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OT - Furnace problems


Sep 30, 2002
Seattle, WA. USA
My furnace went out tonight. It has been a bit erratic but tapping on the controller got it to go on until tonight (I ordered another controller but it is not here yet).

The curious thing it is doing is rotating the vent motor pretty much forever ... not sure what that means. I'm assuming that the vent just rotates until it is lined up and then it asserts a signal of some kind that the controller is supposed to respond to? Presumanbly, the controller is waiting for said signal, not seeing it (because it is brain-dead?) and so it never leaves this startup phase.

Another issue I noticed is that what I assume is a high-temperature sensor shorted against one of the burners (I saw it spark when I was tapping on the controller. One of the screws came off a bracket that it is attached to and it was swinging free. I kindof assume that seeing a spark is not a good thing for the temperature sensor or the controller on the other end. I'm wondering if I should replace it - is there any easy way to check it?

Anyway, if anyone has any suggestions ... it is getting cold here :-)

Bob Welland
Similar thing happened to me and I made things worse tinkering with it. So my solution has nothing to do with the furnace.

The wife and I scrounged up every candle we could find in the freezing house (had 30 or so tea candles plus some big ones), lit them in our bedroom, and it stayed very warm and cozy through the night. We just left the window cracked an inch or so to keep oxygen flowing.

Hopefully it won't get that bad for you tonight!
I've pretty much stopped tinkering ... it made me kind of curious about how the control system works. The overall system is very simple but I presume that you want it to turn off if anything malfunctions in the least bit. Hacking on things that are fed natural gas is really not my cup of tea.

Fortunately, it is still near 40 degrees (at midnight) so perhaps it won't get too cold.

Bob Welland
Are you sure it has to be the control. First I would try (if you haven't) to disconect power and wait for it to reset either at the panel with a breaker or a switch which is commonly at the furnace, and then repower with the thermostat off. Then have someone turn the thermostat on while you monitor what is happening at the furnace. One of the most common things is for the furnace to power on vent motor run and even the initial burn happen and then for it to power back down because it didn't get the proper signal from the flame sensor. If this is the case the vent motor will still keep running and the furnace may try to startup again and if it does this enough it will error out at the control box. If it is the flame sensor, the ones I have worked on are fairly simple to clean with some scotchbrite and put back into service.
To be blunt, you probably have no business screwing with this, and should call a good serviceman.

Here's a few hints, though.

While some controllers do have relays, and therefore IF a relay becomes erratic, tapping on the controller might actually have some effect. Otherwise, troubleshooting using a hammer is usually not the approach.

These systems sometimes have several "traps" which are meant to protect YOU. There can sometimes be a number of safeties, including "spill" switches, "vent" switches, along with "flame sensor" devices.

You could have a plugged or damaged vent, or junk in the vent blower, or even a bad vent switch, which is causing the controller to not fire the furnace

You could have something wrong with the gas valve itself, or even the gas supply TO the valve.

The fact that you are seeing spark from "what you assume" is a sensor is probably a good thing---this is probably the spark ignitor electrode, and is actually trying to fire the burner.

I would guess that "screws coming out" and "brackets swinging free"--especially if you are referring to the flame sensor/ ignitor assembly IS NOT A GOOD THING and probably DIRECTLY related to the problem!!!!

Here's a fairly normal startup sequence. You need to look at the wiring diagram. With power on, and a thermostat closure, and related switches such as vent spill switches closed, the controller will start up the vent motor, and run it for a predetermined TIME to flush out the combustion chamber and get fresh air into it.

After the predetermined time, the controller "looks" for a switch closure on the vent proving switch, to prove that

the vent motor is running and generating proper pressure/ vacuum

the vent is not plugged and therefore 'preventing' proper pressure/vacuum.

Next, the controller starts the spark device, or in the case of "hot surface" heats up the ignitor.

After a predetermined time--5-10 seconds or so--the controller opens the gas valve. You should hear it click as the solenoid fires.

Almost immediately, the burner or pilot should ignite, and in the case of a pilot, after "proving" the flame signal, the "main" should ignite.

The pilot or burner will fire for several seconds while the controller "looks" for a "flame signal."

The usual way that this is done on modern, electronic controls is called "flame rectification." When you apply an AC signal to an insulated electrode, that is immersed in a gas flame, the FLAME ITSELF causes DC rectification--just like a diode--and a tiny current, on the order of a very few microamps flows.

The controller has a circuit to detect this, and if NOT detected after a few seconds of burner time, the controller will shut down the gas valve, and depending on the controller, will go to "retry" for 3 or 4 cycles, then

lock down, at which time you must cycle the power on/off to initiate a reset and start all over again.
Thanks for the info - a service guy is on his way ... this is one thing that I've decided I should not try to "do myself" ...

BTW, I in no way intended to mess with any of the safely stuff - nor would I suggest that that would ever be a good idea.

Bob Welland