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?



  • Can you tell us more about which unit this is? Some have a minimal combustion air intake temperature. Also the power rating, size of your house and insulation may give clues about if that unit is maybe just undersized
    – PlasmaHH
    Nov 28, 2014 at 11:57
  • 1
    Are your furnace ducts in the attic? That might do it, especially if there's a duct or two that are disconnected or the system is leaky.
    – iLikeDirt
    Nov 28, 2014 at 15:35

2 Answers 2


I'm going to suspect poorly insulated ducts running in unconditioned space as a first guess. I don't know the layout of your system, but it's very common to take duct runs up into the attic, and even if they are wrapped with duct wrap (R3-5 typically) that's abysmal. If the ducts are poorly wrapped or not wrapped in spots, it gets worse. R-50+ is far more appropriate for hot air in a cold attic.

As for the furnace exhaust, it should be fairly warm - otherise you get to visit the series of questions that are on here about furnace exhaust (high-efficiency condensing) freezing up in low temperatures and shutting the furnace down.

Fundamentally, if your furnace is running 24/7 and the temperature continues to drop, it's undersized for the load - but one of the best ways to deal with that is to reduce its load by improving insulation and air-sealing (caulking things - the most boring, most bang-for-the-buck way to improve efficiency in most houses that have not already had that done). You could also add a second furnace to take some of the load, but then you'll be burning even more gas. You do not want a furnace that is much larger than your "design load" (ideally, your furnace runs 24/7 and maintains temperature on your coldest days) since an oversized furnace is inefficient .vs. one that's just the right size.

  • I'd add to the first sentence of your last paragraph "...undersized for the load, OR IS NOT OPERATING PER THE MANUFACTURERS LITERATURE"
    – Jim
    Dec 28, 2017 at 16:15

There should be a nameplate on the unit, which among other things will list the expected temperature rise across the heat exchanger. This value tells you how much the furnace should be expected to raise the temperature of the incoming air.

Furnaces do not put out an exact temperature air, the outlet air temperature is always based on the inlet air temperature. For example. If you have a furnace with a 60°F temperature rise rating, and an inlet air temperature of 65°F. You should expect 125°F air coming out of the furnace. If you have an inlet air temperature of 45°F, going into the same furnace. You should only expect 105°F air to come out.

Add to that the fact that some ducts might be run in exterior walls or unconditioned space, and you could be losing heat before the air actually reaches you.

It sounds like either the furnace is undersized, dirty, old, or the home is not insulated well. Make sure the filter is clean, all registers and returns are open and unobstructed, and have the heat exchanger cleaned and inspected by an HVAC technician.

  • It would be interesting to measure the heat rise and see if it's high enough right there...
    – rogerdpack
    Dec 10, 2018 at 4:11

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