Older Forced Air Furnace Won't Fire

wildrage

I have a pretty old Westinghouse series NXAPE, model FGB0192UA 150,000 BTU natural gas forced air furnace. It's used to warm roughly 25% of my home - the rest of which is heated by a Wells-Mclain 250,000 BTU boiler. It typically only really fires when it gets very drafty in the house. I'd say it normally run roughly 45 days a year, for under 5 hours a day.

It stopped working last week. When a call for heat comes in, the pilot lights quickly and stays lit, but the burners do not fire. I've left it in this state for around 5 mins and still no flame (pilot still lit). During this time I hear a humming noise and a very faint smell of natural gas.

It uses a mercury switch for the flame sensor, which I tried swapping out, but I still get the same results. It's odd because the initial problem was that the pilot kept on getting lit then relit, and every once in a while the burner would fire on, but only for 5 mins. Now the pilot stays lit, but nothing else. Furnace never turns off - I think it thinks the burners are running since the pilot is lit and it's calling for gas.

Could I have knocked something when I was switching out the mercury switch? Could the gas valve be shot? If so - wouldn't the pilot not light? Is there anything else I can check / try?

This is an older unit, and by what I see, there are no rollout sensors. The only other sensor I can think of could be the fan control / limiter, but I'm assuming if that was malfunctioning the burners would atleast light - and I replaced that 3 years ago when it was malfunctioning.

At this point I would call a pro, but it's been days and I can't get anyone to come out. Any input would be appreciated.

For reference the gas valve is a White Rodgers 36C84 Type 304, and I'm told the direct replacement is a 36C84-912.

Big Ed_4

Where did you find the mercury switch ? Thinking dirty pilot ...

wildrage

Big Ed_4 said:

Where did you find the mercury switch ? Thinking dirty pilot ...

I picked one up at supply house. White Rodgers Mercury Flame Sensor, 48" Element, 3 Pin Plug, Bulb Style #20, which shows up as a direct replacement for the one I had. Just to decrease variables, I cleaned up the old one and put it back in. Seems to now work at keeping the pilot on.

Check out video here: https://youtube.com/watch?v=gjmTjASvDe8

Jon_blaney

How about some pictures of the control elements, gas valve, relays, etc.

mattmia2

Check the voltage at the gas valve. Is there a control board? Picture of the schematic. It seems a lot like either a bad contact or connection means the gas valve isnt getting enough power to open the main valve or more likely the main valve is bad.

Jon_blaney

If you have voltage, try tapping on the valve. If the burners light, valve must be replace. Do not use.

wildrage

Thanks guys.  Was at a family event last night.  Will have some pictures in a little bit. 

wildrage

@mattmia2






@Jon_blaney Tried lightly tapping on all areas of the valve and got nothing

Jon_blaney

So, voltage at the gas valve?

wildrage

Jon_blaney said:

So, voltage at the gas valve?

Where should i check with the voltometer?

Jon_blaney

The electrical connection at the gas valve. I think you are looking for 24 volts.

wildrage

Jon_blaney said:

The electrical connection at the gas valve. I think you are looking for 24 volts.

wildrage

Jon_blaney said:

The electrical connection at the gas valve. I think you are looking for 24 volts.

I'm getting 25 volts on the call to heat.

wildrage

Would just like to rule everything out before i buy the 400 gas valve.  I'm also not thrilled with messing with the gas line, but I installed a gas hot water heater last summer so it won't be my first time.  I also bought a plugin alarm that detects gas leaks out of my paranoia last summer .

1. Pilot stays on and lit indefinitely.
2. I have 24v to the gas valve
3. The other furnace lights with no problem, so I'm assuming the gas line is fine

With a standing pilot, I'm assuming the only possible issue could be with the gas valve itself?

mattmia2

You will need a tech that can do combustion analysis to stet up the new valve.

You can make sure the mercury switch makes a good connection when the switch is closed and make sure it is making a good connection to the valve and check the pressure to the valve before saying it is the valve, but if it has 24vac and the mercury switch is closing on pilot heating it, and you have adequate pressure to the valve then the valve is bad.

Jon_blaney

You need special equipment to test combustion and gas pressures. Did you price the gas valve? May be time to trade the old girl in. How about a heating air handler run off the boiler you already have?

wildrage

Had the guy come out today.  Me and him messed around with it for about an hour and he determined that it must be the valve.   I sort of like the redundancy of having 2 systems, however, I've had SteamHead out here talking about adding a few steam radiators added and going to 100% steam.  Something I'll seriously look into when I'm in the market for a new boiler. 

JUGHNE

There is a pressure switch within the gas valve. It must have at least 5" of gas line/supply to the gas valve for the main valve to open.

I believe I mentioned in your previous post that the simplest repair is to install the old school standing pilot valve with the simple thermocouple.

It is less money than any component you might throw into this system.

If you had a pro there and he did not check the gas pressure, then he is too young to know about these things.

That furnace is looking short for this life.