1 hour lockout cycle, diagnosing reason

[Beezfun]

Need help diagnosing my furnace once it goes into 1 hour lockout -

Got up this morning, house cold and furnace off. Pulled thermostat and jumpered Rh to W, nothing. Went to furnace, jumpered Rh to W on circuit board, nothing, not even draft inducer starting up. Noticed the green "OK" LED on the circuit board was flashing, but didn't have sense enough to count how many times. Turned off power and pulled all thermostat wires. Turned on power and jumpered Rh to W, now furnace comes on normally and works. Turn off power, reconnect thermostat wires, jumper Rh to W and it works fine. Go upstairs and connect thermostat to base unit, furnace doesn't come on even though the thermostat is calling for heat. Back down to the basement, there's no 24v at terminal W. Turn off power, pull thermostat, check pins on back of thermostat and Rh has continuity to W. Not sure what's going on, so I put the thermostat back on the base and heat comes on, everything working fine.

After staring at furnace documentation, I think what happened is furnace went into 1 hour lockout mode for reasons not yet determined. The 1 hour lockout gets reset by power cycling, so that's why it worked after I turned off the power. I'm looking for help diagnosing the cause next time this happens. Obviously I'll count the LED blinks to see what the error code is, but if it's in lockout mode, what do I do for further * I'm confused about thermostat issue, it's relatively new, seems to work fine, not sure why it had that glitch. My technical skill level is pretty high but never worked on furnaces before.
thanks

[4ftBanger]

We'd really need the make and model of furnace.

Hard lockouts generally are related to ignition failure after multiple retries so it could have been an issue with gas delivery to the burners, or the flame sensor not properly sensing ignition. It seems unlikely that everything would start properly after reset though so I expect you errored out on a either a pressure switch error or a limit switch.

I've had pressure switches fill up with condensation and fail - you can sometimes take it out and hold it upside down to drain it but usually requires a replacement soon after.

If it's a limit switch, check for dirty filter, dirty ac coil or anything that could be restricting airflow. You can measure the temperature of the air leaving the furnace - Most are designed for a max of 55 degree rise, so if it's 70 in the house and you have supply air temperatures over 125 (70+55) then you arent moving enough air through the heat exchanger.

Keep us posted if you learn what the error code is.

[Beezfun]

Thanks for suggestions, comments below inline.

4ftBanger -
> We'd really need the make and model of furnace.

Rheem RGPH upflow, circuit board is UTEC 1028-927

>
> Hard lockouts generally are related to ignition failure after multiple retries so
> it could have been an issue with gas delivery to the burners, or the flame sensor
> not properly sensing ignition. It seems unlikely that everything would start properly
> after reset though so I expect you errored out on a either a pressure switch error
> or a limit switch.

Flame sensor LED was flashing yellow, manual says it should be on solid, so I cleaned off flame sensor and now yellow LED is on solid. That might have been part of problem. In your experience is cleaning enough, or should I replace it now before it *

>
> I've had pressure switches fill up with condensation and fail - you can sometimes
> take it out and hold it upside down to drain it but usually requires a replacement
> soon after.

Good suggestion, I stared at troubleshooting flow chart and schematic and see that the induced draft motor won't come on if it senses a closed pressure switch, so that explains why nothing happened when I jumpered Rh to W. It's still fairly warm here so I can live without a furnace if I screw things up. I think I'll take the pressure switch off and see if it has any moisture in it. Next time this happens I'll look at the blinking LED, two of the codes it flashes show open or closed pressure switch, then I'll know for sure.

> Keep us posted if you learn what the error code is.

Will do, thanks

[4ftBanger]

A standard flame sensor (aluminum/steel rod) can usually be cleaned a couple of times before it has to be replaced and I've cleaned them before and had no flame sensor problems resurface for over a year. Cleaning with sandpaper or other abrasives causes grooves and scoring that make it collect dirt faster - Believe it or not, the Trane rep instructed us not to use anything more abrasive than paper money to clean a flame sensor so we all carried a $1 bill among our service tools.

[MTI STUDENT]

We are also taught at school to use paper such as money to clean a sensor , also in this case would inspect the HSI for white spots , it could have a crack that will cause intermittent problems.

[heatseeker]

sounds like your t-stat is malfunctioning. you can reset the lockout switch by turning the power off and on.

[Beezfun]

heatseeker -
> sounds like your t-stat is malfunctioning. you can reset the lockout switch
> by turning the power off and on.

Bad thermostat was my original thought, but when I took it off the wall it showed continuity between Rh and W pins when calling for heat. And when I physically jumpered Rh to W nothing happened. So I knew something else was going on. Is it likely for a thermostat to fail when it has 24v attached to it, but show continuity when it's not connected to the * I've stared at this schematic and can't convince myself whether or not the lockout mode isolates the 24v so it isn't available, that would make it look like the thermostat wasn't sending down 24v. That would explain why I didn't see it on the board.

[heatseeker]

yes it is not t-stat- have you figured it out *

[Beezfun]

heatseeker -
> yes it is not t-stat- have you figured it out *

It hasn't happened again so can't say for sure. In retrospect, I think the flame sensor was bad enough that it led to the 1 hour lockout cycle. That's why nothing happened when I connected Rh to W. Once I started turning power on and off, and pulling thermostat on and off base, who knows, maybe I got confused about what actually happened. I did look at pressure switch, contacts were open as they should be when furnace is not calling for heat. Unless problem repeats, not much more I can do.

[heatseeker]

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