I have a 24.500 BTU (output) wall heater made by the Williams Furnace Company of La Mirada, CA, that was installed in the early 1980s. It is a very basic unit with no fan or filters and it has never missed a beat until recently when it began shutting off the burner about 30 seconds after the thermostat lights it. The burner then stays off for about another 30 seconds, then relights on its own (the pilot stays lit) and stays lit for another 30-sec. before shutting off again. It will just continue this on-off cycle, lighting approximately every minute, then going out before lighting again.
Starting with the easy things, I disconnected the thermostat and jumped the wires to see if the thermostat was the problem. No difference--same on-off result. Then, even though the pilot remained lit through all the cycling, I tried a new thermocouple. Nothing changed.
That left the gas control valve, which was an ITT unit for which no replacement was available. As a substitute I installed a Robertshaw #710-502 natural gas millivolt valve that fit perfectly in place of the old valve, suspended below the body of the burner. The result was exactly the same with the burner lighting, then shutting off on the same one-minute cycle. The only change was that the pilot no longer remained lit when the burner shut off so the burner was not able to relight on its own. I tried a new thermocouple (the third one) to at least see if I could get back to where I was before with the pilot staying lit, but again no change--just 30-sec. of the burner, then it and the pilot go off.
I'm at a loss. I can't figure what would be causing this problem, particularly with its fairly regular on-off timing. Gas pressure doesn't appear to be the problem as the pilot flame appears normal and the burner lights with its regular whoosh and generates good heat for the 30-seconds that it stays lit. Both the burner and the pilot have good, strong blue flames while they're lit. Can anyone offer any help?
Those units typically don't use a thermocouple. They use a pilot generator also called a thermopile.
A thermocouple connects with a nut and a pilot generator has a braided jacket with two wires.
Some units use both.
Do you have a voltmeter ? You'll need one for testing.
Measure TH/TP to TP. Should be approx 450-600 millivolts.
Call for heat and measure again. Could be around 250-300 millivolts.
Now measure from TH/TP to TH. Should be near the previous measurement (250-300)
Let us know what you find. Higher voltages from what I posted are better.
Unfortunately, the only part of the information plate that is worn is where the model number is. What I can still make out appears to be 5GV-5, and I think there may have been another character at the beginning, before the first 5.
Yes, it does have a pilot generator rather than a thermocouple. The Robertshaw gas valve didn't specify any polarity for attaching the generator, and I've reversed the leads to see if that would make any difference. It didn't.
On the slim chance that I got a bad new control valve, I'm going to reinstall the original ITT unit tomorrow to see if there is any difference. The one thing the ITT valve did differently than the Robertshaw is to leave the pilot lit when it shut off the burner after its usual 30-sec. burn. Even that would be an improvement because at least the burner keeps relighting and making heat half the time, after each 30-second dead period. I'm baffled.
Pete, I do have a multimeter, a cheapie from Harbor Freight, but it should do what I need. I assume the readings are to be taken with the pilot generator and thermostat connected to the control valve terminals. When you say "call for heat," do you mean for the readings to first be taken with the pilot light off and then with it lit? Or do you mean to first take the reading with just the pilot lit and then with the burner lit? Please clarify for me. Thanks.
Last edited by don04; 03-17-21 at 09:57 PM.
Reason: Adding information
This keeps getting weirder. With a new pilot generator from the local hot tub store the meter read 592-millivolts. When the burner is lit, and by the time I can get the probes on the terminals, the generator is putting out about 150mv and dropping. When it gets to about 60mv, the burner shuts off.
I tried another pilot generator, one I got from a retired PG&E gas service guy. used but good when he saved it. It read 496mv with just the pilot, but did the same thing when the burner was lit, dropped steadily to about 60mv and shut off the burner. That makes the forth pilot generator (two of them new) that has done the same thing, exactly what the original 40 year old pilot generator had begun doing.
I know it's possible I got a bad control valve right out of the box, but the fact that the new one is doing exactly the same thing as the old one makes me less inclined to suspect it of being bad. I have a couple more pilot generators to try, but with four in a row doing the same thing, I don't expect any change when I try the others. Anyone have any ideas?
