Furnace thermocouple problem

[California]

Problem solved!

I'm documenting this here in case it might help someone else in the future.

This 1999 heater has a safety device I have never seen before, and since no one else described it I assume this is a new feature.

A sensor named 'Vent Safety Spill Switch' is located on the back of the heater below the chimney connection. Per the installer manual: "The switch, when activated, will extinguish the pilot flame."

I still don't understand if it senses heat or some combustion gasses, but according to the manual it activates if the chimney is blocked, too short, or isn't type B vent pipe. Or if the vent has too many bends or is too close to horizontal.

The safety switch has blade terminals and I had already crimped the wiring harness's connectors tighter to assure a good connection. Finally standing on my head between the heater and the wall I found another set of blade connectors halfway between the safety switch and the gas valve in the heater. They were loose, I crimped the connectors to make them slide snugly on to the blades, and problem solved.

To verify that thermocouples weren't the problem I reinstalled the one I took out last week, and then the one that was in it prior to that. All three thermocouples worked properly, about 45 seconds of holding the override was all it took to light the pilot. I don't think before this repair we ever got it lit in under five minutes, since it was new in 1999.

Thanks to everyone for your comments! They inspired me to keep analysing this mystery. I was ready to replace the heater's main gas valve assembly which as it turned out, wouldn't have fixed anything.

And Junkman, thanks for your concern. That inspired me to check for recalls. So far as I can find there isn't one for this heater.

 

[Egon]

Good to hear the " Thermocouple Mystery " is solved.

Never heard of the safety device you mentioned. Is your heater one of the high efficiency modelds that vent through the wall with a plastic pipe?

Egon

 

[California]

Ergon,

I doubt this heater is a premium high efficiency unit. It specifies ordinary Type B vent pipe. (double wall).

This link has photos showing the front and back of the heater. The 'Vent Safety Spill Switch' is below the vent on the back. It is mounted on a spacer to hold it a half inch away from the cabinet.

I had assumed that was an overheating shutdown safety sensor. After reading their description, I think now it may sense if combustion gasses run backward and spill out of the chamber's intake.

The sensor's wires connect to the gas valve in an unusual way. There is a wide saw-cut straight down through the fitting where the thermocouple screws in. The sensor wires enter this slot and terminate at a (replaceable) button that lies between the tip of the thermocouple and the bottom of its socket.

'Wiring Harness w/ Bluebird' is the description in the parts list for this button and its wires.

I've never seen anything like it!

 

[Egon]

Thanks for the information.

Resettable Vent Spill Safety Device automatically shuts unit off in the event of flue blockage or incorrect vent installation.

Never seen one but now know about it. Glad you were able to figure it out and let us know.

Egon /forums/images/graemlins/grin.gif

 

[Oilman74]

Jump out the two wires that lead up to the flu before the chimney that’s in over fill switch or when the draft is blocked. Or jump it out with a jumper wire where it goes into the gas valve . it’s extra safety but you really don’t need it .

 

[Hay Dude]

A new heater is much more expensive than a few thermocouples. I use to be an HVAC servie tech/installer and one night I went to a house and found a bad thermocouple, when I went to my service truck I found that the other service man had got the only therm thermocouple long enough for this job off of my truck that morning. Rather than drive all the way back to the shop I took sandpaper and cleaned the snot out of the flame snesor and stuck it back in. Miraculously it worked but I knew it would only be a temp. fix, I showed the homeowner what I had done and since it was a mild night I told him I would bring a new one by the next day. The next day on the way home from work I gae it to him and he said he would put it on, I saw him a week later and he said he forgot to do it that night and it was still working. I asume you hae checked that the sensor is in the center of the flame and not the end.

That’s all I do and it works great.

 

[MossRoad]

17 year old zombie thread revival.

Welcome to TBN.

 

[California]

Jump out the two wires that lead up to the flu before the chimney that’s in over fill switch or when the draft is blocked. Or jump it out with a jumper wire where it goes into the gas valve . it’s extra safety but you really don’t need it .

I just noticed this prehistoric thread has recent posts.

Update. Things I've learned since 2004. Maybe this will help someone else encountering this problem:

* After that 'Resettable Vent Spill Safety Device' on the back of the heater tripped a few times, it stayed tripped for hours, then permanent, showing infinite resistance from one terminal to the other. A replacement wasn't available at the local HVAC supply house so I found it online. (I found they are rated in degrees. So it senses temperature, not gasses).

* I used a common automotive blade fuse as a jumper in place of the safety device, until I received the replacement.

* I finally called a HVAC pro to troubleshoot why the heater was tripping its safety device. He said the chimney (Type B vent pipe) didn't extend far enough above the eaves to draw properly. He extended the height with an offset to get above the roof line. Problem solved. It ran for a decade without problems. Then it quit, but cleaning all the blade connectors between the safety device and the gas regulator restored normal function. The heater has now been in use 20+ years and continues to work fine.

 

[MossRoad]

Thanks for the update.

Just to make you feel better, you were 17+ years younger when you posted that originally.

 

[kenmac]

I just noticed this prehistoric thread has recent posts.

Update. Things I've learned since 2004. Maybe this will help someone else encountering this problem:

* After that 'Resettable Vent Spill Safety Device' on the back of the heater tripped a few times, it stayed tripped for hours, then permanent, showing infinite resistance from one terminal to the other. A replacement wasn't available at the local HVAC supply house so I found it online.

* I used a common automotive blade fuse as a jumper in place of the safety device, until I received the replacement.

* I finally called a HVAC pro to troubleshoot why the heater was tripping its safety device. He said the chimney (Type B vent pipe) didn't extend far enough above the eaves to draw properly. He extended the height with another section of Type B. Problem solved. It ran for a decade without problems. Then it quit, but cleaning all the blade connectors between the safety device and the gas regulator restored normal function. The heater has now been in use 20+ years and continues to work fine.

must be on a natural draft furnace, and not a vent motor assist type. Does the B vent run up through the interior of the house and exit the roof, or does it go through an outside wall, and run up the outside wall of the home ?

Reason I ask about how the B vent is run is, when it's cold out, sometimes it takes a little bit to get the B vent warm enough to establish a draft. Or, if you have more lateral run vs vertical height, you can have issues.

What's funny is, I have seen vents terminate just on an outside wall, and thought not code, vent just fine