Location: Blinding You With Library Science! Gender:
Posted:
Jun 3, 2025 - 6:32am
ScottFromWyoming wrote:
Welp, I knew it was something simple: the thermopile wasnât generating enough millivolts to open the valve but it was enough to keep the pilot lit. I had measured it at the beginning but must have knocked it away from the pilot or something phenomenally simple like that.
Gave it a shove to get it up into the flame and the millivolts went up and the valve opened wa-la
Completely unrelated to the thread topic (not the first time I've done this, won't be the last), but I see that word and my brain says "Thermopylae" (and pronounces it that way, too).
Welp, I knew it was something simple: the thermopile wasnât generating enough millivolts to open the valve but it was enough to keep the pilot lit. I had measured it at the beginning but must have knocked it away from the pilot or something phenomenally simple like that.
Gave it a shove to get it up into the flame and the millivolts went up and the valve opened wa-la
Something I tell my guys all the time - "meters are friends, they will save you time and make you look smart"
I did think of polarity, but on solenoids it doesn't matter (unless they have done something weird). I've had the argument discussion here in the shop and even set up a demo with a contactor to prove the point. Not as familiar with the thermopile, but it's basically just a set of thermocouples.
Now I'm vested, so be sure to update here as well!
Welp, I knew it was something simple: the thermopile wasnât generating enough millivolts to open the valve but it was enough to keep the pilot lit. I had measured it at the beginning but must have knocked it away from the pilot or something phenomenally simple like that.
Gave it a shove to get it up into the flame and the millivolts went up and the valve opened wa-la
I did think of polarity, but on solenoids it doesn't matter (unless they have done something weird). I've had the argument discussion here in the shop and even set up a demo with a contactor to prove the point. Not as familiar with the thermopile, but it's basically just a set of thermocouples.
Now I'm vested, so be sure to update here as well!
I'm pretty sure I've tried it already in my "four wires, three screws, limited combinations" phase. But I'll be more diligent about documentation this time.
Keep in mind that the behavior before the new thermostat is the same as after. Polarity shouldn't matter and the connection at the thermostat is pretty straightforward, although I'm going to post an image tonight so that any HVAC redditors can see that it's correct or not.
The gas itself isn't a mystery and although the valve body has several available ports, they're all for convenience: you plug the ones you don't need and use the one that points toward the thing you're sending gas to. In this case it goes straight up into the burner. And I had the old one for reference; it's set up identically.
I am a bit concerned about the overheat sensor but putting in the jumper and bypassing the sensor... there's not any secondary voltage sensor in the valve itself, only an electromagnet that slams shut when the power is interrupted. Here's where I lose any faith in my own ability to work this out but I'll play with the configuration of everything again tonight.
One redditor said that more recent thermopiles are sensitive to polarity and sometimes need to be installed a particular way. I got the impression that this was not a design feature, just a working fact, so he wants me to swap the red/white coming from the thermopile just to see if that does it.
I did think of polarity, but on solenoids it doesn't matter (unless they have done something weird). I've had the argument discussion here in the shop and even set up a demo with a contactor to prove the point. Not as familiar with the thermopile, but it's basically just a set of thermocouples.
Now I'm vested, so be sure to update here as well!
I am definitely not calling your education, bloodline or upbringing into question, Scott - you would know that, I know you are a craftsman who works with his hands and can figure stuff out; so don't take it that way.
I have zero experience or knowledge of US gas stuff, so I am working from first principles here.....
My first thought was that pipes are connected incorrectly - the thermostat thinks it has done the right thing and that there is gas flowing but for some reason there isn't - my first thought is gas lines are crossed and so the gas flow is in the incorrect direction or at the incorrect port. This might translate to signals coming from or going to the wrong place.
Perhaps a different porting layout on the new thermostat compared to the old one?
Or, a different electrical connection layout, and so signals are mixed up?
We have a 75metre deep borehole here, and a level sensing pump protection system which was giving trouble and not allowing the pump to turn on based on a spurious low level signal.
A sparky came, dropped 2 new level sensors down the borehole and into the unrecovereable deep. He then agreed with me that water level was not the real problem, and used a switch to simulate the input from the level sensor, and left.
It failed the next day.
I thought that maybe the sensor was not a simple level switch but perhaps capacitive...I played around with various capacitance values to simulate the sensor = no joy.
Then I thought...maybe resistive?
An combination of resistors totalling 80k Ohms did the job, the pump runs when required, all is good.