Just tried another pilot generator, the fifth and another new one--same result. As soon as the burner lights, the voltage begins to drop, shutting off the gas to the burner at 62-millivolts. This seems like such a simple system, a gas valve and a millivolt generator. What could possibly be causing the voltage in the pilot generators to drop?
Does the pilot remain good & strong when the main burner lights? If the pilot shrinks, it may not be enough to heat the thermopile & generate the voltage needed to keep the gas valve open.
I've made a little progress. In order to see if the pilot flame changed when the burner lit, I left the pilot/generator assembly out in open air instead of its normal place behind the metal shielding near the burner, then I lit the burner manually. The pilot flame stayed the same, strong and blue. But what did change was the pilot generator voltage only dropped to its normal level (burner on) of about 212-millivolts and stayed there. The voltage did not rapidly drop when the burner lit, as it did when the pilot assembly was in its usual position near the burner. That voltage drop would quickly cause the control valve to turn off the gas to the burner.
So, maybe it's an electrical short that causes the voltage from the pilot generator to decrease, perhaps related to the pilot assembly being in contact with the metal burner or the sheet metal surrounding the burner (though it been that way for dozens of years). When the pilot and generator are not touching any of the metal structure of the heater, the voltage from the pilot generator stays as it should and the heater continues to operate.
It's only when the pilot assembly is installed in its usual position between the body of the burner and the sheet metal burner box that the rapid voltage drop happens when the burner is lit. What's hard to figure is why the apparent short only happens when the burner is lit and not also when the pilot flame is simply heating the generator.
The pilot/generator assembly is located in the same place it has always been. If the problem is an electrical short, the question is why has it begun happening and how do I fix it. I sure could use some ideas. If I haven't explained the problem well, please ask questions and I'll try to do a better job describing what's going on. Thanks.
This is a new one on me. The only thing I can think of is maybe an air current coming around that shield is deflecting the pilot flame. Is there any way to get a small mirror in there to watch the pilot when it is installed in it's normal position?
Problem solved. The key was finding that keeping the pilot generator away from the burner flame stopped the rapid drop in voltage when the burner lit, which would then shut off the gas to the burner when the voltage dropped low enough. Surprisingly, it took several tries to find a place where the generator was not too near the burner flame, but close enough that the pilot flame could light the burner when the thermostat was switched on. It turned out that now the positioning of the pilot/generator assembly has become kind of sensitive, where it used to not matter at all. But once I found the right spot for the pilot/generator, the system works normally again.
What makes this so strange is that for the nearly forty years of the wall heater's life it hadn't cared where the pilot and generator were placed. Generally they were just pushed up behind the metal shielding by the burner, completely out of sight and completely in the burner's flame, held in position by the rigidity of the aluminum tubing that carries the gas to the pilot. But apparently, for some still unknown reason, the generator began overheating by being in the burner flame and that's what caused the gas to the burner to be shut off. Why it decided to wait 40 years to do that is the big question. It's not because the pilot generator was deteriorating, because five different generators were tried without any change. It's almost like the heater simply decided it wanted some attention. And it got some; about a week of changing parts and trying to solve the problem.
Now, the place where the pilot/generator assembly sits in order to make everything work normally is completely different. The generator is totally outside the chamber and the pilot is barely into it, only the very tip of the pilot with its upward shooting flame. And there isn't much flexibility in the positioning in order to keep the generator from overheating and still light the burner. Fractions of an inch now make the difference between the system working or not working.
But it works again, that's the important part, so I'm happy. I guess I've learned something, but it's probably information I'll never need again. Hopefully, though, it might help someone else. Thanks to everyone who provided some input.
I need to replace a Gar-Ber R Spin-On Fuel Oil Filter. The filter says to use "clean lubricating oil" on the gasket. What can I realistically use? I've seen HVAC guys use home heating oil as lubrication. Is this a big no-no?
I would like to blow-out the fuel line when changing the filter. Can I use an air compressor to achieve this? I've seen the CO2 cartridges but don't know where to purchase them.
Is it okay to blowout the line with the fuel filter installed? Does it in anyway damage the filter?
Hello -
I have a Trane XR90 system that is having a problem that I can't seem to diagnose. Here is the system...
