Inducer Fan Cycles On and Off


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Old 11-13-12, 12:21 PM
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Inducer Fan Cycles On and Off

Hi,

I have a gas furnace with spark igniter. (Heil/HQCO Forced Air System Model number NHGE075CF01).

What occurs is when I call for heat the inducer fan will come on, stay on for a couple seconds then go off. It will do this several times. It never locks out. Once it goes through this cycle it will ignite fine and runs fine (it never shuts down during a call for heat). It also never fails to ignite. The number of on/off cycles varies. The more recent the last time a call for heat was made the fewer on/off cycles the inducer fan goes through. So if the unit is running throughout the day the inducer fan will cycle on/off once or twice (or not at all) before lighting.

The tube to the pressure switch is good and clear. The pressure switch is new and was tested on my second unit so I know that is not the issue. The air filter is new and clean.

Any insight to this issue is greatly appreciated.

John
 
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Old 11-13-12, 12:31 PM
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When the thermostat is calling for heat the inducer motor should turn on and stay on as long as the thermostat continues to call for heat.


Use a multimeter to see if 24 VAC to the R terminal stays on, which should cause the inducer motor to stay on.

Also check to verify that the 120VAC to energize the inducer motor is staying on.

You may have a bad circuit board that is failing to keep the inducer motor energized, but you need to be sure the thermostat isn't shutting things off and that it's not the inducer motor that is bad and shutting off.
 
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Old 11-13-12, 02:56 PM
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Thanks for the reply. After checking it appears the T-stat and inducer motor are fine. Is a bad circuit board my likely issue?

Thanks
John
 
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Old 11-13-12, 04:27 PM
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Yes. If there is 24 VAC to the W terminal, power should be switched on to the inducer motor. If it isn't you have a bad circuit board.
 
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Old 11-16-12, 06:09 AM
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Hi,

I replaced the circuit board and tested it a couple times. It seemed to work fine (no inducer on/off). During the night, though, it never came on at all. It must have locked out. I reset the breaker and the inducer motor came on for about a second, went off, came back on and the unit ignited fine. It went through the entire heat cycling without an issue. The inducer seems to never be an issue once it ignites.

Any other ideas? The unit never shuts off during a heating cycle so the problem is limited to start up. Its odd that it locked out since it never did that before.

Thanks
John
 
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Old 11-16-12, 09:35 AM
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You need to observe the sequence of ignition on the furnace and observe what is happening when the furnace fails to ignite properly.

Remember this on your initial post:

>


It appears that you are now reporting different symptoms, is that correct?

As I understand your remarks, the inducer motor is now starting reliably and staying on. The burners are lighting reliably and staying lit, but at some point this correct ignition sequence is failing and the furnace is locking out with the inducer motor shutting off.

Is that what you are observing? If not please correct what I am hearing you say.

The thing most likely to cause the furnace to lock out is the pressure switch not staying closed reliably. You can pull the rubber hose off the pressure switch off the furnace and observe what happens when you turn up the thermostat. Does that produce the lockout you are observing?

Are there diagnostic lights on the circuit board? If so, what are they signaling?
 
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Old 11-16-12, 10:45 AM
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Yes, I think my post was confusing. The same thing is occurring with the exception that I believe it locked out once (i assume it locked out since I wasnt awake at the time and I reset the breaker and the unit fired -after the inducer cycled on/off a couple times). Other than that, the same symptoms (inducer comes on & goes off several times then stays on, ignites and no problems throughout the heat cycle).

There are no diagnostic lights on my system which makes things much more difficult.
 
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Old 11-16-12, 11:22 AM
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Sorry. I am unable to understand clearly what you are describing.

Everything needs to be understood with respect to the furnace sequence of operation --- the series of events that starts with the thermostat calling for heat and then goes from event to event until the thermostat is satisfied and the furnace goes through a series of events to shut down.

By jumping from one thing to another I can't understand what might be happening.


You seem to be repeating the idea that the inducer motor is repeatedly turning on and off when the thermostat calls for heat. As I said earlier, the inducer motor should be staying on once the thermostat calls for heat.

There are circumstances that could turn it off later in the ignition cycle, such as the pressure switch opening, and that could cause a lockout. But that doesn't appear to be what you are describing.

