Our furnace supply vents are in the middle of the house and the returns are on the outside - which I understand is 'backwards'.

When the furnace is running, plenty of hot air comes out, but seems to struggle to heat the outside areas of the house.

Between cycles when the fan is 'on', it seems to only blow cold air out the supply vents.

My question: Is it still efficient / worth running the fan in between cycles if they really make the house seem 'drafty', or should I leave them off?

House info:

We have 1200 SQFT single family cinder-block house with hardwood floors. We recently had the crawl space conditioned and 18" of blown fibreglass put in the attic (along with all leak sealing, etc.) Our furnace is a 85%(ish) efficiency Lennox gas / electric blower.

We have been keeping the house at 68F during the day and 62F at night, which is most comfortable when the temperature is 25F and over.

I've been running the furnace up to 70F when it's really cold (20F and below down to 5F during the day, 5F to -10F at night) in an effort to heat more of the house up a bit more.

Updates based on questions in comments:

  • the air is 'cool' but not 'cold' like it's coming from the outside - it feels like it's just pulling air the cold air from the vents that are sitting next to the walls that won't heat up

  • the furnace is in the crawlspace, and within the conditioned crawl space (which is a vapor barrier on the floor and an insulated vapor barrier on the crawlspace wall with the space between the floor joists and the crawlspace walls filled with spray-foam)

  • and I was told and have read that running the fan in between cycles keeps the warm air moving around the house - but I'm starting to doubt that's a good practice in my case...?

  • How cold is the cold air coming out? Obviously while the furnace isn't on, you're not going to get heated air. Is the furnace in the crawlspace? What's the ambient temperature in there? When you say the crawlspace is 'conditioned' do you mean it's been insulated? Are the vents inside the insulation of the house? Eg: sometimes they are run through the attic, above the insulation, so they're constantly being cooled by the cold attic air.
    – gregmac
    Commented Jan 5, 2017 at 2:21
  • And why are you running the fan constantly anyway? I have mine set to run a minimum of 10 mins per hour (so basically the thermostat will run it if it otherwise hasn't run while heating/cooling), but my house has large south-facing windows and there's a very noticeable temperature gradient from the north to south side -- running the fan helps minimize the difference.
    – gregmac
    Commented Jan 5, 2017 at 2:23
  • Thanks for the response @gregmac - I've added answers to the bottom of my question... Commented Jan 5, 2017 at 2:25
  • If it's an old house built back before the 50's it probably had a coal furnace once and supply vents are in the middle of the house and the returns are on the outside.
    – Richard
    Commented Jan 5, 2017 at 2:49

2 Answers 2


You're right--it's not a matter of efficiency, but of comfort. I'm in a similar climate, and the only time I run the fan between heat cycles is if 1) I have the fireplace burning and it gets very warm in some parts of the house, or 2) I have a lot of company over and we need air circulation.

Otherwise, it just makes the house feel cold. I wouldn't do it. Granted, I don't have your reversed airflow path, so I may not have issues with window icing and cold zones to the degree that you do.

I'd look into having flow reversed. It's probably a fairly inexpensive job to reconfigure the furnace plenums, and you could tackle the vent swaps yourself. That is, of course, if the duct capacity is appropriate.

  • Ah! That is an incredible idea - I'm going to talk with my furnace guy about that!!! Many thanks!!! Commented Jan 5, 2017 at 17:23

When your house was built it was common practice to install the return grills on the outside wall under the windows and the supply registers were on the inside walls. Years later the supplies and returns were reversed to the way they are now. Yours are not backward, that is how it was done then. My house was built 18 years ago and I run my fan all the time to keep all the rooms the same temperature. My furnace has a multi speed motor which allows the blower to run at a lower speed for constant air movement and a higher speed for heat or A/C. All furnaces can utilize a multi-speed motor to accomplish this. It is possible to utilize 2-4 different speeds depending on the type of motor used. I would have to ask if your outside walls are insulated, since cinder block has an R value of about 2.5 which would cause the outside walls to be very cold. And no, very few houses built before 1950 had coal furnaces unless you lived in the middle of nowhere.


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