Flame Sensor

[Bain]

A strange thing happened while I was checking out a house today. A gas furnace, easily accessible in an attic space above a garage, shut down while I was checking out the electrical panel. I hopped up the stairs, removed the access panels to do my thing, and put the panels back on.

It was cold, so the furnace fired almost immediately after the panels were back in place. But the thing wasn't operating correctly. The burners fired for about two seconds, then shut down, fired again for another two seconds, and then shut down. This happened time after time after time.

The flame sensor looked okay, but I chipped off a small piece of soot with a fingernail. Then I scraped the sensor with a utility knife. The next time the furnace fired, it worked perfectly, and continued to do so during the remaining couple of hours I was in the house, so there's no sad ending to the story. What I don't understand, though, is what I could have done to make the furnace act up in the first place? There's Murphy's Law, and all that, but I don't believe in coincidences. Any clue?

[Chad Fabry]

Things work until they don't. It's a subtle difference from 3 millivolts to 2 millivolts, but the control panel noticed it.

Or, the furnace short cycles until the flame rod stores enough heat to get past the proving threshold- maybe it needs to short cycle twenty-three times to get there.

Maybe all the pipes in the house are frozen now and you admitted on the internet that you messed with it.

[kurt]

Yes, control module issue. Every time I've had that sort of thing occur, it always ends up being some issue with the control module.

[Bain]

Maybe all the pipes in the house are frozen now and you admitted on the internet that you messed with it.

I looked at the house--which was occupied by an elderly couple I'm betting don't get out much--this morning. I admit to wondering a few times throughout the afternoon if I was going to THE CALL.

"You inspected my house today, and now the furnace is broken."

[Garet]

Yes, control module issue. Every time I've had that sort of thing occur, it always ends up being some issue with the control module.

Not always.

This happened on mine about 5 years ago. The HVAC tech I called said to lightly rub the flame sensor with steel wool to remove whatever crud on the furnace was interfering with the millivolts. It saved me a service call and it's worked fine ever since.

[kurt]

Yeah, I misspoke. Not actually a control module issue, but the control module noting a discrepancy.

[Jim Katen]

Things work until they don't. It's a subtle difference from 3 millivolts to 2 millivolts, but the control panel noticed it.

Or, the furnace short cycles until the flame rod stores enough heat to get past the proving threshold- maybe it needs to short cycle twenty-three times to get there.

Maybe all the pipes in the house are frozen now and you admitted on the internet that you messed with it.

I think Chad nailed it. The dirty flame sensor is a classic problem. If you had let it be, it probably would have lit & stayed on.

BTW, now that it's been scratched by a knife, the flame sensor should really be replaced.

[Marc]

I agree with Chad & Jim. You just happen to be there when the prover was borderline. It may have been borderline since the previous heat season.

Marc

[JohnC]

Here is a YouTube video that may help. It shows how to clean a furnace sensor.

John Callan

[Jim Katen]

Here is a YouTube video that may help. It shows how to clean a furnace sensor.

If a flame sensor needs cleaning, there's a bigger problem that should be addressed. They shouldn't be getting dirty in the first place.

And if you're going to clean a flame sensor, you should use very fine emery cloth, steel wool, or a white scotch brite pad. Alternatively, you could use the rough skin from your manly, calloused hands. If you use anything coarse, the sensor will eventually fail from the damage.

If a spark plug is fouled, you don't just clean it and stick it back in.

- Jim Katen, Oregon

[Marc]

If a spark plug is fouled, you don't just clean it and stick it back in.

- Jim Katen, Oregon

Why not? I've done it hundreds of times successfully. Cars, lawn mowers, string trimmers, portable power plant, etc.

Marc

[Bain]

Things work until they don't. It's a subtle difference from 3 millivolts to 2 millivolts, but the control panel noticed it.

Or, the furnace short cycles until the flame rod stores enough heat to get past the proving threshold- maybe it needs to short cycle twenty-three times to get there.

Maybe all the pipes in the house are frozen now and you admitted on the internet that you messed with it.

I think Chad nailed it. The dirty flame sensor is a classic problem. If you had let it be, it probably would have lit & stayed on.

BTW, now that it's been scratched by a knife, the flame sensor should really be replaced.

Hah, the sensor wasn't scratched, it was cleaned with surgical precision by someone wielding a sharp blade.

