PDA

View Full Version : My furnace went south. Help, please



Tony Zona
11-18-2015, 6:14 PM
As winter bears down, my old Reznor hanging furnace in the shop won't light. It is narrow and has only two vertical heat exchangers, I think you would call them. I got it used years ago from a friend. I'm guessing it was built when John Adams was in third grade.

My web searches were no help. The furnace man I called is too busy. (I may have to find another one sooner or later. Sad, because this one has been very good.)

Here is what I have found so far.

1. The pilot is burning and the propane tank is 25 percent full.
2. The Honeywell thermostat closes a circuit. I tested with a meter.
3. The wires at the furnace from the thermostat also make a circuit when the thermostat closes. (I removed the wires from the furnace terminals and tested only the wire and thermostat.)
4. The electrical breaker is functioning just fine with the furnace getting power line voltage.
5. The furnace transformer is putting out about 29 volts when disconnect from the furnace. It is a 24-volt transformer. I am going call it good.
6. The main gas supply line valve is open.
7. Formerly the furnace would click when I turned up the thermostat. Then the furnace would light and a short time later the fan would start. It does not click now.


What should I do next?

Paul Stoops
11-18-2015, 7:30 PM
I am not familiar with that type of furnace, but is it possible that the thermocouple might be defective? The thermocouple in most gas burners is heated by the pilot light and must be functional to turn on the gas valve to supply fuel to the burners. Just a thought.

Paul -- Auburn, WA

Tony Zona
11-18-2015, 7:39 PM
I thought if the thermocouple is bad the pilot will not remain burning. Is that correct?

ken masoumi
11-18-2015, 7:50 PM
I am not familiar with that type of furnace, but is it possible that the thermocouple might be defective? The thermocouple in most gas burners is heated by the pilot light and must be functional to turn on the gas valve to supply fuel to the burners. Just a thought.

Paul -- Auburn, WA
That might very well be the cause of the problem,I have a natural gas heater that exhebits the same problem every year, it won't light ,only the pilot lights up( and stays on), so every few months I clean the thermocouple with some Emry cloth to remove the build up of soot.it seems to do the trick.

Paul Stoops
11-18-2015, 8:03 PM
I was just looking online and found this: http://www.stevejenkins.com/blog/2012/10/diy-gas-fireplace-wont-light-how-to-clean-your-thermopile-and-thermocouple/
The author mentions a thermopile in addition to the thermocouple -- apparently the former controls the gas valve and the latter monitors the pilot light flame. Don't know if all gas burners have both devices or not. But I know in the past I have had gas burners that had periodic thermocouple problems that were resolved by just changing out the thermocouple.

Paul -- Auburn, WA

Tony Zona
11-18-2015, 8:18 PM
I'll give it shot. I always thought they were good or bad, not weak. I should have tried that. Thanks. I'll let you guys know what happens.

Matt Day
11-18-2015, 9:21 PM
My house furnace stopped working and the pilot light was on. Turns out it was the thermocouple. The thermocouple basically tells the gas valve to come in which then gets lit by the pilot. So just bc the pilot is on doesn't mean it will light.

Michael Weber
11-19-2015, 1:54 AM
There is at least one limit switch in series with the 24 volts that may be bad. When the thermostat is calling for heat do you have the 24 volts across the gas valve terminals? If you have a 24 volt operated system it's unlikely there is a thermopile in your furnace. Check you have voltage across the gas valve terminals, not just at the transformer. If no voltage you have a bad limit switch or open wire somewhere. If you do have voltage at the valve you need a new main gas valve.

Kev Williams
11-19-2015, 4:00 PM
Attached to the furnace box should be a temperature controlled switch, that switches the fan on and off depending on the heat in the flame box. It may be combined with, OR there may be a second OVERLIMIT switch, which if memory serves, turns off the gas if the temperature in the flame box gets above a certain temperature. The 'fan and limit control' switch in the diagram may be like yours. We own a mobile home and I had to change the fan switch in it a few years ago, and it had a separate limit switch- what happened was the furnace fired but the fan never came on, so once it got too hot the limit switch shut down the gas valve. Not exactly sure of how it all works, but you may have a limit switch problem...

http://www.engraver1.com/erase2/furnace.jpg

Tom Stenzel
11-19-2015, 10:35 PM
There is at least one limit switch in series with the 24 volts that may be bad. When the thermostat is calling for heat do you have the 24 volts across the gas valve terminals? If you have a 24 volt operated system it's unlikely there is a thermopile in your furnace. Check you have voltage across the gas valve terminals, not just at the transformer. If no voltage you have a bad limit switch or open wire somewhere. If you do have voltage at the valve you need a new main gas valve.

^--- Yes on the above. When Mike says limit switch he means a temperature switch, not a position switch. Kev fleshed it out a bit.

If the system has a 24 volt transformer it won't date from when John Adams was in 3rd grade. Might date from when he got his sheepskin though.

It's normal for a thermopile system to have the pilot to light with a bad thermopile; the gas is on all the time. The thermopile provides the power to operate the main gas valve. If the pilot goes out the pilot gas will go up the flue.

