I’m in a pretty cold state. It’s been below zero F quite often this winter. My furnace has not been keeping up and my house has gotten pretty cold. I got some space heaters to supplement.

Furnace is about eight years old.

But I started to think that I’ve never had this problem in past winters. Maybe furnace needs a tune up? I always keep a clean filter and air volume from vents was fine. All gas jet thingies were fired up and looked fine.

So I started thinking that this past summer I switched my system to always ON because I thought maybe constantly circulating the AC on the house would keep the temp low with less energy. Plus I didn’t like the system turning on and off when I was trying to fall asleep.

This is a 100 year old house with no insulation. I retrofit the central AC and heat in 8 years ago.

So I’m trying to figure out why the heat won’t keep up and I decide to switch the system to Auto. We had a really cold night and it kept up just fine. The house is hot now!

Is this normal? If not is it indicative of a problem?


Running the fan reduces stratification, meeting that it makes the temperatures in the house even. The thermostat being centrally located in a house is often satisfied before the remainder of the house is able to warm up especially in an old house where the heating registers usually are centrally located what's return air located at outside walls. In a house with no insulation the exterior walls lose heat much faster than the heat can be replaced from the ambient air within the house. Basically it's an indication that you have a poor duct system which is not uncommon in an old house.

If you let me know the state, square footage not including the basement if it's below ground, the furnace model number or the size in btu I can tell you a little more about your system.


Although the above statements are very accurate I did interpret your question in reverse. I read it as, you were having a problem maintaining the temperature with the fan set to auto.

Having your issue with the fan set on would normally be a real head-scratcher. But in this case I would say you need to have a look at your Ecobee. It's a fantastic thermostat, I have one too and there are a lot of features available to tweak your system. If you have multiple sensors and and they are satisfied before the main thermostat the system may shut off prematurely.

This is a screenshot of my ecobee enter image description here

If the sensor upstairs reaches the temperature set point well ahead of the main thermostat I think ecobee uses some kind of average to determine when to shut it off.

To determine if this is the issue I would suggest turning off all the sensors except for the main thermostat run the fan set to on and see if anything changes.

  • The thermostat is on the main floor. Set at 62 F it could not hit 60 with the fan always on. It stayed in the 50s. But set to Auto it’s hitting 62 just fine; even getting up to 68+. I’ll get some measurements and the furnace model. When we had the hvac installed they ran new ducts. First floor vents are in the floor, second floor vents are in the ceiling. Second floor is always warmer than the first. I also was running an Ecobee if any of the data helps. Thanks! – jqning Mar 8 at 5:41
  • How many sensors do you have on the ecobee? Are you using the "follow me" feature or "geofencing"? – Joe Fala Mar 8 at 13:45
  • Joe: I have one sensor, but I am not using follow me. The sensor shows up but does not influence the thermostat. The guy below with the comment about ducts makes sense. I will reply there. – jqning Mar 8 at 17:41

It depends on the model of the furnace. Some have multiple stages in a fixed "on" state the system may not be able to change to a higher stage. If it is a single stage furnace then the duct work may not have sufficient insulation and constantly moving the air through the unheated poorly insulated spaces is cooling the air.

  • That's a good point, we just use spray foam on attic ducts here but I'm sure in other areas people are still wrapping. Even very well wrapped system still needs re-taping in spots every few years. – Joe Fala Mar 8 at 15:01
  • This makes a lot of sense. The house has an unfinished basement which is usually around 50 degrees. The ducts for the first floor are in the basement. The air gets to the top floor up through one duct and then travels through the ceiling to the vents, which are all in the ceiling. The ceiling is insulated with sprayed-in insulation. It makes sense that the unheated air is getting cooled too much as it moves through the basement and also through the ceiling! – jqning Mar 8 at 17:45
  • It's code where I live and I'm almost certain it's code where you live, that duct in unconditioned space needs to be insulated. I'm not sure what the minimum is I just get r24 sprayed on my ducts. – Joe Fala Mar 8 at 17:55
  • It may be code but I have seen many metal ducts that are not insulated especially in older homes. – Ed Beal Mar 8 at 19:37

I have lived in 2 houses over the last 40+ years and in both houses the furnace fan ran 24/7 except on a few days in the spring and fall when my wife likes to open the windows and "air out" the house. My thermostat, an older mercury type, has been set at 72 degrees F. in the winter and 75 degrees F in the summer. The temperature settings are constant 24/7. All the rooms in my ranch home are relatively the same temp with very little temp difference. I have an electronic air cleaner + a pre-filter. The furnace fan is a 3 speed which utilizes the lowest speed for continuous air circulation and the middle speed for heating and the high speed for A/C. In your house, you said that if you set the thermostat to 62 degrees then how could the house temp go to 68 degrees unless the thermostat is defective? Running the fan 24/7 should not reduce the heating in your house unless there is more to the story. Where exactly are the supply registers located in each room and where are return grills in the rooms and at what elevation. If you are thinking about energy conservation the best money spent is on insulation first and a window up grade next.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.