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Tim H.
Tim H., HVAC Professional
Category: HVAC
Satisfied Customers: 3971
Experience:  Experienced in all aspects of HVAC, both residential and commercial.
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I have a Heil natural gas furnance; model # *****; the

Customer Question

I have a Heil natural gas furnance; model # *****; the exhaust blower continues to run and the furnace will not light, we just replaced the main limit switch and it still does will not light.
Submitted: 4 years ago.
Category: HVAC
Expert:  Tim H. replied 4 years ago.
Please do not rate me until we have had a chance to work through a solution. This may take a while. Do not use the ratings to rate the site, just the technician you are working with. For site issues, contact customer service.
In most cases, if the main blower continues to run and furnace will not light it indicates an open limit switch. It can be the pri***** *****mit (which is probably the one you replaced), or a flame rollout or even certain other types of limit switches on some furnaces such as a draft safeguard or secondary limit. Usually this will be evident as a LED flash code from the main board indicating an open limit switch.
Customer: replied 4 years ago.
The unit seems to be in constant purge, because the exhaust blower continues to run
Expert:  Tim H. replied 4 years ago.
What kind of flash code are you getting at the board? Have you checked intake and exhaust PVC pipes for blockage?
Customer: replied 4 years ago.
yes, did not see any blockage in PVC pipes, did not see any flashing flash codes
Expert:  Tim H. replied 4 years ago.
If the inducer continues to run and burners don't light, you have one of the following conditions:
1) a plugged intake or exhaust (this can be as small as a leaf that got sucked inside one of the pipes-may not be visible
2) a disconnected or kinked pressure switch hose
3) a condensate drainage system that is plugged
4) a slow or defective inducer motor
5) a bad pressure switch
Customer: replied 4 years ago.
we will try replacing the pressure switch tomorrow, if we do not freeze tonight!! :-)
Expert:  Tim H. replied 4 years ago.
I would try removing the intake pipe from the top of the furnace OR leave the top door open or off and see if this makes a difference. It may be as simple as something that got sucked into the intake pipe.
Customer: replied 4 years ago.
ok, we will try that first.
Expert:  Tim H. replied 4 years ago.
If that doesn't work, check some of the other things on that list before ordering parts.
Expert:  V12Tech replied 4 years ago.
Heil and Tempstar Furnace Eyelet Failures
Models NUGM, NTGM, NDGM, NCGM with date codes of L9421 through L9723.
These furnaces were subject to premature heat exchanger failure and were thus the subject of a service bulletin due to eyelet failures.
If you own one of these furnaces,they suggest seeking a heating contractor for possible repairs or replacement .
Have the exchanger checked where the exchanger is pinched together with rings. These models had issues.
If the unit was not exhibiting a code that also leads me to suspect the board is bad, this original timer board and replacement both have a diagnostic LED/light.
Expert:  Tim H. replied 4 years ago.
The heat exchanger issue has nothing to do with your problem. You are experiencing an issue with the inducer motor continuing to run. This could be for one of the reasons I have previously mentioned or could even be from a stuck relay on the board. We have not determined that as of yet.
Expert:  Tim H. replied 4 years ago.
If you are truly not getting a LED flash code (and I'd want to make sure you know where to look for this), then it would tend to be more likely a board issue.
Expert:  V12Tech replied 4 years ago.
The pressure switch not closing or staying closed etc will be a fault which should lock it out if normal was my point, which it is not doing, the inducer motor will not run unless a limits open, ala exchanger issue relevance or bad board as earlier stated, the lack of a lock out and diagnostic precluded the other possibilities in my estimation, after 3-5 attempts the control should have locked out and faulted exhibiting a code
Expert:  Tim H. replied 4 years ago.
The board could possibly be bad, but I doubt the heat exchanger is although it's never a bad idea to have your heat exchanger checked out annually. The furnace has a protection device called a flame rollout switch which would trip if the heat exchanger had a hole or defect in it. I want to be si=ure that you are looking in the right place for the LED flash code. This will show up after the furnace has attempted to start. The one way to check for a bad board is to remove the heat signal from the board by turning the heat all the way down or off. With no signal at C-W at the board, the inducer should stop unless the small relay on the board is bad allowing it to run non stop.
Expert:  V12Tech replied 4 years ago.
With a recall whether the exchangers bad or not is immaterial,I would insure this unit is not under recall before putting any money into this troubled design,If it is not from this serial code era, I would recommend Co detectors in addition as the model was troubled.
Expert:  Tim H. replied 4 years ago.
The easiest way to determine if you have a board problem is to take away the call for heat. You can do this by either turning down the heat to well below room temperature or turning the thermostat "system" switch to off. Then go down to your furnace and see if the inducer is still running. If it is, replace the board, if not-we can go on with the troubleshooting. Sometimes other experts become involved in our questions whether we want them to or not. To determine who you'd like to continue to work with, compare his number of accepts and rating to mine and make that decision.
As I said previously, it is always good practise to have your furnace inspected annually by a qualified HVAC contractor.