Miller Nat Gas Forced Hot Air issues
just closed on our daughter home Friday and furnace acting up. Had tech out who said A coil blocking air flow and causing high temp switch to cut out. He removed A coil, thought fixed, but burner kicking in and out on long duration runs. Sometimes furnace continues entire heat cycle/stat satisfies, but gas valve does drop out and the glow plug comes back on and then gas ingnites and may run 10-15 minute or longer of less and do reignite all over again. Sometime needing completely breaker off reset, other times not.
Tech is hanging hat on high limit switch, which had to be ordered and two days out and its zero here, so we are 24hr baby sitting and resetting as needed.
My question, he said it can’t be jumpered due to a resistance being read by controls. Is there a way to test the high limit to confirm it’s bad?
Furnace is in double wide it’s a 1998 Miller Nat gas forced air. High limit part number is 6264370. I am debating on losening mounting screw of switch, sliding a metal washer to create air gap, retighten and see if that helps until parts here
Comments
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It could be the limit breaking down… The limit is a safety , don't jump out or repair , only replace …
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We are desperate to get it to just run without being reset. I am very aware of the dangers of allowing the furnace to possibly over heat. My IR gun says plenum temp is at 97* where high limit is mounted and research tells me the switch is a 190* cut out. We are onsite non stop monitoring.
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My main question is, how to test the switch? Tech says you can’t jumper, due to controls reading the resistance of the switch? If that is the case, I have VOM meter and can test the resistance, but what should it be? I know that apparently it completely opens up when it exceeeds 190*.
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If the A-Coil was plugged so is the squirrel cage fan blade.
Whats the temperature rise between the return and a supply?
IR Thermometers are very inaccurate!
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Tech removed the A coil and also cleaned the squirrel cage fan which was very dirty as well.
How do I check the supply and return temps? IR gun on the upper inlet and then on the bottom outlet covers? What should I use instead of an IR, or just take reading with a grain of salt?
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Thermometer.
Color, material distance all affect IR readings.
Ideally, you're looking for 40 - 60° difference between return and supply.
Did this tech recover all the refrigerant before removing the A-Coil?
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I have digital type thermometers of all sorts I use for cooking. I will try to use one of those to checK supply and return temps. I was not present when A coil was removed, we took ownership of the home on Friday and was told the air was not working. The outside unit appeared to have been damaged significantly.
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Most likely there are several safeties in series on that circuit . Most should be a manual reset , the limit is automatic reset. One way to check the limit is to use a voltmeter and test for voltage across the limit . See if it drops off …. A dirty flame sensor can caused the short cycling too , and will shut the furnace down after a few tries ….
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a probe type thermometer stuck in the supply and return will get you close enough. if it is just a switch yo can test if the problem is the limit opening with an ac voltmeter, if it is opening the voltage will go from 0vac while the burner is firing to 24vac when the burner stops. i'm not convinced you don't have a proof of flame issue.
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When I arrived this morning to relieve my son-in-law he had the night shift watching the house, the furnace was tripped out and 8° below thermostat setpoint. I cycled the thermostat down and then back up again and furnace tried to relight the first time, I saw the glow bar than a flame, and then Gas valve dropped out immediately. It did the exact same thing again on its own, and then the third time it stayed on and now seems to be running continuously for about ten minutes and drops out again. A dirty flames Sensor seems like a possibility, I’m not sure how to clean it or to test it. I would’ve thought the tech would’ve done that, Especially since we had a dirty a coil, squirrel Cage fan, and filter
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Unfortunately, most techs today are clueless!
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I am reading 120vac on both sides of high limit switch
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you measure voltage across the switch , not to ground
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it could be a regulator or other gas supply issue too. or even a tank that is too low to keep pressure at the current ambient if it is lpg.
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I read zero voltage across switch while running or off. 120vac both sides running or tripped off
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No voltage across any limit/switch with power on indicated that it is closed.
I would look at the flame sensor…..may need cleaning or replacing.
