Furnace ignites, but won't stay running. It appears to be due to the 5 burner not lighting - where the thermocouple is located. When the TC doesn't heat up, the gas shuts off. Why would the 5th burner not ignite?
I thought about that, but we have never had it in the past. This problem actually started yesterday, but yesterday and this morning I was able to resolve it by simply cycling power to the furnace (switch on front). This no longer works.
After the fifth burner lights off, does it look and act like the rest of them ? Are they all uniform in appearance ?
I have watched the cycle numerous times (~10) and yes - after the 5th burner lights it looks/acts no different than the others. From upstairs, everything seems normal with the furnace now that I rotated the TC 180 degrees to be in the path of flame of the 4th burner. But if you are downstairs watching the system, what you see is the igniter glows, the 4 burners start within 2-3 of each other and then about 7 seconds elapses before the 5th burner starts. I don't think I want to leave the TC like this.
No, you don't want to leave it like that. As a test method it was ok to do that but not for permanent operation.
On the burners there are carryover bridges that carry the flame down the burner rack to ignite all of the burners. All these have to be carrying the flame, even the 2 to 3 second in the first four is too much delay as the light of should be nearly undetectable from one end to the other.
You have already determined that gas will flow through the 5th burner orifice so the carry over must not be getting to it as it would light off if it were. Look closely at how the burners connect to one another and you should find your problem there. It is a matter of getting them lined up correctly and making sure all the passages are clear. Also it does take the right amount of gas pressure to get it to light off correctly, ( you can't be way off on it )
If I gave you instructions on how to clock your meter to find out what the gas pressure approximately is, do you think you could do it ? It only involves turning off other gas appliances and timing the meter while your furnace runs for 1 minute while watching one of the dials on it.
I was thinking also the same lines and I have the utility coming to check the pressure at the meter. They have indicated that they will only check outside, not at the furnace.I also have a gas dryer and water heater and they seem to be working fine, but I just did a little test. I turned on the dryer and then raised my thermostat setting to start the furnace. This furnace behaviour was: 2-3 second for the first 4 burners (basically unchanged) and then 15 seconds for the fifth! This would make me think it is a pressure issue - don't you agree?If they tell me the pressure is up to standards, I've really no idea what to do next.
Yes, they will check your incoming pressure and that could be where the problem is but it really sounds more like a regulator problem to me. As if it is taking the regulator a while to catch up with the new pressure demand when the furnace fires.
If you start your furnace and let it run while you are looking at the gas meter in your home and time how long it takes for the smallest dial to make one revolution ( usually the 2 foot dial) and let me know I can tell you if you are getting the approximate pressure to the furnace burners, just make sure the dryer and water heater aren't firing when you are timing it. Also need to know the BTU input of the furnace. You can get that info off of the label inside the furnace by the model number.
The utility was here and says the pressure is fine behind the meter. Told me to clean the TC (flame sensor) with emory cloth. I don't see this making a difference because 1) it doesn't seem dirty 2) that 5th burner takes to long to light.Furnace label says 125,000 BTUs. Two foot dial took 60 seconds to make one revolution. Dryer was not on, can't garauntee that the water heater wasn't, but not likely since no hot water was used in the last 90 minutes (it should be "recharged").
Was it a two foot dial ? It will say it near the dial .
I'm confused. I said "Two foot dial..". There is another small 1/2 foot dial, but I ignored that one.
A two foot dial spinning for 60 seconds indicates that you are using about 120,000 during that time period. So it looks like the furnace manifold pressure is pretty close.
I'm betting its your gas control valve in the furnace, the regulator isn't working properly, that's what all of this tells me, not excluding making sure that the pilot carry-overs are clear. The only test I can't do is to be on the manifold pressure when it is lighting off, this is where I am guessing it is slow to come up at.