My point is that returning to first principles often helps.....
I think the new thermostat probably has a different connection layout or requires a different input signal to work properly when compared to the old one.....search online for documentation or check package inserts etc.....you can figure it out I am sure...possibly removing the overheat sensor makes it obnoxious and uncooperative, or it is looking for a certain type or value of sensor?
These things are sent to teach us new tricks.....
Keep in mind that the behavior before the new thermostat is the same as after. Polarity shouldn't matter and the connection at the thermostat is pretty straightforward, although I'm going to post an image tonight so that any HVAC redditors can see that it's correct or not.
The gas itself isn't a mystery and although the valve body has several available ports, they're all for convenience: you plug the ones you don't need and use the one that points toward the thing you're sending gas to. In this case it goes straight up into the burner. And I had the old one for reference; it's set up identically.
I am a bit concerned about the overheat sensor but putting in the jumper and bypassing the sensor... there's not any secondary voltage sensor in the valve itself, only an electromagnet that slams shut when the power is interrupted. Here's where I lose any faith in my own ability to work this out but I'll play with the configuration of everything again tonight.
One redditor said that more recent thermopiles are sensitive to polarity and sometimes need to be installed a particular way. I got the impression that this was not a design feature, just a working fact, so he wants me to swap the red/white coming from the thermopile just to see if that does it.
Typically if the pilot light is on, the thermocouple is okay. It's a safety feature so you don't flood the basement with propane.
Normally, the TH terminal gets a 24V signal from the thermostat (usually 24VAC), and this causes it to open. The thermopile gets a constant milivolt signal to keep the pilot on. There is also a high limit switch that should be between the thermostat and the TH/TR connection. I'd guess this limit switch is open, so when you get the call for heat, it drops all voltage and the solenoid shuts down.
Check with a meter to see if you are getting 24V all the way to the solenoid, then walk back up the chain.
Also - something look off on your wiring, the thermostat should terminate one line to TH/TP and the other to TH. That connection on top looks like a common or ground.
It's a millivolt system, thermopile etc
Right now, the high limit switch is bypassed using the jumper that was already in place in the new valve. If I hook up the limit switch, light the pilot and move the thermostat to "heat," the gas clicks off just the same as now. If the pilot is lit and I disconnect one leg of that switch, the gas clicks off too.
AFAICT, the top screw is TH/TP, the middle one is TP (obscured by the spade) and the bottom one is TH. This photo is upside-down relative to mine but...
I sometimes used to fire a gas kiln that had an optical sensor that told when the pilot was lit. If it wasn't, the gas shut down. I think that included the pilot, since it wasn't always on, unlike a furnace. So how does your system check the pilot and what does it do if it thinks it is not lit?
My death-trap gas wall furnace here won't fire if the pilot isn't lit but I don't know if it shuts down the pilot along with everything if the gas is on to the pilot but it isn't lit. I do know that getting the pilot to light and stay on is a pain in the arse. I think it uses a thermocouple.
Typically if the pilot light is on, the thermocouple is okay. It's a safety feature so you don't flood the basement with propane.
Normally, the TH terminal gets a 24V signal from the thermostat (usually 24VAC), and this causes it to open. The thermopile gets a constant milivolt signal to keep the pilot on. There is also a high limit switch that should be between the thermostat and the TH/TR connection. I'd guess this limit switch is open, so when you get the call for heat, it drops all voltage and the solenoid shuts down.
Check with a meter to see if you are getting 24V all the way to the solenoid, then walk back up the chain.
Also - something look off on your wiring, the thermostat should terminate one line to TH/TP and the other to TH. That connection on top looks like a common or ground.
TL;DR: When the thermostat calls for heat, the valve shuts off all gas, including the pilot. Help!
Thought this was a sharper photo but oh well: This is in the apartment that is on the back of our garage. Currently unoccupied so not a crisis, just a PITA.
Red wires come from the thermostat (TH) and the thermopile (TP)
White wire with spade comes from the thermopile
White wire w/o spade comes from the thermostat
The markings indicating which wire goes where (TH TP)(TP) and (TH) are confusing but: (TH TP) is the screw on top in this photo, (TH) is the screw on the bottom. (TP) is hidden by the spade connector, in the middle. I'm assuming I have that decoded correctly but am open to new ideas. Of course last month when I replaced the thermopile, I failed to photograph the thing before disconnecting it because I am not the sharpest spoon in the bucket. The internet has a vast trove of information but finding out which wire goes where has been surprisingly difficult. So, so many YouTubes that are purportedly about wiring up a replacement thermopile explain how a Thermopile works but then just say "hook up the wires and light the pilot!"
