During a cold spell last week when temperatures got down to -23f (-31C) my furnace started showing issues with blowing enough air and the air was not very warm. Last winter it did not have any issues at those temperatures. I've done a lot of DIY diagnosing the last few days, here are my observations:
There is not an air intake problem, air is getting to the blower and burners just fine. Filters are newly cleaned (electric).
Exhaust vent pipe near furnace is very hot, too hot to touch.
Out going ducts are moderately warm just after the furnace, definitely not hot and does not seems close to as hot as they should be in my inexperience estimation.
The blower seems to be working properly, it sucks air from the intake side strongly.
Burners are running fine.
Thermostat works, furnace will turn on and off appropriately as I change the temp on it.
I pulled the vent covers off most of the vents and that has improved things considerably.
Air coming out of the vents is not very warm but good enough to heat the house with the covers off (does not heat the house enough with them on).
The furnace is a York 8.T 80,000/64,000 BTU model from 2005, model number PT8A12N080UH11B.
The house is 30 years old and we have been here a year. Right now I'm thinking there is some kind of blockage inside the furnace which is causing air to go out the exhaust instead if the ducts. Or the inside of the furnace is getting hot but not enough duct air is moving through so the heat is going up the exhaust.
Is this something I should try to fix myself or should I hire a pro? I have a significant amount of experience with DIY over the last 17 years. Cars and electrical mostly however.
After posting this I noticed that the furnace control board has an LED that was flashing. Checked the manual and it was an error code saying the pressure switch contact did not close after the inducer was energized. This means nothing too me but before I was going to call a pro I wanted to take a look inside. Pulled out the blower motor and everything looked spotless. Heat exchanger pipes are very clean and there is plenty of room for air flow, no need to worry about internal blockages.
After putting it all back together the error code went away and the LED is green (everything working). Maybe cutting the power caused the electronics to reset and everything is fine now. Seems to be working but who knows. If the issue comes back I will be having a pro come take a look at it.