Recirculating central vacuum air

Discussion in 'HVAC Heating & Cooling' started by Mikey, Jul 28, 2008.

  1. Mikey

    Mikey Aspiring Old Fart, EE, computer & networking geek

    Messages:
    2,736
    Location:
    Central Florida
    Now that we've tightened up the house, things requiring makeup air (central vac, exhaust fans, clothes dryer, fireplace, etc.) don't work very well.

    We open windows for the exhaust fans, dryer, and fireplace, but I'd like to see if I could plumb the output of the central vac through a HEPA filter into the AC return plenum.

    I've got a 2" PVC outlet pipe about 20' from the plenum, which is a box built of ductboard. Any comments and/or advice on how to make the transition?
  2. Marc46

    Marc46 HVAC Contractor

    Messages:
    102
    Location:
    Central Florida
    Assuming I understand what you are wanting to do,........which I believe is to balance the suction from the vac, which is probably exhausted in the garage or outside, and eliminate the negative air pressure in the home when running it,........right?

    I would buy or make a 4" to 2" reducer to mount on your vac pipe, and then run it to your return in either 4" flex,......dryer vent,.......T-fin,.......or even pvc. Terminate it in your return with a 4" dampered collar, so you can adjust the "pull" per se.

    You could eliminate all of the problems by putting in a fresh air return, but I understand why you most likely do not want to do this. We are in the same area!:D
  3. Mikey

    Mikey Aspiring Old Fart, EE, computer & networking geek

    Messages:
    2,736
    Location:
    Central Florida
    Right.
    That's along the lines of what I want, but I also want an in-line HEPA filter to make sure all the vacuumed crud is trapped. I suspect a lot of the fine stuff isn't caught by the vacuum, since they tout the benefits of exhausting air to the outside.
    Yeah, I've thought of that, too. That's probably the best for everything except the vacuum, since I don't want to recirculate dryer, stove, or bathroom exhaust, but I thought the vacuum air could be easily filtered and recirculated.

    I think this whole area is going to be a big deal in the HVAC game in the near future.

    Where are you at? Probably not Polk City :D.
  4. Marc46

    Marc46 HVAC Contractor

    Messages:
    102
    Location:
    Central Florida
    You are correct,...........it is already becoming a big deal as homes get tighter, and especially if icynene insulation keeps getting hotter.

    I am in Ocala.

    If you like,......I can PM you with my number and e-mail. If you have a digital camera, and could shoot some pics for me, I would be able to recommend something for what you want to do.
    Might even arrange for a filter setup at wholesale if one of my suppliers is in your area!

    Let me know.

    Edit: I think possibly you are misunderstanding what I mean by a "fresh air return." They are required by code for commercial applications, which is mainly what I do now.

    In the simple form,.........you would simply run a "duct" to a source of outside air, that ties into your return.
    In a perfectly sealed home,.........with a perfectly balanced, and designed A/C system, you should have neither positive, or negative pressure inside your home.
    When you turn on anything that creates a "vacuum" you create a negative pressure, or in the case of a non-vented gas fireplace, you run short of combustion air.

    With a fresh air makeup, you are allowing your home to become positively pressurized to a slight degree. When you turn on a range hood, or exhaust fan for example,.........the positive pressure is there to negate the outflow, that would otherwise create a negative pressure. It is all based upon the CFM of the exhaust appliances you have in your home, and that is how it is sized.
    It also provides needed combustion air for ventless fireplaces, without having to crack windows.

    The downside is the re-introduction of humidity into the home. There are many methods to do the same and avoid the negative, but they can become complex for an existing house.

    My point is,........you would not be pulling exhaust fumes of any kind into your A/C system with a fresh air return. As long as the intake is not located next to your exhaust fan outlet per se!
    Last edited: Jul 28, 2008
  5. Mikey

    Mikey Aspiring Old Fart, EE, computer & networking geek

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    2,736
    Location:
    Central Florida
    I used to own a lot on the Leeward Air Ranch in Ocala. Never got around to building for several reasons.

    I'll PM you with my contact info & send a couple of pics vie e-mail.

    Not clear why the home becomes positively pressurized if the fresh air return is totally passive.

    I didn't really mean "recirculating" stove exhaust, etc., but was, as you pointed out, concerned about replacing conditioned air with hot, humid outside air. Or cold, dry outside air on the 9 days we have it :). The vacuum exhaust is just hot mostly due to compression; it may cool on expansion, and the particulate matter can easily be filtered.
  6. Marc46

    Marc46 HVAC Contractor

    Messages:
    102
    Location:
    Central Florida
    My old partner's parents lived out at Leeward.
    The father died, and my partner went back to NJ in 1995.
    Small world!

    The heat of compression is why I recommended opening it up to 4",.......plus the availability of standard A/C fittings.

    What you are wanting to do is no big deal. I just need to eyeball your setup.
  7. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,313
    Location:
    New England
    What you want is a HRVF (heat recovery ventillation fan). This is a highly engineered item, and tough to implement on your own. Panasonic makes one, and others. Do a google search, and you should find some.

    They recover both the heat and humidity so the makeup air is as close to the exhaust air's qualities as possible. There is a heat and humidity exchanger in the box. No need to exhaust conditioned air without recovering what you paid for.

    Canada has higher requirements on tighter houses, so they are more common there.
  8. Marc46

    Marc46 HVAC Contractor

    Messages:
    102
    Location:
    Central Florida
    This was part of my comment before,........many solutions that avoid the waste. Expensive, and not simple to integrate into an existing home at times.

    All depends what he wants to spend on his "little" project.
    There are several beyond what you are referring to,........check Trane.
    I would not deal with a non-HVAC entity myself.

    As is always the case,......one can turn a minor issue, into a major undertaking, and expense.
  9. alternety

    alternety Like an engineer

    Messages:
    671
    Location:
    Washington
    A small point in nomenclature, and some other info. There are two basic types of air circulation systems on the market. If your house is real tight, you should have one that runs periodically to provide fresh air. New construction may require a system to be installed. I don't know if national code requires it but I know the inspector here seemed to want one (I designed one in from the start).

    Anyway, back to the types. A heat recovery unit does just that. Incoming and exhaust air pass each other in an impermeable heat exchanger. Humidity is essentially unaffected.

    The energy recovery ventilator system transfers both heat and humidity through an exchange medium that moves moisture. Read some manufacturers info on the web to decide what you need. It is a climate thing.

    These devices really don't apply for strictly makeup air. You could make or use a heat exchange core and run the exhaust of the vacuum system through it. It would need to be clean. In energy recovery systems there is usually some exchange of incoming and exhaust air. Not suitable if the system is used for bathrooms, and probably vacuums.

    If you have combustion equipment with required venting (e.g., furnace, water heater without a powered outside air draft system) and a tight house you MUST provide makeup air for anything that exhausts air from the building. If you create a negative pressure in the house and these appliances run, you will be pulling carbon monoxide into the house. This will make you sick or kill you. Note: retail CO detectors are mostly useless, by law.

    I have seen a recirculating exchanger that automatically adjusts for balanced indoor pressure. That might work quite well for most things (not combustion systems) but may not have the volume for the vacuum. That unit (you will have too Google around I don't remember the name) has a relatively low flow; which is why I could not use it. It is below required exchange rates for my house volume.
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