When the thermostat asks for heat, the power and OK LEDs light, the inductor blower runs for about 30 seconds, then shuts off with the OK LED flashing twice. The gas valve does not open (no clicking) and the burner of course does not light. If I open and close the lower door to cycle the door switch, same thing happens. If I also open the upper door, the furnace will run normally- gas turns on, burner ignites. This has been a problem with this furnace for 8 years, except that it ordinarily happens just once, the first time we turn it on at the beginning of the cold season. Then I'd just go open the doors and close it up, and it would run the rest of the season. This time, it's not yielding so readily.

I had a service technician out about 4 days ago for annual maintenance. He apparently cleaned what needed cleaning, and declared everything to be in good working order. I described the problem (above) to him, because the furnace was working at the time and I couldn't demonstrate it. He said the original vent installation was incorrect in that it is 2" for the entire length (maybe 20 feet total), but measured flue pressure and said it was ok. Couple of days later the furnace has become very obstinate about running.

About 5 years ago I had a tech look at the problem, and I *think* he replaced the vent system pressure switch. The furnace ran fine afterward, but the next year, same problem. He left a pressure switch on top of the furnace, which is marked as a 0.4 wc; the one in the furnace is a .8 wc. If I look up the part number of the .4, it seems to be the correct one for this furnace, but I'm not sure.

Any ideas? Should I go ahead and put in 3" venting? Change the pressure switch?

 

Usually there is an index that describes the meaning of the various diagnostic codes t6hat can be found on the inside of the fan compartment cover or perhaps on the ignition control module.

Look for what that code means and post it here.

 

I don't see a listing of flash codes posted in the furnace, but an Internet search says
2 Flash: Pressure switch or induced draft blower problem detected.
While that makes sense, how do I actually isolate the problem? Bad or incorrect switch, bad blower, or can it be the improper vent plumbing? Does the fact that it operates with the upper panel removed point to anything in particular?

 

Check on the back of the blower door for a schematic, it should be on there. You need to know specifically what your unit has for those codes, not what is off the internet.

 

Yes, there is a schematic. No, there are no diagnostic codes listed.

 

Amazingly enough, rooting around in my old files I found the installation instruction manual. 2 blinks means pressure switch is open.

 

Now that I've found the manual, I may be answering my own questions. Turns out the manual says the 2" vent is correct for this furnace, so I should be able to drop that consideration. If I follow the troubleshooting flow chart, that leaves me with either a blocked vent or a defective pressure switch. I'll check out the vent tomorrow, and if it's not blocked, I'll replace the pressure switch and see what happens.

 

Make sure your drain trapand all hoses are not plugged also.

 

The probability of a bad pressure switch is about 5%. Basically it's a waste of time and money to replace a pressure switch without testing it or checking out the other far more numerous likely issues.

So if your own diagnostic efforts don't pan out, try HVAC tech's suggestion of inspecting the trap. If that doesn't solve the problem, post your various efforts here and we'll suggest other things to check as possible problems.