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We just moved into our first house in September and it's old but surprisingly well insulated. We don't like it overly warm either so in the winter the thermostat is often set to 18°C/64.4°f The furnace is a fairly new gas furnace (Lennox ML193UH045XE36B) and unless it's well below -10°C it only needs to kick on every few hours. We've been finding that the air feels a bit stale and some areas end up with quite bad temperature dead zones. If the thermostat is at 18° sometimes one area will be up to 20°C/68°f while another will drop as low as 17°C/62.6°f.

We also both have moderate allergies and use air purifiers around the house and find the cleaner air isn't circulating very well.

I installed a new thermostat (replacing a very old mercury switch one) with a newer Honeywell RTH221B digital programmable one and afterwards I was fiddling with it and flipped the fan switch from auto to on. We were both surprised at how nice the air felt temperature and allergen wise and after referencing the blower motor power usage chart I figured it would probably only cost us $10/month to just leave it on a lot of the time

Unfortunately I was quite wrong. When the basement was finished the previous owner had the main duct from the furnace narrowed because it was easy to hit your head on. It goes from 8×15in (120in²) / 39×20cm (780cm²) to 4×20in (80in²) / 10×51cm (510cm²) reducing the duct cross section by 35%! I'm sure the other duct work isn't amazing either but I think this bottleneck makes the external static pressure at least 0.7-0.8wg so the furnace blower has to run essentially at max power in order to push air through the ducts. Because of this that month our power bill went up exactly as much as you'd expect if the blower was running at the worst possible efficiency (cost us an extra $44 if I recall), and that was with a MERV 4 filter. Long term changing the duct back is a possibility but I don't think this is guaranteed to improve things that much. (I'm giving all this information in case there's something quite obvious I've missed.)

Basically my question boils down to:

Is there a way, (including replacing the thermostat) to program the fan to, for example, turn on the blower on for 5 minutes and then turn off again for 15, repeat, without actually having the furnace produce heat?

Thank you!

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    There are all sorts of smart thermostats which should be programmable to do exactly what you're asking. You could probably do it with a "programmable" (smart, but not "smart") thermostat, too. Then it boils down to a a shopping question of "which thermostat(s) give me this functionality", and that's explicitly off-topic here.
    – FreeMan
    Commented Dec 15, 2023 at 14:48
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    I have a Lennox and run the fan continuously. It continuous setting can be set to four speeds, 25%, 50%, 75% and 100% of heating mode speed. If you set it at the lowest setting it should use less power and still circulate and filter the air. Check the manual. Commented Dec 15, 2023 at 16:03
  • Just leave the fan set to ON. Is is not worth the extra wear on the fan from starting more frequently. But some people don't like to leave the fan ON in winter because it will make a cold draft if the furnace is not heating. Commented Dec 15, 2023 at 17:14
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    Incidentally, the duct change doesn't make that big of a difference in cost, you'll only knock off about $10 of that $44 even if you dropped the static pressure to 0, which is unlikely
    – Joel Keene
    Commented Dec 16, 2023 at 7:39
  • VtC because this is attracting nothing but shopping recommendations.
    – FreeMan
    Commented Jan 16 at 19:26

2 Answers 2

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I don't think most (any?) of the "dumb" programmable thermostats allow what you want, but I know (because I have one) that the ecobee3 has a setting where you can ask it to run the fan X minutes per hour. It will run the fan for X minutes if the system doesn't call for cooling/heating during that hour. If it does call for cooling/heating during the hour, it won't run the fan by itself. I'm sure other smart thermostats have such a setting as well.

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    Nest has the same feature.
    – isherwood
    Commented Dec 15, 2023 at 17:03
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    Carrier "cor" thermostats also have this feature (they are built on the ecobee platform). Many higher-model Aprilaire thermostats also have it.
    – Moshe Katz
    Commented Dec 15, 2023 at 17:25
  • OK, having said that, IDK if the ecobee (or Nest) will do exactly what the OP asks, which is basically run 5 minutes on and 15 off. The ecobee does it on an X minutes per hour basis, so it'd do 15 on and 45 off.
    – Huesmann
    Commented Dec 16, 2023 at 14:19
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Nest (which I have) lets you run on a schedule or as a one-time operation. Either way, you are limited to 15-minute increments. So, no 5 minute option (I'd use this too, if it existed). Fan control with a Nest thermostat - Google Nest Help

Ecobee, on the other hand, seems to do it not with a schedule but by setting a minimum run time of 5-55 min per hour. Looks like you can do a one-time run in increments of 15 minutes. ecobee Fan Runtime Explained (9 Key Things To Know) Fan settings on the ecobee Thermostat

Had I known this, I probably would have bought the Ecobee. 🤷‍♂️

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  • To be fair, a 5-minute interval is probably not productive. You need to turn over a significant portion of the room for it to be worthwhile.
    – isherwood
    Commented Jan 16 at 20:40

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