Also, the furnace was 17 years old. A dirty flame sensor isn't necessarily indicative of a larger problem. The thing was simply fouled after 17 years of use.

[Bain]

If a spark plug is fouled, you don't just clean it and stick it back in.

- Jim Katen, Oregon

Why not? I've done it hundreds of times successfully. Cars, lawn mowers, string trimmers, portable power plant, etc.

Marc

In the name of the Holy Mother, I actually agree with Marc on something. Who would have thought that could happen?

[Marc]

First Les, now John B. Maybe I need to see a counselor. The eye is already twitching. []

Marc

[Chad Fabry]

First, I meant microamps, not milliamps in my first post, unless the furnace is really old and has a milliamp gas valve then the flame rod works differently than with a newer furnace controlled with a module.

In a current(pun intended) furnace, an AC voltage is sent through the flame rod and it uses the flame itself as a conductor to get to ground. The flame has a huge surface area so the current flows easily from the rod to the ground it finds in furnace chassis- it's AC current so it wants to go back to the flame rod but the flame rod surface area is so small that there isn't much of a path. In essence, the flame rod acts as a rectifier and converts the AC to DC. It's the DC current that module detects.

By scraping the flame rod, you change its surface area and it's not too hard to change it enough to reduce the DC output by increasing the AC return path. We're talking millionths of an amp and one millionth of an amp can make the difference between fire and ice. (apologies to Robert Frost)

Cleaning spark plugs is a very temporary repair, especially if they're abrasive blasted.

[Marc]

First, I meant microamps, not milliamps in my first post, unless the furnace is really old and has a milliamp gas valve then the flame rod works differently than with a newer furnace controlled with a module.

In a current(pun intended) furnace, an AC voltage is sent through the flame rod and it uses the flame itself as a conductor to get to ground. The flame has a huge surface area so the current flows easily from the rod to the ground it finds in furnace chassis- it's AC current so it wants to go back to the flame rod but the flame rod surface area is so small that there isn't much of a path. In essence, the flame rod acts as a rectifier and converts the AC to DC. It's the DC current that module detects.

By scraping the flame rod, you change its surface area and it's not too hard to change it enough to reduce the DC output by increasing the AC return path. We're talking millionths of an amp and one millionth of an amp can make the difference between fire and ice. (apologies to Robert Frost)

Cleaning spark plugs is a very temporary repair, especially if they're abrasive blasted.

Backing Chad on that, except that it isn't surface area. It's temperature. Without the prover at high, glowing hot temperatures, electrons can't leave the prover and migrate to nearby grounded surfaces. It the same principal as vaccuum tubes. Electrons can't flow back to the prover from the chassis because the chassis is too cool to emit any electrons and the applied voltage is too low for them to jump (spark) the distance. What results is a pulsed and varying DC current.

Marc

[kurt]

Cleaning spark plugs is a very temporary repair, especially if they're abrasive blasted.

How about burning it with a portable torch?

I've found taking my lawnmower plug out and burning it 'til it's white works wonderfully.

[:-paperba

[hausdok]

Hi,

When I was learning about cars they would still clean (file) plugs and points and we'd have to realign them with a little tool.

I was taught that using an abrasive cleaner, sandpaper or a file would raise tiny microscopic ridges all over the surface; and, as Chad says, would be a temporary fix. My teachers preferred to use a sharp blade to scrape the surfaces clean in one direction only. It worked; not as long as a new part would but it worked longer than sandpaper or a file would.

This thread is of particular interest to me 'cuz it looks like I'll be paying for a repair on something for the first time in nearly 15 years.

I forgot to turn a furnace back on a few weeks ago and suddenly remembered it at about 9:00 pm that night. I didn't want the folks to be without heat, so I tried to call the selling agent and tell her to tell the homeowner where the switch was and how to turn it on. I got a voice mail but no agent, so I left a message; then I emailed the same message to her. I got a 5-word response back from her iPhone in minutes - "I'll take care of it."

The next evening I got a call from the listing agent; he was at the house trying to make the furnace come on - apparently they'd gone all night and all day without heat thanks to the fact that the selling agent didn't "take care of it," as she'd promised.

While I was on the phone with him, the agent said that the furnace had suddenly come on while he was jiggling some wires connected to the gas valve. I figured that I must have knocked one loose somehow. He thanked me and I asked him to extend my apologies to the homeowner and hung up.

