Gas Furnace Slow To Start - Is all OK?

DSPGuy · Feb 11, 2025

I've recently noticed in my Comfortmaker furnace, that after the inducer blower comes on, the flame igniter will cycle on for about 10 seconds then off for about 10 seconds several times before the gas valve clicks and feeds gas to the burner and it lights. I'm not seeing any diagnostics codes flashing, just the "All's Well" heartbeat. The igniter is glowing orange when it's being cycled on. The furnace model is N9MP1100J20C1 and serial number is A060759194.
It sits in this state of inducer blower on, with the igniter cycling on/off for about 40 seconds before it eventually sends gas to the burner and lights the flame. Other than this, the furnace seems to be performing without issue. Is this normal? If not, what could be causing it? This is in a used home I recently bought, built in 2006. So I don't know if it has always done this.

Snoonyb · Feb 11, 2025

Welcome.

The appliance uses 24v, so do you have that voltage at the gas valve, when the igniter is on?

DSPGuy · Feb 11, 2025

That is an excellent thing to check, which I hadn't earlier thought of to try. So I just tried it. And as luck would have it, it's not exhibiting the problem right now. The thermostat says go. The draft inducer blower fires up for 10 seconds. The ignitor starts to glow. After about ten seconds, the gas valve clicks and the burner lights. All on the first try. All textbook. No cycling like I earlier described.

The gas valve control voltages look reasonable. Before the igniter warms up, it's at about 0.7 VAC. Then when the igniter has been glowing for a few seconds, the gas control voltage jumps to 24 VAC and the gas valve immediately opens as expected.

I expect the problem to return before long because I believe it's been happening pretty consistently since I moved in. When it does, I'll report back here. Thanks for the excellent suggestion.

Snoonyb · Feb 11, 2025

There are a couple of voltage depended components, unfortunately, and from the tech Manuel, the timing duration of event, seems a bit under the stated of 17sec.

Just something to keep in mind.

DSPGuy · Feb 11, 2025

Snoonyb, My timing estimates to this point have been purely guesses. Quite rough. I could measure them accurately and report back. Thank you.

Snoonyb · Feb 11, 2025

I understand, and I was just quoting from the Manuel, as a point of edification.

PJB12 · Feb 12, 2025

The symptoms you present, DSPGuy, indicate that the pilot or pre-flame is not hot enough to trigger a Flame Sense on the first cycle. It's common with partially clogged pilot or pre-flame orifices. It is also common with a dirty flame sensor.
Repeated tries usually trigger the cycle due to residual heat on the flame sensor.

First Try This- If the flame sensor is a metal rod, clean it with very, very fine emery cloth or an ink eraser. That may do the trick. If it is a glow window or hot surface sensor, don't touch it.

Why Clogged Orifice-
Gas tends to have moisture in it. The moisture can cause clogging of the pilot orifice by corroding the aluminum orifice thimble just a bit.

Worth Checking-
It is doubtful that a breeze is blowing the flame partially away from the sensor. Look for a sideways flame. On some (not all) furnaces this can indicate a cracked heat exchanger. On others, it can indicate blocked combustion air intakes.

It's probably time for a general furnace cleaning and tune up. A furnace tune up person can clean or replace the orifice thimble easily. Or, she can clean or repalce the flame rod if needed.

But if the cleaning person tells you that you have a cracked heat exchanger and have to get a new unit "Now!", get a second opinion. Some unscrupulous companies fake it to sell new equipment.

Ask friends or family for referrals. Or, if your gas utility company offers it, have them do the cleaning. They have nothing to gain by selling you something that you don't need.

And be SURE you have working carbon monoxide detectors on every level of the home, but at least 15 feet away from gas & solid fuel burning appliances.

Hope This Helps,
Paul

dsteinhorn · Feb 13, 2025

PJB12 said:
The symptoms you present, DSPGuy, indicate that the pilot or pre-flame is not hot enough to trigger a Flame Sense on the first cycle. It's common with partially clogged pilot or pre-flame orifices. It is also common with a dirty flame sensor.
Repeated tries usually trigger the cycle due to residual heat on the flame sensor.

First Try This- If the flame sensor is a metal rod, clean it with very, very fine emery cloth or an ink eraser. That may do the trick. If it is a glow window or hot surface sensor, don't touch it.

Why Clogged Orifice-
Gas tends to have moisture in it. The moisture can cause clogging of the pilot orifice by corroding the aluminum orifice thimble just a bit.

Worth Checking-
It is doubtful that a breeze is blowing the flame partially away from the sensor. Look for a sideways flame. On some (not all) furnaces this can indicate a cracked heat exchanger. On others, it can indicate blocked combustion air intakes.

It's probably time for a general furnace cleaning and tune up. A furnace tune up person can clean or replace the orifice thimble easily. Or, she can clean or repalce the flame rod if needed.

