on millivolt systems, and if he ever sells the xtra binders they're a great resource at a very reasonable price. The, "touched it and now it works" is a powerpile trait. To really understand what is wrong millivolt readings must be made on all system components. And, is the bleed vent blocked? This is a good example of while this forum is useful for a homeowner the impossibility of describing things you don't understand makes the hands on work of the pro necessary. And, safety being the most important, the limit/vent/controls/relief inspections are always money well spent.

Unknown

This is not unusual

with millivolt systems. The pilot can be on and due to resistance somewhere in the valve circuit the burner will not come on. Just bumping the valve will sometimes bring it on.

Make sure the pro who comes does a complete millivolt set of readings. Sometimes if they find it running they think you are crazy. Truth as you know you are not it was not running and now it is. There is a definite protocol to test this and find the problem. Truth if it happened once it will happen again so make the tech check the millivolts.

Things to watch for:

Loose splices in wires (redo them)

Loose connections on the valve

Faulty relay contacts max millivolt drop across contacts 10 millivolts

Faulty high limit switch in the relay max millivolt drop across switch 10 millivolts

Make sure he does a pull in on the gas valve. This is done by blowing out the pilot and letting the safety drop out. Then relight the pilot and see if the valve pulls back in between 179 to 320 millivolts. Anything above 320 replace the gas valve.

You could have a bad pilot generator it should when disconnected from the valve and with the pilot lit measure over 540 millivolts. This is a 750 millivolt generator so I like to see no less than 700. If you can't get above 540 replace the generator.

Have him also clean the pilot while he is there and it is a good idea that while the pilot is out to replace the generator. This is especially a good idea if it has not been serviced in a while.

This is a good time to oil circulators if they are the type that need oiled.

Finally have them do a combustion test on your boiler for efficiency and safety (prevention of high levels of carbon monoxide).

tim smith

millivolts

Make sure he pulls all connections on wires and cleans the terminals and connectors. Check millivolts under load at the main coil on the valve when running. Should be somewhere over 150 millivolts at main coil while running otherwise some connection or device is eating to many millivolts, and this will cause pilot to drop out or main valve. Good luck. Tim.