A couple of minutes later the listing agent called me back and said that the furnace had just shut down and wouldn't restart. Thinking it was the flame sensor, I told him not to touch anything and I'd be right there. I got over there and ended up dinking around with it for about an hour before I gave up. It just would not come on - not even for a second - it was like it wasn't listening to the thermostat or something.

I put new batteries in the thermostat, checked all connections, checked the flame sensor - even replaced the fuse and nadda. Well, not nadda; I did get a faint flash at the LED on the control board for about 1 second after I plugged in the new fuse; but then the LED turned off again and the furnace went back into hibernation and refused to listen.

Knew I hadn't done anything to damage it and was pretty sure that the agent had probably done something to queer the control board, but I really didn't have many options there - it had worked before I'd turned it off and opened it up so there was no way anyone was going to believe me - I told him to send me the HVAC tech's bill.

Called the agent last night to find out what the HVAC guy had come up with. Haven't heard back yet. Any ideas?

ONE TEAM - ONE FIGHT!!!

Mike

[Bain]

As for cleaning the flame sensor, I relayed what happened yesterday to a dear friend who has a masters HVAC license and a very successful business. He told me his men clean sensors on a regular basis, with pretty much a 0% call-back frequency.

You know I love all my big-brained friends here, but in real time, sandpaper apparently does the job just fine.

[Marc]

Hi,

When I was learning about cars they would still clean (file) plugs and points and we'd have to realign them with a little tool.

I was taught that using an abrasive cleaner, sandpaper or a file would raise tiny microscopic ridges all over the surface; and, as Chad says, would be a temporary fix. My teachers preferred to use a sharp blade to scrape the surfaces clean in one direction only. It worked; not as long as a new part would but it worked longer than sandpaper or a file would.

This thread is of particular interest to me 'cuz it looks like I'll be paying for a repair on something for the first time in nearly 15 years.

I forgot to turn a furnace back on a few weeks ago and suddenly remembered it at about 9:00 pm that night. I didn't want the folks to be without heat, so I tried to call the selling agent and tell her to tell the homeowner where the switch was and how to turn it on. I got a voice mail but no agent, so I left a message; then I emailed the same message to her. I got a 5-word response back from her iPhone in minutes - "I'll take care of it."

The next evening I got a call from the listing agent; he was at the house trying to make the furnace come on - apparently they'd gone all night and all day without heat thanks to the fact that the selling agent didn't "take care of it," as she'd promised.

While I was on the phone with him, the agent said that the furnace had suddenly come on while he was jiggling some wires connected to the gas valve. I figured that I must have knocked one loose somehow. He thanked me and I asked him to extend my apologies to the homeowner and hung up.

A couple of minutes later the listing agent called me back and said that the furnace had just shut down and wouldn't restart. Thinking it was the flame sensor, I told him not to touch anything and I'd be right there. I got over there and ended up dinking around with it for about an hour before I gave up. It just would not come on - not even for a second - it was like it wasn't listening to the thermostat or something.

I put new batteries in the thermostat, checked all connections, checked the flame sensor - even replaced the fuse and nadda. Well, not nadda; I did get a faint flash at the LED on the control board for about 1 second after I plugged in the new fuse; but then the LED turned off again and the furnace went back into hibernation and refused to listen.

Knew I hadn't done anything to damage it and was pretty sure that the agent had probably done something to queer the control board, but I really didn't have many options there - it had worked before I'd turned it off and opened it up so there was no way anyone was going to believe me - I told him to send me the HVAC tech's bill.

Called the agent last night to find out what the HVAC guy had come up with. Haven't heard back yet. Any ideas?

ONE TEAM - ONE FIGHT!!!

Mike

From your description, I'd say it's a control issue, which could be any of many different things. If by 'switch' you meant the 'heat-off-cool' switch, then that's where I'd begin. I'd do it by jumping the white wire to the red wire which bypasses the thermo and calls for heat. I'd then see what happens.

Marc

[plummen]

As for cleaning the flame sensor, I relayed what happened yesterday to a dear friend who has a masters HVAC license and a very successful business. He told me his men clean sensors on a regular basis, with pretty much a 0% call-back frequency.

You know I love all my big-brained friends here, but in real time, sandpaper apparently does the job just fine.

What he said! [:-party]