Thought I would pass this along in case anyone else ran into a similar situation and was short on ideas.
Our primary heat source is a coal stove and we have a forced air lp furnace as a backup. I had a problem with the shaker grate on the coal stove last week so I wasn’t able to use coal until I got it welded. No problem, I thought, the furnace can keep things warm for a couple days.
It is fairly new, a 6 yo high efficiency Bryant that has the exhaust and combustion air vents PVC piped through the wall. Well I was busy painting the kitchen and finally noticed it felt cooler than usual around 9. Checked the thermostat and it was 65, supposed to kick on at 68, hmmmmmm. I checked the led on the control board in the furnace and sure enough it was blinking an error code related to the combustion air pressure. Could have been one of 10 possibilities. Went through about half of them with no luck, so I brought in a load of firewood and built a wood fire in the stove (don’t need the shaker grates for the wood fires). Decided to sleep on it and try again in the morning.
The next morning I decided to blow out the combustion air pipe before calling the tech. On my unit the pipe is connected to the bottom right of the fan housing and there is a capped off port on the top left of the housing. I took the cap off this port and directed my leaf blower into this port. My wife watched for any debris to come out of the pipe outside. Left it blow out for about a minute, nothing big came out, only PVC shavings from when the pipe was cut to fit during installation. Replaced the cap and turned the furnace back on and wallah…..it worked and had been working ever since. I make sure to turn it on once or twice a day to make sure it fires up.
There are a couple small tubes from the combustion air pipe going to a couple sensors, so my theory is that one of those PVC shavings was blocking one of those tubes just enough to set the error code. Got the shaker arm welded in the coal stove so the furnace is again a back up.