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I have a house with four heating zones, three of which are pertinent to this question. A standard summer day might look like this:

  1. Top floor - 23c-24c (slightly too warm, AC on most of the time on warm day)
  2. Main floor - 22c (just fine, AC on sometimes)
  3. Basement - 18c (slightly too cool, also too humid)

Note that my home is in southern Ontario.

I'm thinking I should run the furnace fan 24/7 in the summer (and perhaps winter) as it would move warm air from upstairs into the basement and move cool air from the basement into the top floor. It may also help regular humidity in the basement, though I'm not sure.

A quick Google Search suggests running the fan all the time might cost me 45$ a month or something. This is not a negligible sum, but if it prevents the AC from running to cool the top floor it might be worth it.

Also of note is that I have a large, efficient wood burning fireplace on the main floor. In the winter this will heat the main floor well, but not other floors. Circulation could be nice here.

Does it make financial sense to run my furnace fan 24/7? Or perhaps 50% of the time? Either way, should it run as fast as it can go (assuming I tolerate the noise)?

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    I had a customer who wanted HVAC fan running 24/7 (from 4am Monday to 5pm Friday there was someone in the building all the time between office, sales room and warehouse). New system - fan wasn't running at night except if heat or AC was running. Took the computer guy (me) to figure it out - even though temperature was set same for all time blocks (occupied/unoccupied), thermostat was not obeying "fan always on" in "unoccupied" times. I made the unoccupied times the minimum 15 minutes each & fan ran 23.5/7 and customer was happy. TL;DR If you do this, it may be more complex than you expect!
    – manassehkatz-Moving 2 Codidact
    Commented Sep 27, 2021 at 1:09
  • @manassehkatz-Moving2Codidact this is a residential Trane unit and I "believe" I just set the unit to "circulate" with an uptime of 100%. Perhaps it won't be that easy though! Do you have an opinion on whether this is a good idea?
    – Behacad
    Commented Sep 27, 2021 at 2:32
  • The cost is, relatively speaking, not much. The issue I dealt with was a commercial install but with (in my opinion) a cheap (likely not US-made based on the low quality of the instructions) thermostat - the problem was the thermostat, not the HVAC itself. A lot of places have run fans 24/7 (or as close as they can get) for the last 18 months to provide better airflow/filtration but unless you have a really good system and high-quality filters, I think the effects are more "comfort level" (which is what my customer wanted) than anything else.
    – manassehkatz-Moving 2 Codidact
    Commented Sep 27, 2021 at 2:36
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    I just had a new HVAC system installed. My installer indicated that we should leave the fan running 24/7 as newer systems are designed for it. Ours has a variable speed blower and blows low until heat or AC kicks on, at which time the blower ramps up. After a couple of weeks, we decided we can't stand it on all the time and set it back to "auto". If you have a newer, variable speed blower, it may work well for you. OTOH, you may also decide you don't like it blowing all the time... TL;DR: try it and find out.
    – FreeMan
    Commented Sep 27, 2021 at 16:47
  • @FreeMan Yes this is a newer furnace with variable speed blower. I don't think "trying" it will do much for me, since I know I can accept the noise at 50% power. At this point this is an economical decision. Does running the fan actually prevent my AC/heat from running by spreading the load around?
    – Behacad
    Commented Sep 27, 2021 at 22:35

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If the ducts are in a cold enviroment such as attic or crawl space, having the fan on all the time will circulate cold air throughout the homes interior, causing the heating system to cycle more often. My father in law had a new system installed in a dual zone ranch style home, the installer left the system in fan on mode resulting in a $200.00 increase in the monthly heating cost.

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