I've moved all the wires to every combination I can think of: Both whites on the THTP screw, a red on each of the others. Am I doing this completely wrong?
This is a Williams gravity wall furnace, gas of course. Last month it was burning a lot because we had subzero temps and frozen pipes. One day it wasn't burning but the pilot was lit. The internet said that sometimes the thermopile gets weak enough that it won't actually open the main valve but the pilot stays lit. So I replaced the thermopile ($35). That had zero effect. Next suspect was the overheat sensor. Since we'd been forcing the thing to run hot for a week or more, trying to unfreeze pipes, the idea that it could be tripped made a lot of sense. Some heaters' sensors have a reset switch but ours did not. So I looked up the correct part number as best I could and that replacement ($65) does have a reset button. But it doesn't have any effect either.
So the next and only remaining part is the valve. ($140) Put this in, hooked it up and naturally detected a slight gas leak so I determined that the connector going to the pilot wasn't snug so I tried to tighten that and it was that one-attempt brass piece that I think I shared earlier. So I ordered a couple of those ($15) and put one in, really snugged that SOB down and no leaks. Ha ha! Here we go! Lit the pilot, that's fine, bright blue flame. Go to the thermostat and slide it to HEAT and it clicks and the new valve audibly clicks a millisecond later, shutting off all gas, including to the pilot.
Disconnected and reconnected all the wiring everywhere, in lots of configurations. Replaced the batteries in the thermostat because the display wasn't super bright. Took the safety sensor out of the mix, put the old one back in, took it out, then finally bypassed it completely (in the photo there's a black wire running from THTP to a connection on the back of the valve; normally there are two wires running from those two posts up to the sensor which is fastened at the top of the furnace.
Out of options, I replaced the thermostat last night, knowing it wouldn't make a difference and I was right. But it's a nicer thermostat so I'm not upset about that.
I'm going to post this to Reddit but I expect a LOT of people telling me things that aren't germane to the problem, mostly having to do with my intellect and possibly upbringing.
Unencumbered by any facts...
I sometimes used to fire a gas kiln that had an optical sensor that told when the pilot was lit. If it wasn't, the gas shut down. I think that included the pilot, since it wasn't always on, unlike a furnace. So how does your system check the pilot and what does it do if it thinks it is not lit?
My death-trap gas wall furnace here won't fire if the pilot isn't lit but I don't know if it shuts down the pilot along with everything if the gas is on to the pilot but it isn't lit. I do know that getting the pilot to light and stay on is a pain in the arse. I think it uses a thermocouple.
TL;DR: When the thermostat calls for heat, the valve shuts off all gas, including the pilot. Help!
Thought this was a sharper photo but oh well: This is in the apartment that is on the back of our garage. Currently unoccupied so not a crisis, just a PITA.
Red wires come from the thermostat (TH) and the thermopile (TP)
White wire with spade comes from the thermopile
White wire w/o spade comes from the thermostat
The markings indicating which wire goes where (TH TP)(TP) and (TH) are confusing but: (TH TP) is the screw on top in this photo, (TH) is the screw on the bottom. (TP) is hidden by the spade connector, in the middle. I'm assuming I have that decoded correctly but am open to new ideas. Of course last month when I replaced the thermopile, I failed to photograph the thing before disconnecting it because I am not the sharpest spoon in the bucket. The internet has a vast trove of information but finding out which wire goes where has been surprisingly difficult. So, so many YouTubes that are purportedly about wiring up a replacement thermopile explain how a Thermopile works but then just say "hook up the wires and light the pilot!"
I've moved all the wires to every combination I can think of: Both whites on the THTP screw, a red on each of the others. Am I doing this completely wrong?
This is a Williams gravity wall furnace, gas of course. Last month it was burning a lot because we had subzero temps and frozen pipes. One day it wasn't burning but the pilot was lit. The internet said that sometimes the thermopile gets weak enough that it won't actually open the main valve but the pilot stays lit. So I replaced the thermopile ($35). That had zero effect. Next suspect was the overheat sensor. Since we'd been forcing the thing to run hot for a week or more, trying to unfreeze pipes, the idea that it could be tripped made a lot of sense. Some heaters' sensors have a reset switch but ours did not. So I looked up the correct part number as best I could and that replacement ($65) does have a reset button. But it doesn't have any effect either.
