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Old 12-04-2008, 06:55 PM   #1
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Heil furnace issue


My Heil gas furnace is cycling on and off. It is model NUGK075DG06. when I set the thermostat to call for heat the furnace fires and stays lit for approx. 45-50 sec. It shuts off for around 5-8 min. and fires back up.

I checked the pressure switch and it is @ 26.6 volts while the furnace is firing. I cleaned the ignitor/flame sensor w/ steel wool. I was told that the flu could be stopped up so I checked and it has a grate in the end. I'm suppose something could get in it but I am skeptical.

I replaced the flame sensor and no change. PLEASE HELP!!!

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Old 12-04-2008, 07:04 PM   #2
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Heil furnace issue


Check your filters and check for a closed damper, sounds like it's either going offf on high limit (overheating due to lack of airflow) or the MV signal from the flame sensor is being lost in your circuit board, IE you'll need a new board. Could also be a number of other problems, stuck pressure switch, failed limit, excessive flame rollout, malfunction at the gas valve. But check the filters and damper first.

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Old 12-04-2008, 08:17 PM   #3
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Heil furnace issue


Have a new filter. Would the circuit board also be the ignition module? I had to replace that about this time 5 years ago. I have to ask a stupid question I quess. Where would the damper be? I don't see anything resembling a damper. Thanks for your help.
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Old 12-04-2008, 08:21 PM   #4
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Jump the press switch and see what happens. i say this because if the switch was closed during firing than the voltage should be zero.

Try that and get back to me.
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Old 12-04-2008, 09:38 PM   #5
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Heil furnace issue


I jumped the p switch and it still shuts off after about 45-50 sec.
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Old 12-04-2008, 09:56 PM   #6
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Heil furnace issue


Either the board again or something in the 24v circuit. Check the burner limits and jump them.
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Old 12-04-2008, 10:27 PM   #7
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I tried that and still nothing. You think it could the ingnition module?
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Old 12-04-2008, 10:33 PM   #8
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I THINK you should have gotten all the things you did on the board before this.

You've eliminated everything else.
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Old 12-04-2008, 10:44 PM   #9
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In most cases, if the flame sensor is sending a signal of X amount of MV which is specified according to make and model of furnace (carrier is 93MV, Trane is 58 MV, Goodman/Amana is in the 40's) But, the flame sensor may be one of a number of issues, 1. if it's a 90% or better furnace, the inducer assembly collector box may be clogged and not allowing the condensation to be evacuated from the assembly. 2. the fan limit switch may be failing prematurly. 3. the inducer motor may be failing, on the side of the motor it should have a rating nameplate which will indicate FLA, if the amerage reading you get from measuring the amperage off the 120V line to the motor is over 80% of the listed FLA, the motor is failing and needs to be replaced. 4. The inducer motor capacitor may not have sufficient Microfares, typically they are 3MFD, if less than 80% the capacitor needs to be replaced. 5. the flame sensor wire connecting to the ignition module (I believe with your make and model it's a rectangular white-rodgers board) may be loose from it's connection. 6. Last but not least, turn off all power to the unit, take the control board off the side of the cabinet or motor assembly and examine the back of the board for any signs of burns or breaks in the circuitry. I know it's not the best of answers, but, there are some things you can't do yourself, it's what keeps guys like me employed. If you feel overwhelmed with any of these diagnostics, you're better off calling a qualified tech, and don't get insulted if they're in and out in 5 minutes, it what we do
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Old 12-04-2008, 10:50 PM   #10
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Thx for your help.
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Old 12-04-2008, 10:51 PM   #11
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And a damper would look like a metallic lever on supply side of your ductwork, if it's in line with your duct work (if your ductwork runs up and down the lever should be up and down) the damper is open, if it's horizontal, it's closed, some people have them located in the attic, some right nexe to the furnace, you may have to take a good long look, but if there is one, make sure it's open, but I'm a bit more confident in thinking it's your control board.
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Old 12-04-2008, 10:53 PM   #12
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Heil furnace issue


J theo is right you need to know your limits - call a tech you don't want to get hurt or cause more problems that will cost you money
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Old 12-04-2008, 10:53 PM   #13
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Yes there are some things a HO can't do. Reading DC micro amps in the flame signal is one of them.
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Old 12-05-2008, 05:22 PM   #14
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Heil furnace issue


I had a tech out and it is the fan/limit switch. The old one is a Cam-Stat limit switch. The parts store says that the Honeywell L4064B 2236 should work fine. One question... the old one is about 7 in. long and the Honeywell is 8 in. Will this pose any problem?
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Old 12-05-2008, 05:47 PM   #15
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No. Cam- stats are pretty well obsolete. The Honeywell is a better one. Be very careful how you change it. The Honeywell usually has a jumper between the fan and limit circuits which may need to be removed for separate fan and limit circuits. If you fail to remove it 120 volts will enter your 24 volt circuit and smoke will appear. Follow the wiring diagram carefully.

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