Do you have a multimeter and understand how to use it? If so, you want to measure the voltage between the C terminal and the W terminal. When the W terminal has 24 VAC applied to it by turning up the thermostat, the inducer motor should turn on and stay on.

Is that what is happening?
 
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Old 11-16-12, 01:04 PM
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you could have a bad connection in the low voltage too, which could allow your inducer to stop and start

i could make a list of things that can cause this

you need to get a meter on it while its running and try to catch it when it acts up, if not i will work on my long list lol
 
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Old 11-16-12, 01:08 PM
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Thanks for the replies. The more time I spend trying to diagnose the more I see different things occur. I'll start from the beginning and note the different things that are occurring. I know how to use a multimeter and have been checking things as I go.

Call for heat. Inducer motor clicks on/off repeatedly. After several on/off cycles it stays on, unit ignites and everything runs fine. After checking the voltage to the inducer motor, T-stat, tube, pressure switch, vent leading outside and finding all ok I changed out the circuit board.

Now I'm noticing something a little different. Several times the above situation occurred. Then on several occasions during testing this occurred: Call for heat, inducer motor comes on and stays on but there was no attempt to ignite(no spark). Entire unit heated up very quickly. The system never detected any type of fault and never shut down. When I saw this occur I quickly changed out the limit switch with one that I know works properly but that didn't solve the problem.
 
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Old 11-16-12, 01:17 PM
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>



Check to see if the 24 VAC on the W terminal stays on while the inducer motor is
turning on and off.

If it's shutting off, check to see if the voltage to the R terminal is staying on or shutting off when the voltage shuts off at the W terminal.

Also, is the inducer motor turning on and off erratically and randomly, or with some kind of pattern?


(The voltage at the R terminal should be 24 VAC whenever the 120VAC to the furnace is turned on. The thermostat takes power from the R terminal and should turn on the inducer motor when the thermostat is calling for heat by energizing the W terminal with 24 VAC)
 
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Old 11-16-12, 01:56 PM
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There is not pattern, although it seems like after replacing the board yesterday the inducer is only switching on/off once when it works properly. I checked voltage previously and all thing pointed to a proper working inducer motor. Its harder to check now that the inducer motor is staying on more often now when testing.

More often now what is occurring is the inducer is coming on and staying and the blower motor is turning on. That's it, nothing further is happening, no attempt to light. The entire furnace is getting very hot even though it never fired and it does not shut down from overheating.

Does this "overheating" issue make sense? Could this be a defective limit switch near the burners?
 
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Old 11-16-12, 02:49 PM
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Getting very hot from what? There has got to be a cause for that.


After the inducer motor comes up to speed, it should cause the pressure switch to close.

When that happens the hot surface ignitor should heat up, the electric gas valve would then turn on and the burners would light.

Check the pressure switch with a meter to see if it is closing.
 
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Old 11-16-12, 02:58 PM
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when the inducer was running and you had no spark, was the pressure switch closed and did you have a 24V signal to the spark module?
 
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Old 11-16-12, 03:20 PM
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I hear an echo, boilernut!
 
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Old 11-16-12, 03:32 PM
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oops! old age setting in again lol
 
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Old 11-16-12, 05:06 PM
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I really appreciate you guys helping me sort this issue.

Since changing the circuit board yesterday the inducer motor has only cycled on/off once before working fine during many tests. The different issues that I'm experiencing are 1. didn't turn on last night (i suspected a lockout because once I reset the breaker it worked fine) and 2. several times while testing the unit the inducer fan and the blower came on and the unit got very hot to the touch but never ignited or tried to ignite. To be clear the inducer motor and blower came on at the same time, no delay. Sequence was this: breaker on(t-stat already calling for heat when shut down)->inducer and blower on immediately->unit got very hot->never ignited->I shut it down

I know its supposed to go like something like this: call for heat->inducer motor->check pressure->gas released->spark->ignite->blower

During all tests I'm getting a closed pressure switch (even when it failed to ignite and got hot). I've never seen it shut down mid-cycle. The problem seems to only be during start up. It's like the circuit board fixed one issue but another popped up.

Reading this it sounds like I have no idea how to read a multimeter but I really can.
 
 

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