But if the cleaning person tells you that you have a cracked heat exchanger and have to get a new unit "Now!", get a second opinion. Some unscrupulous companies fake it to sell new equipment.

Ask friends or family for referrals. Or, if your gas utility company offers it, have them do the cleaning. They have nothing to gain by selling you something that you don't need.

And be SURE you have working carbon monoxide detectors on every level of the home, but at least 15 feet away from gas & solid fuel burning appliances.

Hope This Helps,
Paul

We had a cracked heat exchanger in our house in Chicago. Condensation from combustion exhaust collected and dripped into the cumbustion chamber leading to unpleasant results including repeated cycling of the ignition sequence. The unit was likely a "2nd" installed on the cheap by a contractor who took a small cottage and dramatically expanded it to a large four bedroom home. Luckily, the installation date was printed on the unit although the serial number was scratched and undecipherable, so the manufacturer covered the replacememnt under warranty of the heat exchanger, even though we were second owners of the hodge-podge remodel (the house was cursed in many other ways as well). If the heat exxchanged is cracked, follow-up on passible warranty work. Our next house in Sacramento was a new build but the HVAC sub-contractor installed a "2nd" (probably damaged in transit) compressor unit which failed totally after three years. The serial number was also scratched and untraceable but the HVAC sub-contractor was still in the area. The main contractor contacted the sub with our issue and the sub-contractor replaced the unit gratis, to avoid a suit. LASTLY, our most recent place in San Diego had a similar ignition cycling issue as DSPGuy mentioned. The solution was a corroded orifice which the service man quickly repaired -> and then: last spring during a routine AC tune-up, there was no coolant in the San Diego system and leaks were found in the evaporator and condenser coils in a system that was 7-years old. Both coils were covered by warranty but not labor. The evaporator coil was available in-stock but the condenser coil would have to be fabricated by the factory with a minimum of six and more likely twelve week lead time as estimated by the regional rep in June last year. My wife was unwilling to go that long without AC, so we repalced the entire system with a dual-fuel (gas and heat pump) system with zoning upstairs and downstairs because of the uncomfortabla temperature stratification we expeirenced each season.

DSP: Fingers crossed that this is most likely a simple fix as suggested in the previous posting.

DSPGuy · Feb 13, 2025

Snoonyb said:
I understand, and I was just quoting from the Manuel, as a point of edification.

Is there a way for me to get the tech manual? I've googled it and haven't run across it. Maybe I've missed it. Maybe it's not readily available to the non-professional public.

Also, I've not seen it cycling with the ignitor firing but the gas not coming on lately. I think it's one of those things where you just have to be there at the right time. I'll keep watching for it.

dsteinhorn · Feb 13, 2025

DSPGuy said:
Is there a way for me to get the tech manual? I've googled it and haven't run across it. Maybe I've missed it. Maybe it's not readily available to the non-professional public.

Also, I've not seen it cycling with the ignitor firing but the gas not coming on lately. I think it's one of those things where you just have to be there at the right time. I'll keep watching for it.

An ignition sequence should be reliable, repeatable, and consistent; otherwise, there is something amiss as described in previous posts (low gas pressure, low voltage, etc.). Do you hear the igniter sparking or see it glowing through the little viewing window?

Snoonyb · Feb 13, 2025

DSPGuy said:
Is there a way for me to get the tech manual? I've googled it and haven't run across it. Maybe I've missed it. Maybe it's not readily available to the non-professional public.

Also, I've not seen it cycling with the ignitor firing but the gas not coming on lately. I think it's one of those things where you just have to be there at the right time. I'll keep watching for it.

Look here; https://www.manualslib.com/products/Icp-N9mp1100j-20c1-9669561.html

Scroll down to pg 45.

DSPGuy · Feb 13, 2025

PJB12 said:
The symptoms you present, DSPGuy, indicate that the pilot or pre-flame is not hot enough to trigger a Flame Sense on the first cycle. It's common with partially clogged pilot or pre-flame orifices. It is also common with a dirty flame sensor.
Repeated tries usually trigger the cycle due to residual heat on the flame sensor.

First Try This- If the flame sensor is a metal rod, clean it with very, very fine emery cloth or an ink eraser. That may do the trick. If it is a glow window or hot surface sensor, don't touch it.

Why Clogged Orifice-
Gas tends to have moisture in it. The moisture can cause clogging of the pilot orifice by corroding the aluminum orifice thimble just a bit.

Worth Checking-
It is doubtful that a breeze is blowing the flame partially away from the sensor. Look for a sideways flame. On some (not all) furnaces this can indicate a cracked heat exchanger. On others, it can indicate blocked combustion air intakes.

It's probably time for a general furnace cleaning and tune up. A furnace tune up person can clean or replace the orifice thimble easily. Or, she can clean or repalce the flame rod if needed.

But if the cleaning person tells you that you have a cracked heat exchanger and have to get a new unit "Now!", get a second opinion. Some unscrupulous companies fake it to sell new equipment.

