Day and night furnace runs for 15 minutes and then restarts

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Old 11-24-18, 08:23 AM
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Day and night furnace runs for 15 minutes and then restarts

I live in California. Outside temperature now is 60 degrees; indoor is 68.

House is a two-story built in 1989.

Furnace is Day and Night 394J natural gas forced air updraft, about 29 years old. The electric pilot assembly is a three-wire LH-680-005. Gas valve model is White Rodgers 36E93-301. The control board model is Carrier HH84AA011.

Starting from two days ago, it whould shut down before reaching the set temperature. The sequence of events observed:

1) thermostat calls for heat

2) sparks, pilot ignites immediately, pilot is about 3 inches tall, with 80% going to the right of the hood burning the spark ignitor tip, the other 20% to the left of the hood heating up the bimetal

3) 2 minutes later, main burner ignites, spark stops

4) 1 minute later, blower comes on

5) 15 minutes later, main burner shuts off, spark starts sometimes immediately, sometimes one minute after

6) at this point, sometimes the pilot lights up immediately, sometimes it takes 10 minutes to light up

7) it then repeats from step 3, but main burner will shut off after a minute or two, not 15 minutes.

The filter looks clean. I also temporarily removed the filter and bypassed the high limit switch. The problem remains. So I don't think it's a high limit problem.

I googled and found a few youtube videos listed on Carrier, Bryant, Day Night, Payne - Gray Furnaceman Furnace Troubleshoot and Repair which suggests the electric pilot assembly can be problematic. I took it out and inspected it. It looked clean and the orifice was not blocked. I used compressed air to blow it clean. The bimetal did look dirty so I used a 220 grit sandpaper and polished the bimetal to a shine. I put it back and it worked for 30 minutes without shutting off. I thought it was fixed.

However the next morning the exact same problem happened. At this point I just ordered a replacement part from ebay (the local HVAC supply house does not have it.) It will arrive in a day or two.

In the meanwhile I would like to get some advice from this helpful board.

1) What do you think the problem is?

2) Is it time to get a new furnace and A/C? The supply house guy basically told me furnaces last only 15-20 years and mine is long overdue.

3) Can A/C be replaced in winter (albeit California winter)? I was just wondering how the tech can test the temperature/pressure reliably in winter while A/C mostly operates in the summer.

Thanks a lot.
 
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  #2  
Old 11-24-18, 09:52 AM
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With an almost 30 year old furnace.... it's time to consider a replacement.
At the very least you should have the heat exchanger checked for integrity.

If that's the original igniter assembly then that's great. Lasted a long time.
Just changing that assembly should take of your problem.
 
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Old 11-25-18, 07:08 AM
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Thank you PJmax. I will report back if the new ignitor fixes the problem.
 
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Old 11-25-18, 11:17 AM
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I replaced the igniter. It now takes only 30 seconds (old one takes 2 minutes) for the bimetal to make the switch to turn on the main burner. Unfortunately the furnace still shuts down after 12-15 minutes. So the problem remains.

I also temporarily jumped R and W to make sure the thermostat is good. And it is good.

Any ideas please? Control board, or main gas valve? I looked at the schematics, and it was not obvious to me how the control board could just shut the furnace off, while the circuit between R and W is closed. Perhaps it's the solenoid on either the main valve or the hold coil is bad?

If it's the control board, I can still handle it. If it is the main valve, it obviously goes beyond my capabilities for sure.
 
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Old 11-25-18, 11:31 AM
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You need to check any safety limit switches. It sounds like the furnace may be over heating.
 
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Old 11-25-18, 11:35 AM
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Thank you. Earlier I had temporarily bypassed limit switch and the symptoms were exactly the same. I also took the burner out and looked inside using a small mechanics mirror. There were no dark burn marks or any visible cracks. When the blower comes on I also did not see any flame roll-out.

Also it looks someone else complained the EXACT same problem: https://www.doityourself.com/forum/g...shuts-off.html but unfortunately there was no follow-up.
 
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Old 11-30-18, 11:56 AM
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Update: I replaced the gas valve and everything is working now. I think it's the hold coil that went bad.

The new valve has a factory-set manifold pressure of 3.5 WC, matching the info on the plate. The burner seems fine. However just to be safe, I ordered a manometer and will make sure the pressure is accurate.

Question for the board: do you always adjust the manifold pressure on a new furnace, or when you replace a valve?
 
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Old 11-30-18, 11:53 PM
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As part of the commissioning of the furnace..... the manifold pressure is supposed to be checked. Since it's new..... many installers don't bother since the valve was tested in that furnace.
The pressure must be checked on a valve replacement.
 
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Old 12-03-18, 11:49 AM
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Thank you, PJmax. You are absolutely right. The manifold pressure was set at 3.63 WC. I turned it down to 3.5 WC just to be safe.
 
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