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Thread: High (?) condensate volume and low internal pressure

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    Aspiring Old Fart, EE, computer & networking geek Mikey's Avatar
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    Default High (?) condensate volume and low internal pressure

    Just out of curiosity, I stuck a bucket under the condensate drain and found the system is wringing out about 5 gallons per day from our 1800 sq ft home in central Florida. Now, admittedly, it's humid outside, but should it be that humid inside? Is this a normal condensate volume?

    However, I have the feeling that when the system is running, the house is slighty depressurized -- I can detect the smell of chimney when walking by the fireplace. I have no idea why that might be. The house is reasonably well sealed (I thought), and the ductwork inspected visually for leaks and sealed up with duct sealant at all joints, registers, etc. The house and AC should be an essentially closed system, but I must be actively sucking outside air in and pushing conditioned air out. Looking for ideas.

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    In the trades Dana's Avatar
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    Unbalanced &/or leaking duct systems in less than air-tight houses can drive fairly high rates of infiltration. Most houses that haven't undergone some level methodical air-sealing measures will leak well over 5 air exchanges per hour (ACH) with an unbalanced duct system, and 2-3 when it isn't running. If air is coming down the flue it doen't necessarily mean that the whole house is under negative pressure (though it might be), only that the great outdoors was the lowest impedance return path for pressure differnences between rooms. It also means that the flue damper isn't a tightly sealed type.

    If some rooms have only supply ducts & no returns, without door grilles or jump ducts for equalizing the pressure, that would cause these types of pressure differences. Any rooms with supply-only need to be retrofitted with jump duct. sometimes a grille in the bottom of one side of a partition wall, and the top side of the other can perform this function, or ceiling (or floor) mounted grilles with flex ducts connecting the two rooms or room/hall will work. In some rooms where privacy is less of a concern a door-mounted grill would do it, but getting all rooms as close to the same pressure as possible is key.

    It's probably worth contacting two different types of contractors- an home air-sealing contractor (usually a service offered by an insulation installer- often foam-insulation installers), and an outfit that does duct sealing. If the ducts are all inside a reasonably maintained pressure boundary of the building the amount of infiltration even from leaky or unbalanced ducts can be made fairly small. But sealing the ducts & air handler puts the conditioned air where it was designed for rather than random places. In many homes in FL the air handlers & ducts are in a ventilated attic outside the pressure boundary of the house, where duct leaks are all but guaranteed to use the great outdoors as the return. Sealing the attic itself is often easier & preferable to attemping to fully air-seal the attic floor boundary, including all duct, lighting, & plumbing penetrations. (Vented attics in FL are mostly a mis-application of a solution to problems found only in cold climates, and end up introducting more moisture to the building than it ever purges, with only the slightest effect on shingle temps or cooling loads.)

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    Aspiring Old Fart, EE, computer & networking geek Mikey's Avatar
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    That gives me something to think about; thanks. In this case, there are 2 returns, a big 20x25 in the main hallway, and a 14x14 in the master bedroom that looks like an add-on. All internal doors are almost always open, so extra return ductwork seems superfluous. In fact, I was wondering if the add-on duct in the MBR lowered the return impedance and unbalanced the (presumably) carefully engineered original ductwork design.

    There are 6 known attic-to-indoors leaks -- all can lighting installed before AT (boy, there's a misnomer) cans were available -- but most other accessible plumbing and electrical penetrations were sealed prior to adding R-30 fiberglass insulation to the attic. That seems to have been very effective, but has the downside of allowing the AC to idle for extended periods. This allows the air in the attic ductwork to heat up and absorb moisture from somewhere, so it smells like a swamp when the AC resumes operation. It's one damn thing after another...

    It hurts to do it, but I'll start looking for a contractor who knows what he's doing...
    Last edited by Mikey; 09-22-2010 at 01:51 PM.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    I don't think 'extra' returns would be any problem, they would just provide an easier path for the air to return to the air handler to be pushed around again. Where it becomes a problem is if a room can't easily return air, or the return ducts are simply too small in the first place.
    Jim DeBruycker
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    Retired prof. engr. gator37's Avatar
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    Mikey
    Just a question but do you know if your evaporator (indoor unit, air handler or evaporator side of a package unit) is working correctly. I have seen evaporator coils start to freeze and still produce air flow but very humid air then when they shut off the coil begains to thaw giving you alot of condensate. Rooms smells musty also. (Some of the causes are fan motor, low refrigerant, dirty coil, blockage or dirty filter). How do you know what the humidity level and negative pressures are, did you measure it? Of coarse the other side of the argument is the system is over sized and not running long enough to drag out the moisture before shutting down.
    Not sure what Fl requires now but I have never seen a carefully engineered HVAC for the average residence. Most of the time I see a carefully contractor rule of thumb systems (and some not so carefully) done unless it is a very expensive residence and the owner went the extra mile and hired an engineer. But Fl requirements may have changed since I grew up there but that has been a few years ago.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Lots of condensate out the drain tells me the thing is working; maybe harder than it needs to, though. But, the thought 'cut it with a knife' can describe the humidity levels in FL much of the year. I've never tried to measure mine, but on a humid day (your norm), there's a good stream escaping. I doubt you have evaporator freeze up if you have a good flow out the drain.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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