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Normally the flame sensor location would be on the opposite side of the igniter … It will have a single wire attached to a rod that sticks into the fire .. Single screw to remove , clean the bare metal part of the rod with fine steel wool …
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I remove the burner today after receiving the new high limit switch and replacing it and it is still doing the same thing. The burner tube which also held the igniter had quite a few stink bugs accumulated. I gently removed all of the insects vacuumed things out and was sure not to touch or break the igniter. I fully examined the entire burner tube area and no such thing as a flame sensor exists. There’s only two wires that go into the burner tube and they are the ones connected to the igniter. I examined the igniter which did not appear to have any cracks in it or debris on it. After cleaning all the bugs out it did light and run but it’s back doing the same short cycling that it was doing previously.
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Something is shutting it down
Time to break out the meter and follow the diagram.
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is this an atmospheric burner or a power burner? sometimes the ignitor is also the flame sensor probe.
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yesterday the furnace ran for nearly 2 hours blowing hot air the entire time and gas valve only dropped out once, but cycled through on its own and didn’t shut of at all. Then 2hrs 10 minutes shutdown and needed reset to reignite and run
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You must have a combination hot surface ignitor and flame sensor.
The old HSI may work for ignition but not perform well as a flame sensor.
A new HSI may fix this. Cleaning a HSI is chancy as they are pretty fragile.
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make sure all of the connections of the sheetmetal back to the control are good, the flame signal returns through the burner and the cabinet.
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If a power burner is one with blower fan, then yes. Could it just be a bad igniter that is also the flame sensor?
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This is a 1998 furnace, so it’s possible that the igniter is also the flame sensor?
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it also has to prove the blower somehow, usually with a centrifugal switch on the blower motor.
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When HSI's were first introduced, Rheem/Ruud used them for dual purpose.
They then supplied retrofit flame sensors that tied into the HSI to solve problems.
A new HSI may solve your problems.
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I ran jumper wires to have redundant grounding of controls and burner and didn’t make a difference. I don’t see a centrifugal switch anywhere.
I just found “ way buried” a indicator flashing four code. It’s a United Technology controls0 -
United Tech 1018 series Hot surface ignition control.
I have a new igniter on its way in an hour0 -
is there a label inside one of the cabinet doors that says what the flash codes mean?
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no label for trouble codes, just wiring diagram.
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the old igniter had a small white spot, appeared to be possibly a crack. I put the new igniter in and still does the same thing…..paid tech was certain it was the high limit switch. Replaced and same results. I cleaned burner of some bugs. Checked the orifice, grounded controls.
Sometimes gas valve drops out immediately
Sometimes after a minute or two
Sometimes runs 15 minutes to 100 minutes before a gas valve drop
Makes great heat when burners on.
We have a contractor putting in anew furnace tomorrow, I give. It’s been hovering around zero everyday and keeping the house above freezing for the last 4 days is exhausting. We’re going to keep the old furnace for possible future garage heat….
my last ideas are possibly the control unit? Also, someone mentioned centrifugal switch inside combustion blower as possible causes, but enough is enough. Desperately need heat!
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if you have the manual or can find a copy online with the furnace model and find out what 4 flashes is that will tell you where it is failing. the ignitor could be cracked and is intermittently becoming open too.
does the limit connect directly to the ignition control or is it just in line with the thermostat connection?
is the blower part of the burner or is it a separate part on the exhaust? if it is part of the burner it probably has a centrifugal switch, if it is on the heat exchanger to the exhaust it probably uses pressure switches.
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I’ll have to find a manual
limit switches are wired into control module
blower is separate and attached to burner. The blower has a internal switch. The motor has a diagram showing it normal open switch, close on up to speed0 -
if the limit switch is separate in to the control then the wiring could be bad or loose or the solder joint to the terminal on the board could be broken. since the control is flashing a code it is more likely one of the other safeties although if the centrifugal switch on the motor drops out and drops the gas valve then it would lose flame signal because the flame would go out. you might be able to figure something out by seeing exactly in what order things drop out, like is the flame out or the control flashing a code first or does the ignitor gout then the flame drops or does the flame drop then the ignitor goes out, things like that. leaving a phone recording video when it happens could help.
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