Burners Ignite When Inlet Restricted

Old 12-07-21, 06:49 PM
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Burners Ignite When Inlet Restricted

2005 Heil
Model # H9MPDO75F1281
Serial # A051790263
Honeywell Smart Valve SV9541M


Last winter my Heil furnace burners stopped lighting. The igniter would come on, and then the pilot light lit but the burners would not ignite. I would know that the furnace was going to eventually fail because on the previous lightings the burners would take a long time to ignite and then eventually, on following attempts, stop lighting completely. (If working properly the burners would ignite within 8 seconds or so.) The furnace would then go into a soft lockout. (6 flashes + 1 flash)

Since the initial problem, Ive had the same crew work on the furnace and in that time they have: cleaned all pertinent parts (burner, inlet to igniter etc.), installed a new igniter, returned to adjust the new igniter, checked all grounds (listed as a possible problem), installed a new combustion blower (went bad at that time), and checked for blockage in the exhaust. It looked as if the problem was finally solved but the same issue started up again this fall.

The serviceman returned on the morning of 12/6 and he decided that the gas valve was getting stuck and therefore not allowing the burners to kick on. He ordered a new smart valve and was to contact me when it came in.


When he left the furnace was working properly but soon failed again shortly thereafter. The furnace would attempt to ignite, go into soft lockout and then attempt again after about 5 minutes. After this went on for a few hours I went to watch the furnace when it was attempting to light and when I inadvertently put my hand over the air inlet the BURNERS IMMEDIATELY LIT. I bumped the thermostat up to heat the house well and after the furnace met the temperature it returned to soft lockout on the next attempt to light.

Later that evening, since the furnace had still not run, I again placed my hand over the air inlet when it was attempting to light and the burners lit for a 2nd time. I went to bed and apparently the furnace had not run again all night as it was 60 degrees in the house the next morning. For the third time, I placed my hand over the air inlet when attempting to light and once more the burners lit. I then blocked ⅔ of the inlet with a plastic yogurt lid and the furnace has run properly for about 14 hours at the time of this post (burner igniting quickly as it should.)

The serviceman, however, thinks its just a coincidence that my hand was over the inlet when the burners lit. I think its quite a coincidence that a furnace that had not run for 8 hours would suddenly ignite after putting my hand over the inlet. He told me hed talk to his boss and workmates and see what they thought but I never heard back from him. BTW - its 32 degrees today and he never bothered to check to see if my furnace was still running.

Look, I realize that Im just a know-nothing DIYer but there has to be something to the restriction of air into the inlet. Can someone, anyone, please give me some feedback as to what might be the cause of the burners not igniting? Does the inlet have anything to do with the function of the Smart Valve? I would be very grateful for any input as it looks like I'm on my own.

Thanks in advance!

Old 12-07-21, 07:21 PM
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That's supposed to be a two pipe furnace. Does yours just have one....... exhaust only ?
The fresh air tube just connects to the top of the cabinet.
There is no tube connected to the input..... inside the furnace..... correct ?

How many pressure switches does that furnace have ?
Old 12-08-21, 04:15 AM
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Thanks for your reply.

There is no pipe feeding into the inlet on the top of the furnace and never has been since originally installed. The inlet has just always been open to the basement. There is only one PVC pipe connected to the furnace and that is the exhaust.

I looked over the parts list and only one pressure switch is listed.

Hope that helps.

Furnace still running properly as of 24 hrs

Last edited by reluctant DIYr; 12-08-21 at 04:18 AM. Reason: update info

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