So the next and only remaining part is the valve. ($140) Put this in, hooked it up and naturally detected a slight gas leak so I determined that the connector going to the pilot wasn't snug so I tried to tighten that and it was that one-attempt brass piece that I think I shared earlier. So I ordered a couple of those ($15) and put one in, really snugged that SOB down and no leaks. Ha ha! Here we go! Lit the pilot, that's fine, bright blue flame. Go to the thermostat and slide it to HEAT and it clicks and the new valve audibly clicks a millisecond later, shutting off all gas, including to the pilot.
Disconnected and reconnected all the wiring everywhere, in lots of configurations. Replaced the batteries in the thermostat because the display wasn't super bright. Took the safety sensor out of the mix, put the old one back in, took it out, then finally bypassed it completely (in the photo there's a black wire running from THTP to a connection on the back of the valve; normally there are two wires running from those two posts up to the sensor which is fastened at the top of the furnace.
Out of options, I replaced the thermostat last night, knowing it wouldn't make a difference and I was right. But it's a nicer thermostat so I'm not upset about that.
I'm going to post this to Reddit but I expect a LOT of people telling me things that aren't germane to the problem, mostly having to do with my intellect and possibly upbringing.
I am definitely not calling your education, bloodline or upbringing into question, Scott - you would know that, I know you are a craftsman who works with his hands and can figure stuff out; so don't take it that way.
I have zero experience or knowledge of US gas stuff, so I am working from first principles here.....
My first thought was that pipes are connected incorrectly - the thermostat thinks it has done the right thing and that there is gas flowing but for some reason there isn't - my first thought is gas lines are crossed and so the gas flow is in the incorrect direction or at the incorrect port. This might translate to signals coming from or going to the wrong place.
Perhaps a different porting layout on the new thermostat compared to the old one?
Or, a different electrical connection layout, and so signals are mixed up?
We have a 75metre deep borehole here, and a level sensing pump protection system which was giving trouble and not allowing the pump to turn on based on a spurious low level signal.
A sparky came, dropped 2 new level sensors down the borehole and into the unrecovereable deep. He then agreed with me that water level was not the real problem, and used a switch to simulate the input from the level sensor, and left.
It failed the next day.
I thought that maybe the sensor was not a simple level switch but perhaps capacitive...I played around with various capacitance values to simulate the sensor = no joy.
Then I thought...maybe resistive?
An combination of resistors totalling 80k Ohms did the job, the pump runs when required, all is good.
My point is that returning to first principles often helps.....
I think the new thermostat probably has a different connection layout or requires a different input signal to work properly when compared to the old one.....search online for documentation or check package inserts etc.....you can figure it out I am sure...possibly removing the overheat sensor makes it obnoxious and uncooperative, or it is looking for a certain type or value of sensor?
These things are sent to teach us new tricks.....
TL;DR: When the thermostat calls for heat, the valve shuts off all gas, including the pilot. Help!
Thought this was a sharper photo but oh well: This is in the apartment that is on the back of our garage. Currently unoccupied so not a crisis, just a PITA.
Red wires come from the thermostat (TH) and the thermopile (TP)
White wire with spade comes from the thermopile
White wire w/o spade comes from the thermostat
The markings indicating which wire goes where (TH TP)(TP) and (TH) are confusing but: (TH TP) is the screw on top in this photo, (TH) is the screw on the bottom. (TP) is hidden by the spade connector, in the middle. I'm assuming I have that decoded correctly but am open to new ideas. Of course last month when I replaced the thermopile, I failed to photograph the thing before disconnecting it because I am not the sharpest spoon in the bucket.
I've moved all the wires to every combination I can think of: Both whites on the THTP screw, a red on each of the others. Am I doing this completely wrong?
This is a Williams gravity wall furnace, gas of course. Last month it was burning a lot because we had subzero temps and frozen pipes. One day it wasn't burning but the pilot was lit. The internet said that sometimes the thermopile gets weak enough that it won't actually open the main valve but the pilot stays lit. So I replaced the thermopile ($35). That had zero effect. Next suspect was the overheat sensor. Since we'd been forcing the thing to run hot for a week or more, trying to unfreeze pipes, the idea that it could be tripped made a lot of sense. Some heaters' sensors have a reset switch but ours did not. So I looked up the correct part number as best I could and that replacement ($65) does have a reset button. But it doesn't have any effect either.
