I have a Lennox central gas furnace. It is only about 3 years old, and was on the very high end of the models available in terms of energy efficiency and such. I sprung for a more expensive model firstly because I got rebates from all my energy companies, and secondly because I actually could afford and wanted to have such efficiency!
Up here in Minnesota we can have some bitterly cold days. It's not uncommon for temps to be well below 0. For example, today the ambient outdoor temperature was minus 5 degrees.
I've noticed that when it gets this cold outside, the heated air coming from the ducts is a lot cooler. It's not cold, there's still very definite warmth to it - this is not an issue with the furnace blowing air without the flame going or anything like that. However, it is certainly nowhere near as warm as it normally is.
When the temps outside are around, say, 30 degrees or even as low as the high teens, the furnace pumps out nice toasty warm air and keeps the house warm running less than 25% of the time. However, once temps drop below 0, the furnace can run 24/7 just to maintain a temperature of 66 degrees! If we get lower than say 10 or 15 below, the furnace basically loses the battle, and will run 24/7 even though the house temp will slowly, slowly, slowly creep downward. Again, the air coming out of the ducts when this happens is significantly cooler, but not cold. Luckily lately we haven't had days that cold in long stretches, but last winter there was a period of maybe a few weeks where the average temps were below 0, and I ended up turning off the water mains and temporarily living with a friend, because it was clear the furnace wasn't going to warm the house back up.
The furnace has two PVC pipes going to the outside - obviously one is an intake and one is an exhaust. The air coming from the exhaust pipe feels a lot warmer than the air coming out of the ducts - however I admit this might be deceptive since it's so bitterly cold outside that anything probably feels warm.
All of the flame systems are working fine. The sensors are good, the ignition goes off without a hitch every time, the burning flame is clearly visible through the observation port, and my gas bill shows that the gas company certainly feels that I'm using a lot of gas. LOL
Any ideas why the air coming out of the system is so much cooler when it's extremely cold outside?