In a so far unsuccessful attempt to fix this problem, I have replaced the control board (used KIT18110, which is an upgrade over what I had previously, and this kit included a new ignitor, which I also installed), and I also replaced the flame sensor. But the problem persists.
Here is a description of the problem: The system will be sitting there idle with a slow flashing LED. Then when the thermostat calls for heat, the LED flashes faster, the igniter lights up, the burners light, and the draft inducer starts blowing. All good so far. Then about 30-60 seconds later, the main blower kicks on, and as soon as that happens, the burners go out, and the LED starts flashing four times, then repeats. That code indicates an "open limit switch."
My understanding is that means some internal safety thermostat is over temperature, so opens (disconnects) its circuit, and that's how the system knows it's potentially overheating. I understand this can be caused by restricted airflow, or alternatively, by switches that have gone bad.
Because this happens immediately on the heat call (when the burners have only been on less than a minute), I really don't think the system is actually overheating. Airfilter is new (even tried it without a filter), and registers are all open in the house.
I could only locate two thermostat-looking switches -- one on top of the burner chamber, and one that goes into the inducer motor (maybe, not sure if that's a thermostat):
I unplugged each of those and just connected the two wires for each directly together to bypass the switch altogether, which should simulate the thermostat circuit being closed. But I get the same error... four flashing lights. (I know you'd never want to permanently disconnect these safety switches, it was just a test to see whether those switches were indeed turning off the system. I've already reconnected them.)
Are there any other thermostats/safety switches I can look for to find the culprit?
Any other ideas why this is happening in this system. This is extremely frustrating, and it's about to get cold.
I actually had an HVAC company in the other day, and they were not helpful, just said I needed a new board, and wanted like $950 to replace the board. I ordered it myself instead (they were nice enough to leave me with the exact part number of the kit) for $150 and installed it in like an hour. It was a drop in replacement, so I just labeled all the wires. The FIRST time I fired it back up, it worked fine. The second time the thermostat called for heat, this issue showed up again and the system was just blowing cold air.
If anyone could offer any suggestions or advice, I would greatly appreciate it. Thank you in advance.
Dave
03-16-21, 09:38 PM
In a so far unsuccessful attempt to fix this problem, I have replaced the control board (used KIT18110, which is an upgrade over what I had previously, and this kit included a new ignitor, which I also installed), and I also replaced the flame sensor. But the problem persists.
Here is a description of the problem: The system will be sitting there idle with a slow flashing LED. Then when the thermostat calls for heat, the LED flashes faster, the igniter lights up, the burners light, and the draft inducer starts blowing. All good so far. Then about 30-60 seconds later, the main blower kicks on, and as soon as that happens, the burners go out, and the LED starts flashing four times, then repeats. That code indicates an "open limit switch."
My understanding is that means some internal safety thermostat is over temperature, so opens (disconnects) its circuit, and that's how the system knows it's potentially overheating. I understand this can be caused by restricted airflow, or alternatively, by switches that have gone bad.
Because this happens immediately on the heat call (when the burners have only been on less than a minute), I really don't think the system is actually overheating. Airfilter is new (even tried it without a filter), and registers are all open in the house.
I could only locate two thermostat-looking switches -- one on top of the burner chamber, and one that goes into the inducer motor (maybe, not sure if that's a thermostat):
I unplugged each of those and just connected the two wires for each directly together to bypass the switch altogether, which should simulate the thermostat circuit being closed. But I get the same error... four flashing lights. (I know you'd never want to permanently disconnect these safety switches, it was just a test to see whether those switches were indeed turning off the system. I've already reconnected them.)
Are there any other thermostats/safety switches I can look for to find the culprit?
Any other ideas why this is happening in this system. This is extremely frustrating, and it's about to get cold.
I actually had an HVAC company in the other day, and they were not helpful, just said I needed a new board, and wanted like $950 to replace the board. I ordered it myself instead (they were nice enough to leave me with the exact part number of the kit) for $150 and installed it in like an hour. It was a drop in replacement, so I just labeled all the wires. The FIRST time I fired it back up, it worked fine. The second time the thermostat called for heat, this issue showed up again and the system was just blowing cold air.
If anyone could offer any suggestions or advice, I would greatly appreciate it. Thank you in advance.
Dave