So the next and only remaining part is the valve. ($140) Put this in, hooked it up and naturally detected a slight gas leak so I determined that the connector going to the pilot wasn't snug so I tried to tighten that and it was that one-attempt brass piece that I think I shared earlier. So I ordered a couple of those ($15) and put one in, really snugged that SOB down and no leaks. Ha ha! Here we go! Lit the pilot, that's fine, bright blue flame. Go to the thermostat and slide it to HEAT and it clicks and the new valve audibly clicks a millisecond later, shutting off all gas, including to the pilot.
Disconnected and reconnected all the wiring everywhere, in lots of configurations. Replaced the batteries in the thermostat because the display wasn't super bright. Took the safety sensor out of the mix, put the old one back in, took it out, then finally bypassed it completely (in the photo there's a black wire running from THTP to a connection on the back of the valve; normally there are two wires running from those two posts up to the sensor which is fastened at the top of the furnace.
Out of options, I replaced the thermostat last night, knowing it wouldn't make a difference and I was right. But it's a nicer thermostat so I'm not upset about that.
I'm going to post this to Reddit but I expect a LOT of people telling me things that aren't germane to the problem, mostly having to do with my intellect and possibly upbringing.
TL;DR: When the thermostat calls for heat, the valve shuts off all gas, including the pilot. Help!
Thought this was a sharper photo but oh well: This is in the apartment that is on the back of our garage. Currently unoccupied so not a crisis, just a PITA.
Red wires come from the thermostat (TH) and the thermopile (TP)
White wire with spade comes from the thermopile
White wire w/o spade comes from the thermostat
The markings indicating which wire goes where (TH TP)(TP) and (TH) are confusing but: (TH TP) is the screw on top in this photo, (TH) is the screw on the bottom. (TP) is hidden by the spade connector, in the middle. I'm assuming I have that decoded correctly but am open to new ideas. Of course last month when I replaced the thermopile, I failed to photograph the thing before disconnecting it because I am not the sharpest spoon in the bucket. The internet has a vast trove of information but finding out which wire goes where has been surprisingly difficult. So, so many YouTubes that are purportedly about wiring up a replacement thermopile explain how a Thermopile works but then just say "hook up the wires and light the pilot!"
I've moved all the wires to every combination I can think of: Both whites on the THTP screw, a red on each of the others. Am I doing this completely wrong?
This is a Williams gravity wall furnace, gas of course. Last month it was burning a lot because we had subzero temps and frozen pipes. One day it wasn't burning but the pilot was lit. The internet said that sometimes the thermopile gets weak enough that it won't actually open the main valve but the pilot stays lit. So I replaced the thermopile ($35). That had zero effect. Next suspect was the overheat sensor. Since we'd been forcing the thing to run hot for a week or more, trying to unfreeze pipes, the idea that it could be tripped made a lot of sense. Some heaters' sensors have a reset switch but ours did not. So I looked up the correct part number as best I could and that replacement ($65) does have a reset button. But it doesn't have any effect either.
So the next and only remaining part is the valve. ($140) Put this in, hooked it up and naturally detected a slight gas leak so I determined that the connector going to the pilot wasn't snug so I tried to tighten that and it was that one-attempt brass piece that I think I shared earlier. So I ordered a couple of those ($15) and put one in, really snugged that SOB down and no leaks. Ha ha! Here we go! Lit the pilot, that's fine, bright blue flame. Go to the thermostat and slide it to HEAT and it clicks and the new valve audibly clicks a millisecond later, shutting off all gas, including to the pilot.
Disconnected and reconnected all the wiring everywhere, in lots of configurations. Replaced the batteries in the thermostat because the display wasn't super bright. Took the safety sensor out of the mix, put the old one back in, took it out, then finally bypassed it completely (in the photo there's a black wire running from THTP to a connection on the back of the valve; normally there are two wires running from those two posts up to the sensor which is fastened at the top of the furnace.
Out of options, I replaced the thermostat last night, knowing it wouldn't make a difference and I was right. But it's a nicer thermostat so I'm not upset about that.
I'm going to post this to Reddit but I expect a LOT of people telling me things that aren't germane to the problem, mostly having to do with my intellect and possibly upbringing.
I did something similar when I was organising a concert for Toy Love (of Chris Knox fame if anyone is familiar with Kiwi music) at our school. I had a tin of glue in my bag that I used to put up the posters which then leaked onto my lunch sandwiches .. took ages to get rid of the taste.
