HVAC folks? Furnace won't kick in - thermostat clicks?

[Al Willits]

That helps, thanks.

Ya, looks like a spark to light pilot, with a capillary style safety, meaning that thin tube that goes from the valve into the furnace, is filled with mercury/freon/whatever and when the pilot is lit by that orange wire (spark lead), it heats up the end of that tube, which is probably sitting in the pilot flame.

That causes a bellows on the other end (at the valve) to trip a switch that allows main burner to come on.

Not sure how far you want to go into this, but that tube (pilot safety) can sometimes get weak, or what's usually more common, is the pilot gets a bit dirty and won't always heat the cap tube hot enough.

Trick of the trade was to remove (furnace off) the pilot line from the valve and using a aspirator bulb, blowing though the pilot line, 9 out of ten times it would clean the pilot well enough to function again.
Compressed air will work, but it doesn't take much.

That gray box is your fan and limit control, chances are if you pull that white knob out, the blower will run continuously.

Looks like they're running the W on the thermostat to the Y on the furnace...haven't a clue why, sorry.
Y is usually used for A/C, wondering if they use it to energize a relay or something?
If your on Natural gas, can you see the pilot when it lights?
Is it blue or is there some yellow to it?
Orange is OK, just stuff in the air.
Propane may have a bit of yellow tipping on the pilot, but it shouldn't be very much, if it looks like a candle, its probably dirty.

PLEASE, if your not sure of what your doing, get a service tech out there, add all the numbo jumbo about me not taking any responsibility (wonderful world we live it...suing at the drop of a hat).
Do not change anything, and be careful, if in doubt, stop and either get back to us or call someone.

It doesn't sound serious, but I doubt it'll get better.

Al

[Art Mulder]

... with a capillary style safety, meaning that thin tube that goes from the valve into the furnace, is filled with mercury/freon/whatever and when the pilot is lit by that orange wire (spark lead), it heats up the end of that tube, which is probably sitting in the pilot flame.
...
Trick of the trade was to remove (furnace off) the pilot line from the valve and using a aspirator bulb, blowing though the pilot line, 9 out of ten times it would clean the pilot well enough to function again.
...
If your on Natural gas, can you see the pilot when it lights?
Is it blue or is there some yellow to it?

Hey Al,
- Yes it is Natural Gas
- Yes the pilot is blue (with some white), no yellow as I recall.

furnace-guts-annotated.jpg
- Here's another photo, just double checking my understanding of things
A- is the capillary saftey - yes I know it has mercury, it is labelled as such
B- is the Spark lead - red wire that ignites the pilot
C- is the pilot line that you are wondering about blowing out
D- is the off switch for the gas. I'd turn that off as well as power before trying any of this!

Just to reassure you that while I am not a licensed gas fitter, I am a pretty handy guy who has worked on electronics and hardware like this over the years. I just did't know what all the parts do, just some of them.

And now that you mention it, I remember seeing a furnace tech in previous years using a bulb to blow out things on the furnace

And how about this: "I, Art Mulder, hereby indemnify Al Willits from any responsibility. If I do anything, I accept responsibility for my own actions." Whearas and so forth and the party of the first part and habeus furnace-us...

Happy Weekend, folks!

[Al Willits]

"""""""""""
And how about this: "I, Art Mulder, hereby indemnify Al Willits from any responsibility. If I do anything, I accept responsibility for my own actions." Whearas and so forth and the party of the first part and habeus furnace-us...
""""""""

I'll copy that and keep it in a safe place...

Looks like you got it.
Not sure about the white, must be the Canadian gas...hehe

That mercury safety probably just unplugs from the valve btw.
If ya want, try blowing the pilot line out and see if that white disappears, that safety has to get fairly hot to work.

It could also be in the venting of the furnace, but what I've suggested are things I think you might be able to check, key word here is free, if these don't work I might get someone out to check the pressure switch and combustion air motor, also the vent going outside, like say by someone else, its possible a wind could cause problems.

I don't know how many times your furnace will try to relight, but usually wind will cause only intermittent problems.
When you said in your first post the pilot wouldn't light, was it the pilot or the main burner??

Al

[Art Mulder]

When you said in your first post the pilot wouldn't light, was it the pilot or the main burner??

Pilot.

The way the thing normally works, is that the thermostat will give a click, and the yellow "system" LED will light. This causes the furnace to light, and 10-30 seconds later you hear the muffled "whump" as the main burner kicks in. Then another 30-60 seconds(?) after that the fan kicks in.

(I know, you know all this... )

With my problem yesterday evening, the thermostat would give the initial click, but then immediately give another click, so what I'd hear would be "click-click". The display would blink at the same time, and then nothing would happen for about 20-30 seconds and then it would try again. (so the system light did NOT light up.)

thanks again, Al.

[Larry James]

Art, hope you have it working - if not, here is some additional information.

This looks like a White-Rogers gas valve and ignition system - often called pilot-relight when used with a thermocouple. You have a version designed for induced draft furnaces.

The basic generic sequence is ...

1- Thermostat calls for heat, circuit between R (24 hot) and W (switched 24) energizes a relay and the relay (large silver box with terminals) starts the vent blower motor. Power may also be switched to the spark ignitor at this time, but gas is not yet following. Note: the gas valve does not have a pilot position, only on and off for this system.

2-When the vent motor up to speed and the combustion air switch ( the round control mounted to the right of blower in your picture) senses that combustion air is flowing and closes a switch proving air flow before the gas valve is allowed to open. Note: this switch may have 2 or 3 terminals. Also, the system may have a time delay to allow blower to purge combustion chamber before ignition.

3- After air flow is proven, the spark will light the pilot burner and the mercury sensor will heat up. The black control on the end of the sensing tube is plugged into the gas valve, as Al mentioned. This control usually (?) has a single pole double throw switch (SPDT) inside - like the switching action of a 3-way light switch that you likely have in your house.

4-When the sensor has heated up, proving there is a pilot flame available, the main gas valve is energized by the sensor switch. The pilot should light should ignite the main burners.

5-After a delay that allows the heat exchanger to warm up the blower should circulate warm air. Note: The blower may have a time delay as well as a bimetal sensor - the blower may turn on in this system even if the burner is off.

Al noted, "Looks like they're running the W on the thermostat to the Y on the furnace...haven't a clue why, sorry. Y is usually used for A/C, wondering if they use it to energize a relay or something?" This could be related to what is called a holding circuit that holds the pilot on after the mercury switch changes position (or not). EDIT: more on yellow wire-this setup may be used to supply 24 volts to the thermostat in the off cycle.


There were many variations to this system. That's why I called it "generic." I haven't repaired furnaces for over 25 years, but I repaired many systems with the mercury sensor. The most common problem was with the sensor, or quoting Al again, "...more common, is the pilot gets a bit dirty and won't always heat the cap tube hot enough."

I also give information under the:
"Art Mulder, hereby indemnify" clause. And how about this: "I, Art Mulder, hereby indemnify Larry James from any responsibility. If I do anything, I accept responsibility for my own actions." Whearas and so forth and the party of the first part and habeus furnace-us...

Best to find a "old timer" to repair the system, cause like I used to tell customers - "I can't fix them over the phone."

Let us know what the problem was when you have it working.

Keep warm, Larry