Ask friends or family for referrals. Or, if your gas utility company offers it, have them do the cleaning. They have nothing to gain by selling you something that you don't need.

And be SURE you have working carbon monoxide detectors on every level of the home, but at least 15 feet away from gas & solid fuel burning appliances.

Hope This Helps,
Paul

Paul, Thank you for the time you've taken for the in-depth advice. However, you mention a pilot. This furnace does not have a flame type pilot. It uses an electronic igniter. (Red-hot glowing rod heated by current through resistance.) Do your comments still apply here? By the way, as soon as the gas valve opens, the burner ignites immediately and cleanly. There is what I believe to be a flame sensor on the end opposite of the ignitor, but my guess is that this sensor's purpose is to verify that the burner (not the ignitor or pilot) is producing the expected heat. Let me know if I'm misunderstanding something here. I've attached a photo of the items I'm referring to, annotated with what I believe the items to be. There are two sensors shown in it, A and B, which I'm not certain as to their purpose. Both of them show 0 ohms resistance under all conditions I've tested them (burner on and off). Thank you.

Snoonyb · Feb 13, 2025

They are both ID'ed on pg45, as sequential.

DSPGuy · Feb 13, 2025

Today, I've been able to catch it misbehaving again. Several findings:
1) Today I'm seeing 13.78 VAC at the gas valve input when the valve is supposed to be closed. In the past I was seeing 0.7 VAC. This suggests something very strange is going on with the processor board. Possibly the voltage supply section.
2) It was cycling again. When it's doing it, the voltage at the gas valve input is going back and forth between 13.78 VAC and 26.4 VAC. The valve is not opening at 26.4 VAC, even if I thump it with a screwdriver handle. Please see the attached video.
3) Today I'm seeing 6 flashes on the diagnostic LED, indicating "Failure to ignite or flame sense lost while running" I can believe the 'Failure to ignite'.

DSPGuy · Feb 13, 2025

dsteinhorn said:
An ignition sequence should be reliable, repeatable, and consistent; otherwise, there is something amiss as described in previous posts (low gas pressure, low voltage, etc.). Do you hear the igniter sparking or see it glowing through the little viewing w

It's easy to see the igniter glowing with the front access panel of the furnace removed.

DSPGuy · Feb 13, 2025

We may want to call off the dogs on the 13.78 VAC readings. I was monitoring the valve control voltage by poking two needles into the back of the valve control connector, each to fit tightly against one of the two wire termination connectors. I found that moving that around seemed to cause interruptions to the valve operation. Not sure why. Seemed pretty benign to me. I'm an electrical engineer familiar with things like this. But once I removed those needles, things seemed to behave much more normally. I followed the blue connector wire down to the control board and measured the voltage there. It was either 0.7 VAC or 26 VAC, as expected. But things are in flux. So although I hope it's not the case, I may find tomorrow reason to believe that the 13.78 VAC reports were in fact real. Sorry for the unintended confusion this may have caused. But if one of you pros recognize the ~14 VAC as a known, real world problem with a known solution, it may be a useful thing that I reported it.

Snoonyb · Feb 13, 2025

Since ICP is a CARRIER CO, maybe this will help;

ICP America's Customer Service Department provides a full-service solution from engineering support to QA reporting. To speak with a Customer Service Specialist please call toll free 877-293-2000, Monday-Friday 7:00AM – 4:00PM PST.

International Comfort Products (ICP) manufactures ICP furnaces, as well as other heating and cooling systems. ICP is a subsidiary of Carrier.

DSPGuy · Feb 13, 2025

Snoonyb said:
Since ICP is a CARRIER CO, maybe this will help;

ICP America's Customer Service Department provides a full-service solution from engineering support to QA reporting. To speak with a Customer Service Specialist please call toll free 877-293-2000, Monday-Friday 7:00AM – 4:00PM PST.

International Comfort Products (ICP) manufactures ICP furnaces, as well as other heating and cooling systems. ICP is a subsidiary of Carrier.

Thanks for the suggestion. I may use it at some point. But I'm pretty persistent in trying to solve things myself and usually eventually get to the solution. If what they are offering is free advice on troubleshooting, I'm all for it. If they will want to sell me services, I try to avoid that until I've independently turned over my own last stone, which I haven't done yet.

DSPGuy · Feb 13, 2025

At this point, I'm seeing more and evidence that there is a problem in the connection between the control wires and the control input on the gas valve. When it's up and running, if I shake the connector around a small amount, the gas valve will chatter open/close and the flame will go out. A loose connection there could explain the igniter going on without the gas valve opening, which is the main problem I'm chasing. Will look into that soon. Just been tied up with changing out the car battery today before I get stranded somewhere. Will report back on what I find.

Snoonyb · Feb 13, 2025

Oh, I agree, I will also take the short version when severely inclement conditions are prevalent.

Gas Furnace Slow To Start - Is all OK?

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