During call for heat burners ignite and stay on for 45 seconds. 24vac is lost to the gas valve and gas valve is off for about 40 seconds and the cycle continues.
No error codes. I checked flame current during on cycle and it is 5 micro amps.

Any ideas anyone?

Thanks

Mike

 

Does the venter motor continue to run when this happens?

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Check the voltage drop across the pressure switch during the cycle; an intermittent voltage there indicates that the switch is opening.

Check all the connections.

Make sure that the furnace is properly grounded.

Check the line voltage when it's running.

If everything seems fine i'll find the service manual.

 

the inducer fan motor runs all the time during the gas valve short cycling.

 

I will do the checks you mentioned. I took off some duct work two weeks ago to clean my evaporator and I remember seeing the heat exchanger tubes and I think they were collapsed in certain areas. I think I might just take the heat exchanger out and inspect it.

If this is the case , how can this make the gas valve loose 24vac during off cycle?

 

The pressure switch confirms that there's a proper draft through the heat exchanger; if it opens when heating, the burner shuts down for safety reasons and the heating cycle restarts.

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You should pull the blower and take a look for underneath rather than removing the heat exchanger; if there are holes or cracks you should replace that furnace rather than repair.

 

ok. thank you sir. I'm going to test the pressure switch after the inducer motor starts by bypassing it and then see if the gas valve stays on. Even though there are no error codes during the short cycling maybe I can catch the pressure switch failing. Not going to remove the heat exchanger. I will remove the blower assembly as a last resort.

 

There could be an issue that's preventing the pressure switch from staying closed - don't leave it bypassed for more than a minute or so.

 

If you have a meter.... use that to monitor the pressure switch. This way you can positively confirm the pressure switch.

 

When I was monitoring the pressure switch with a volt meter during heating cycle, I saw no change . It stayed at 0 volts. I suspected it was failing to quickly for the meter to see it. So after the inducer fan motor came up to speed i bypassed the switch and the gas valve stayed on and did not cycle.

The port on the vestibule was partially clogged with dirt and rust. I cleared this port with a thin piece of wire. The pressure switch worked better and stayed closed longer but it would still fail intermitently. I installed a new pressure switch and problem solved.

Thanks

Mike