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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 8702
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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My furnace will not light consistently. I find it blowing

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My furnace will not light consistently. I find it blowing cold air, continuously. However if I open the firebox and toggle the power, next time it calls for heat, it will light. Then I can put the cover back on and it will run fine until it warms up the house. Then next time it calls for heat, it blows cold and never lights.

As far as I can tell, it never even tries to power the igniter or turn on the gas valve in any way when it won't light. I never see the glowing in the firebox or hear the "click" that happens when it operates normally. However, the draft inducer runs continuously and the blower runs continuously.

I have replaced the igniter and flame sensor unit. Didn't make a difference.

Furnace: Comfortmaker EnviroPlus 90 RPJ-II
Installed in 1997
Control board: Honeywell ST9120C 4040

I've checked the draft vents. It is blowing/sucking fine with no obstructions.

The 24 volt pressure valve circuit reads 24V when I measure across the blue and black wires on the control board. Blue one goes to pressure valve, black on is 24V hot.
Welcome to Just Answer.

Thats good work on your end so far! In this case if the igniter is not coming on when it should, the problem is most likely in the control card OR...the high temperature cut out switch that my be going bad and tripping...and by cycling the power you are resetting the control card.

Warm the house up to 75F, then Locate the high temperature cut out switch and put a jumper wire across it.... TEMPORARILY WHILE WATCH IT EVERY SECOND... let the furnace warm the house up to 76F, then cycle the furnace off with the thermostat and see if it restarts by itself after 10 minutes when you set the thermostat back up to 77F and the thermostat is again calling for heat.

If the furnace ignites, then the high limit switch needs to be replaced.... if the furnace fails to start the odds are into the 95% range that the control card is bad.

Repeat this test a bunch of times until you are contortable with what see happening.


If you wish to rate my assistance so far *positively, I will hold the question open for unlimited follow up on the same dime so to speak.


Customer: replied 3 years ago.

I tested the high temperature switch as you described:


1. Turned on furnace.

2. It didn't light, but induction motor was blowing.

3. Tested Ohms between 1 and 6, which should be the loop between the main limit, aux limit and rollout switches. It read 0 ohms. Continuity beep sounded. Those seem fine?


Decided to re-test pressure switch. This time I put a continuity tester across the pressure switch. No continuity.


As soon as I took the cover off the firebox, it immediately sounded the continuity alarm and the igniter stared glowing.


Once it starts I can put the cover back on and it will run until it finishes.


What do you suggest?






Hello again,

It could still be a high limit switch problem...checking with an ohm meter while a device is still wired to the system can produce deceptive results because of electrical feed backs that can make an open switch appear closed. (not seen with DC circuits but common with AC electrical controls circuits).

That leaves us somewhat inconclusive on the high limit control issue. You may want to try the use of a jumper as outlined earlier if the pressure switch tube cleaning fails (described below)


Regarding the cover over the fire box... if you mean the panel that encloses the gas valve and burner head area..... then the pressure switch is opening when you leave the panel in place, and closing when you remove the panel... that is reverse of the normal operation confirmed by the fact that when you replace the panel after the burner fires, the gas valve keeps running. We need to verify that by using a jumper. I will detail that later.


This could also be a defect in the control intermittent defect. That is difficult to confirm even on site with instruments... we are into dangerous territory here.

It doesn't make a lot of sense... often however the small rubber tubes that run to the pressure switch get condensate in them, or insects or dirt, and the tiny orifices in the tube attachment nipples can clog preventing normal operating pressures from being active and that could cause some problems with the pressure switch. Do that cleaning first.




testing the pressure switch function with a jumper. If there are just two wire terminals on the pressure switch. you can test the function this way.


remove the wire from one of the terminals, try to start the furnace. it should not start, there should be no power to the glow plug.


Next cycle the power to the furnace to reset it, then fit a jumper across the two terminals on the pressure switch within 2 seconds of the combustion air fan starting...the igniter should energize 15 to 30 seconds later. (that delay is called the purge cycle) if the furnace works that way, the chances that the pressure switch is the problem are high.. this is all still a bit of an anomally, it could still be a problem with the control card.







Use a paper clip wire to poke into each of those tiny ports where the hoses attach to the furnace and to the pressure switch.... it could also be a leaking diaphragm in the pressure switch or burnt switch contacts.


If the switch sticks closed at the end of a heating cycle, and stays stuck closed next time the combustion air fan comes on, the programming on the control card will not let the furnace fire, the pressure switch needs to be *open *before the combustion air fan starts.. then close within a few seconds *after the combustion air fan starts in order for the control card program to recognize the pressure switch closing as a sign of a slight vacuum in the fire box,


All of this function needs to be verified on site by a licensed HVAC service person. These are critical safety devices.

Let me know how that goes, we can proceed from there.


Customer: replied 3 years ago.

Regarding the firebox, I do mean the cover where the burners and igniter is housed.

I turned it up to 68, opened the box cover to get it to fire, replacing the cover after it ignited. Let it warm the house up to 68 and turn off. The pressure switch was open at this point. Then I increased it to 69 and it wouldn't fire and the pressure switch never closed. As soon as I pulled the cover off the pressure switch immediately closed and the igniter started. All of this is without power-cycling.

Next I took all the vacuum hoses off one at a time and cleaned them out. Didn't notice anything come out. However the hole going into pressure valve the burner box side of the vacuum felt clogged. It seemed to open up after moving the wire up and down a few times.

Put everything back together and tried increasing the temperature again. The induction fan kicked on, but the igniter never started. Vacuum valve was still open. Again, as soon as I pull the little hose off the burner box side of the vacuum valve, the vacuum valve closes and the igniter kicks on.


Customer: replied 3 years ago.

I just tried it again, it wouldn't start as "normal" in this broken situation. The vacuum valve was open. I shorted the vacuum valve with a jumper and the igniter started about 10 seconds later.

Hello again, this is a bit of an odd situation, replacing the vacuum valve (called a 'pressure switch' when you order it) has an 80 to 90% chance of solving the problem.

The other symptoms combined do not point directly to a bad pressure switch.

Most service people at this point would try a new pressure switch and make sure all of the ports were clean at the hose connections. then go from there if necessary.

Let me know how that goes. If you choose to rate my assistance so far positively, I will hold the question open without time limit on the same dime so to speak for any follow up needed later.

Phil and 2 other HVAC Specialists are ready to help you
Customer: replied 3 years ago.

The new vacuum valve didn't fix it. It still isn't lighting consistently.


The good news is that I haven't power-cycled it in weeks. Whenever it doesn't light, I simply suck on the induction fan side of the vacuum valve and it starts right up, just as soon as the valve clicks.


I noticed the other night that the exhaust pipe is dripping water. Could it be that the exhaust is partially blocked by water or that the drain is clogged and it is causing water to fall into the fan?


The induction fan is only 3 years old. Can they weaken? The last one that died just quit working all together.


Thanks for your advice.