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Fixing a central-fan-integrated supply ventilation system

joeinva | Posted in Energy Efficiency and Durability on

I’m in climate zone 4A (VIrginia). My house has an upper and lower HVAC system. The lower system has a central-fan-integrated supply ventilation, roughly like the first diagram in this document:

[Editor’s note: It appears that the poster intended to include a different link — this one: Central Fan Integrated Supply Ventilation—The Basics]

except that:

1) I don’t have a motorized damper
2) I don’t have a balancing damper
2) I don’t have a FanCycler
3) I don’t have a filter on the external air supply

Unsurprisingly, my house tends to be a bit humid in the summer, which I control with basement dehumidification, and dry in the winter, which I suffer through. As far as I can tell, one reason that’s the case would be uncontrolled supply of exterior air.

(It is also true that the house, while of recent construction (2011), is probably relatively leaky. And my other HVAC system is an an unconditioned attic. It’s on my list to make the rest of the house tighter.)

Now, my question is essentially – how do I fix my overventilation situation? Just find someone to install #s 1-4 above?

I should also note that I have, in addition to the usual assortment of bathroom ventilation fans, two gas-burning appliances – a gas cooktop and a furnace. The furnace does not have an exterior air intake – after reading the manual, that’s not how this model is to be installed. So I definitely need a way to supply make-up air, most especially in the winter.


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  1. joeinva | | #1

    Apologies - that's the wrong link, and apparently the correct one is considered spam. Searching for "rr 0304 building science" will find the correct document.

    [Editor's note: Here is the link: Central Fan Integrated Supply Ventilation—The Basics]

  2. GBA Editor
    Martin Holladay | | #2

    Unfortunately, your builder installed a system that I call "the lazy man’s version of a central-fan-integrated supply ventilation system." Here's what I wrote about these misguided installations in an article titled Designing a Good Ventilation System:

    "To reduce costs, some builders install the lazy man’s version of a central-fan-integrated supply ventilation system — one that includes a passive fresh air duct to the return-air plenum, but without a motorized damper or AirCycler control. What’s wrong with this approach?
    • During the swing seasons, when the furnace fan isn’t operating, the house won’t get enough fresh outdoor air, and homeowners may complain of stuffiness.
    • During the rest of the year, when the furnace fan is operating regularly, the house will be overventilated, resulting an a severe energy penalty. During the winter, all that unnecessary cold air will need to be heated; during the summer, all that unnecessary hot air will need to be cooled and dehumidified."

    Your hunch is correct: you will need a competent contractor to install the missing components of your ventilation system. Unfortunately, finding that competent contractor may be difficult. The components of a central-fan-integrated supply ventilation system are worse than useless unless the system is properly commissioned.

    To commission the system, your contractor will need to know how to calculate the ventilation rate required to meet the ASHRAE 62.2 standard, and will need to own diagnostic equipment capable of measuring air flows. Good luck finding such a contractor.

  3. joeinva | | #3

    That's about what I expected, unfortunately.

    Are there any trade organizations that such a contractor might belong to? I've called Research Products Corporation, AirCycler, and SCI, which are the three manufacturers listed on the Fancycler web site, and got a few local contractor names who have at least purchased these products before. (2 of 3 of the manufacturers were helpful.) I haven't sorted through those names yet, but I don't have too much optimism based on the web sites I've seen so far.

    In the meantime, while I am looking for this unicorn of a contractor, presumably sealing up the house would do no harm and might help.

  4. GBA Editor
    Martin Holladay | | #4

    GBA readers: Can anyone recommend a competent ventilation contractor in Virginia to help Joe?

    Joe, it might be useful to tell us the name of the town or city where you live, if you are willing to share that information.

  5. joeinva | | #5

    I would certainly appreciate any suggestions. I'm in Loudoun County, Virginia, which is part of the greater DC metro area these days. It's possible contractors in neighboring counties might be willing to come over, so I'd take any recommendations in Fairfax County, Prince William, Clarke, or Frederick County, MD, as well.

    Martin, thanks for all your help. I have found your articles invaluable.

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