Last Thursday, we had the 4th service call on our furnace since last October in an attempt to fix the problem that’s causing it to flash the pressure switch error code. This time, we had a different technician come to look at it and he had some new ideas on what could be causing the problem.
He started by cleaning out the trap in the drain. Since this had already been done on the first service call last fall, only a small amount of nasty-looking gunk came out of it and it wasn’t as blocked as he’d expected, so this was likely not the cause of our problems. He then tested the drain itself by pouring a pitcher of water down it to make sure there were no backups. Since the drain accepted the entire pitcher with no problems and that’s more than the furnace would ever drain, he ruled that out as well.
According to this technician, a pressure switch error could be caused by a blockage in the intake, exhaust or drain. Since he thought he’d heard water backing up in the furnace when he first turned it on (something that’s not supposed to happen) and the drain appeared to not be the problem, he proceeded to investigate the intake and exhaust to see if there was a problem there.
Next, he looked at all of the PVC piping for the intake and exhaust to make sure that everything was sloped downhill so there would be no place for water to gather and pool. Finding nothing that looked suspect there, he then took a sawzall to the exhaust pipe, cutting it in half so he could look up and down it with a flashlight to see if there were any obstructions. Had a bird built a nest in the exhaust pipe, it could be blocking just enough air flow to cause the pressure switch to flip. Unfortunately, though, there were no obstructions in that pipe, so it seemed to not be a problem.
Since the tech wasn’t able to duplicate the problem and the sound of water backing up in the furnace didn’t appear to be happening any more, he was having some trouble pin-pointing the cause and left our house on Thursday with the intake unhooked from the furnace so that it would draw air from the room rather than outside. The idea was that, if the furnace didn’t have the problem while the intake was unhooked, we would wait several days, reattach it, and if the problem returned, we’d know the blockage was in the intake. If the problem occurred even when the intake pipe was detached, then we’d know we could rule that out as the cause.
Less than a half hour after the technician left our house, the furnace shut off again, with the light blinking 3 times, indicating a pressure switch error. Since then, we’ve had the problem numerous times. Last night and again this afternoon, it’s been so bad that we haven’t been able to get the furnace to come on at all.
According to the technician, the next steps will be to replace the pressure switch itself and then get Trane technical support on the phone to walk him through some diagnostics on the furnace to see if they can determine the problem. At least there’s a plan going forward … I just wish it were a more obvious problem!
An interesting side note: the technician commented on the number of registers we had closed in the attic and said the recommendation is that no more than 15% of the registers be closed at any time. Closing more could result in the heat exchanger cracking, which wouldn’t be covered under warranty.