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Effectiveness/efficiency of HVAC circulate mode?

CrisPA | Posted in Energy Efficiency and Durability on

Last time I replaced a thermostat, I got an Aprilaire that has a ‘circulate’ timer. If heat or AC has not been used for an hour, the stat will run the fan for 15 minutes and then restart its timer. The claimed benefits are reduced stratification and improved AQ (more trips through the filter). On the other hand there’s the cost to run the fan and wear and tear.

Anecdotally, there does seem to be a reduction in stratification that’s most noticeable when the weather is in between heating and cooling temps.  It’s also nice to get a little almost-free warmish air in our corner bedroom after the night-time setback.

Has anyone seen data on real costs and benefits of circulate mode?

Thanks,
Cris (4A)

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Replies

  1. Expert Member
    BILL WICHERS | | #1

    This is more of a comfort issue than a question of efficiency. It is true that air will stratify, so upper levels in a multi level house may get too warm while the lowest levels will be too cold. Circulating the air will help with this, but so will careful adjustment of dampers to optimize where warm air goes during the heating cycle. In a really tight house, where the heat cycle isn’t running as frequently, the circulate mode may be more beneficial.

    You’re probably using several hundred watts worth of electricity to run the blower alone in the circulation mode. If it makes a noticeable improvement in the comfort of your home, than it may be worth it to you to run but it’s a personal preference and also somewhat dependent on your particular house too.

    Bill

  2. Expert Member
    Dana Dorsett | | #2

    The wind-chill effects of room-temp air circulation can lower rather than enhance comfort. Stratifcation effects would have to be pretty severe to make it "worth it".

  3. CrisPA | | #3

    Good points. A 3/4hp blower is going to draw ~ 550w, so not insignificant.
    The wind-chill effect is not a bonus in the winter but might act as a whole-house c*iling f*n in the summer. Whatever you may think about that...

    Thanks, Bill and Dana.

  4. bennettg | | #4

    I have timed circulation set up to run about 1/3 of the time. I have a variable-speed air handler with a 1/2 HP ECM. It runs at about 50% CFM in "fan only", which translates to ~80W for a 3 ton air handler. The above numbers seem more like the ballpark for an air handler with a PSC motor. Mine is rated @ 350W for full airflow @ 0.5" WC.

    I feel having the circulation is a positive, particularly in the shoulder seasons when there are not so many calls for heat or cool. I can feel the cool air when it runs in winter, but the air streams from the registers are directed away from "living" space where we sit, etc., and so aren't bothersome. I don't think the circulation does all that much for stratification, at least for my two-story-with-big-opening-between-the-floors house. It does help with circulating more air through the bedrooms at night, helping lower the CO2 levels there.

    If/when I get the house sealed up to the point that the overall indoor air quality begins to go downhill, the circulation timer would facilitate supply-side fresh air ventilation.

  5. ElectricHoosier | | #5

    We also have a 1/2 HP ECM air handler with our 2 1/2 tons heat pump and use the low speed fan (450 CFM- 50 to 60 watts) 100% at night in winter for circulation/comfort/air cleaning. Aprilaire 2400 filter changed once a year ($18-$20). The rest of the time it runs 5-10 minutes per hour, when heat pump is idle. We can't use low speed fan in summer (as desired) to eliminate upper/lower level stratification becaues it just evaporates the water on the A coil and sends the humidity level way up. Ceiling fans are used in the summer. 450 CFM low speed fan is hardly noticable. The same goes for 700 CFM in 'comfort' heat pump mode. Think slow, soft heat. The air handler is going on its 18th year of heavy use and still works great! I bet the compressor fails first.

  6. CrisPA | | #6

    Bennett,

    It is indeed a PSC motor on an old Goodman 80% furnace that might be twice as big as the house needs. I do think the circulation does help with air quality in the bedrooms.

  7. joshdurston | | #7

    I have an ECM fan, if I run con't fan i get a barely discernible bump in my total daily kw/h consumption (can check it on my utility website). There is a bump but I'd say it's close to 1wk/hr per day, raising my average daily from about 14, to about 15kw/hr.

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