Furnace pilot many cycles before lighting - most of the time

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Old 01-01-18, 12:46 PM
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Furnace pilot many cycles before lighting - most of the time

Frustrating problem on Bard furnace.
Hoping someone can help. Here are the details.

Thermostat call for heat
Air clearing fan kicks on
Few seconds later ignitor starts
Pilot lights just for a second then goes out as well as ignitor and air clearing fan stop
Within a couple of seconds, air fan starts again
It continues this on and off cycle for 1 to 8 times before it finally stays lit and operates normally.

Noticed that it seems to be most problematic after the heat has been off for a few hours.

Honeywell Control board was changed about 10 years ago - worked for years
Honeywell Gas valve changed about 7 years ago - worked for years.

Began the cycle above

I changed out ignitor (not spark wire). Installed proper LP orifice.
No change
Furnace installer adjusted pilot and said it was good. It was not. After some time when the furnace was off, it began with the cycle above again.
Installer came back and said it was the Honeywell S8610 unit gone bad. Swapped new unit and all worked; until it had been off for a while and the cycling began again.

Checked ground wire from new ignition control module; but that was good.
Also checked vacuum switch that is made once the air clearing fan starts.

I'm stumped, upset, and poorer with nothing to show for it.

Any advice would be appreciated.
Thanks
 
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Old 01-01-18, 12:54 PM
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Can you see the pilot once it lights ? It needs to be a good sized flame to enable the burner to start.
Does your unit use the spark rod as a flame sensor or is there a separate flame sensor ?
 
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Old 01-01-18, 04:41 PM
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Yes, I see the pilot light and mine uses the spark rod as the flame sensor.

The oddity is that when it goes through that cycling process, it lights the pilot and within that second shuts down the pilot flame, ignitor, and air clearing fan.

Thanks
 
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Old 01-01-18, 04:46 PM
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I did try adjusting the pilot up at one time and down at another. Both times it seemed to light fine, but the next time it sat for a long time - it again started the cycling.
 
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Old 01-01-18, 05:42 PM
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It almost sounds like the flame sensor is not sensing the flame and shutting down the system. You can try to clean your flame sensor,
 
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Old 01-01-18, 06:35 PM
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The oddity is that when it goes through that cycling process, it lights the pilot and within that second shuts down the pilot flame, ignitor, and air clearing fan.
That's not a flame sense issue. When the flame sensor doesn't sense the flame it shuts down the pilot lighting sequence but it doesn't shut down the draft inducer.
 
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Old 01-01-18, 07:16 PM
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I just installed a new sensor 2 weeks ago - with no change.
 
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Old 01-02-18, 01:28 PM
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Can this have anything to do with the Nest thermostat. I have read some details relative to having no C wire to the thermostat and mine does not have that C wire. According to Nest - it should work without one.
 
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Old 01-02-18, 01:47 PM
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You can try removing the nest and jumping the stat wires and see if it runs normally.

If it does you have the culprit.

Just a thought.
 
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Old 01-02-18, 02:05 PM
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I had a similar issue a few years ago and it ended up being a small piece of debris in the exhaust blower port that the hose pushed onto to connect to the vacuum switch. Stuck in a drill bit to clean out the hole in the blower and reconnected the hose and away we went.

You said you checked the switch, but were you just verifying the switch was operational or that it was actually being tripped when the fan started?
 
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Old 01-02-18, 03:02 PM
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I'm assuming that means to jumper between R and W but want to confirm so that I don't mess anything up.
 
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Old 01-02-18, 03:12 PM
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I had checked earlier on the switch by putting vacuum to it and pinching the hose; which worked and showed that there was no vacuum loss. The switch stayed that way until I release it. I did check to see that it was getting tripped with the fan - and it is.

I just did another test of voltage at the circuit board on the furnace.
I first checked my voltage at the transformer - 26.5
I initiated a call for heat with the thermostat and the the R terminal on the board went to 25volts

I then started over and put the meter connection on the W (heat) terminal; then called for heat.
The voltage began to rise and then immediately cut off - which is a part of the cycling issue that I've been having - which shuts down induction fan and spark; immediately restarting the cycle. As stated earlier, these cycles can be 1 to 8 times before it completes lighting. This time it only cycled the one time and the next cycle lighted fine.


I let it cycle through this
 
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Old 01-02-18, 04:26 PM
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It's not a pressure switch/inducer problem. If the problem was there..... there would be NO ignition at all.

Check from C to R during the call for heat. That should not drop more than several volts. 27 down to 24-25 is fine.

Now check from C to W. If this is the voltage that is sagging real low then it is a stat issue. You can temporarily jump from R to W to confirm proper burner operation.
 
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Old 01-02-18, 06:02 PM
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I checked from C to R while calling for heat - it dropped to 25.2

C to W went to 23.5

Since some Nest issues have been related to not having a C wire - does that mean I should think about using a spare wiring harness wire and connect C on the circuit board to C on the thermostat?
 
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Old 01-02-18, 06:08 PM
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C to W measured at 23.5vac is fine. Did the furnace not run at that voltage ?

I always use a C connection to any electronic stat.... especially a wifi type that needs constant power.
 
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Old 01-02-18, 07:00 PM
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As far as the furnace running, it finally did, but again it was following several of the cycles.

So is it as simple as I stated above - run a wire from C on the board to C on the thermostat. If so, why does that make a difference since it doesn't change the voltage at R.
 
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Old 01-09-18, 05:44 PM
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Update, I worked with the Nest thermostat guy and bypassed the thermostat all together and the furnace still continued to go through the cycling process - induction fan starts, spark starts, pilot quick flash, all shuts down and starts the cycling again - from 2 to 8 times before it finally lights.

Any other options that anyone can think of?
 
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