Normally when the system has a 24 volt transformer the thermocouple holds the pilot gas valve open.

A broken wire or bad switch sounds like a culprit. If the furnace is as old as you say it probably doesn't have much in the way of backup safeties, look for one overtemp switch.

And of course keep a look out for an on-off switch somewhere that got knocked to the off position.

-tom

Tony Zona
11-20-2015, 7:11 AM
Still struggling.

There is no voltage at the gas valve when the thermostat calls for heat.

This furnace is old and severely undersized, and I'm running out of time to waste on it. I'll make one more go.

Thanks for all the direction. I really do appreciate it.

Michael Weber
11-20-2015, 1:25 PM
Well then, if the transformer is putting out voltage but it's not getting across the valve then like everyone,s suggesting, there is an open condition between the transformer and the valve.This includes the wires to the thermostat as well as the thermostat as well. If you want to confirm that you can briefly jumper a wire from the transformer to the valve. Just make sure you identify which wire is which from the transformer to the valve. You don't want to cross them as it might damage the transformer. One side of the transformer will likely go directly to the valve without going through any limits and the other side will probably run in a non direct route through limit controls. Connect a wire from the above latter option directly to the valve. Or, you could just remove the two wires from the transformer and the valve and route jumper wires from both terminals from the transformer to the valve. No polarity to worry about doing that. If tie valve does not click open then it is bad.
Also check the wires from the thermostat to the furnace. I've seen thermostat wires get damage or broken. These are typically solid wire and can break at connections especially if nicked while striping insulation. Not sure about what thermostat you have but it's also possible it has gone bad. If there is only two wires to the thermostat you can short those together to test the thermostat assuming the wires are good.

Jim Laumann
11-20-2015, 2:35 PM
Tony

I have a newer Reznor - it's got a couple of fuses (the 12V automotive style) on it's circuit board. Does yours have fuses? have you checked them?

Jim

Scott Donley
11-20-2015, 3:37 PM
If you have a thermostat that takes a battery, replace them. I know on mine the led still reads but won't turn on the furnace even though the pilot stays lit. Don't ask me how I know this. :)

Larry Browning
11-20-2015, 6:45 PM
Tony,
Did you try replacing the thermal couple? The sell them everywhere and only cost 8 bucks. I am having the same problem with my water heater that drives the radiant floor heat in my shop. The pilot lights but the burner won't kick on. When I first lit the pilot, the burner did come on and it heated the water. About 2 hours later I checked on it and it would no longer light, but the pilot stayed lit. Everyone is telling me it is the classic thermal couple problem. I bought a new thermal couple and will install tonight.

Myk Rian
11-20-2015, 7:57 PM
If the pilot goes out the pilot gas will go up the flue.
Say what? Where did you learn that?
When the pilot goes out, the gas shuts off. That's why you have a little red button to push when lighting the pilot.

Tony Zona
11-20-2015, 8:09 PM
No fuses anywhere. I looked because I had not thought of that.

Tom Stenzel
11-20-2015, 10:18 PM
Say what? Where did you learn that?
When the pilot goes out, the gas shuts off. That's why you have a little red button to push when lighting the pilot.


From the Michigan Consolidated Gas worker who re-lit all the pilot lights in the house after the gas meter was replaced. I was a lad and followed him around. When he lit the furnace the pilot lit right up. When I asked about the gas when it wasn't lit, guess what he said? That's right, straight from the Gas Man's mouth.

My neighbor's house had the same system. As did the house next to his.

Little red buttons? How post WWII! Remember when the pilot on the oven would go out and the main gas stayed on filling up the house with gas? I sure do. The house in Detroit almost blew up once when that happened.

When Tony said old I'll take him at his word!

-Tom

John Sanford
11-21-2015, 3:15 AM
Dude, when your furnace packs its bags and goes south in November, perhaps you should ask yourself what it knows that you don't. Lay in a stock of hot chocolate, or follow it to Florida.

I'm just sayin'..... ;)

Tony Zona
11-21-2015, 8:12 AM
:eek: I'm thinking about that.

:D

Myk Rian
11-21-2015, 2:20 PM
Many companies will do a free inspection, as does the gas company.
If you call a service company, be prepared for a sales pitch. Research the company you call, first.

Bill ThompsonNM
11-23-2015, 1:28 AM
From the Michigan Consolidated Gas worker who re-lit all the pilot lights in the house after the gas meter was replaced. I was a lad and followed him around. When he lit the furnace the pilot lit right up. When I asked about the gas when it wasn't lit, guess what he said? That's right, straight from the Gas Man's mouth. My neighbor's house had the same system. As did the house next to his. Little red buttons? How post WWII! Remember when the pilot on the oven would go out and the main gas stayed on filling up the house with gas? I sure do. The house in Detroit almost blew up once when that happened. When Tony said old I'll take him at his word! -Tom
Of course he said Propane...and propane is heavier than air and just fills up your house bottom up... That's why you're not supposed to bring those big propane tanks in your house where they leak until they get up to a spark.....