High (?) condensate volume and low internal pressure

Discussion in 'HVAC Heating & Cooling' started by Mikey, Sep 22, 2010.

  1. Mikey

    Mikey Aspiring Old Fart, EE, computer & networking geek

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    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.
  2. Dana

    Dana In the trades

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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.)
  3. Mikey

    Mikey Aspiring Old Fart, EE, computer & networking geek

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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: Sep 22, 2010
  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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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.
  5. gator37

    gator37 Retired prof. engr.

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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.
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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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.
  7. Mikey

    Mikey Aspiring Old Fart, EE, computer & networking geek

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    Yeah, it's working all right. There may be a good argument that the system is oversized -- now. The house was built in 1978, had single-pane aluminum windows, 5" of glass wool in the attic, and a 3-ton AC. I replaced the AC with a Carrier "Infinity" system, supposedly the best thing since ice, with a SEER of "up to 21". They stuck in a 3-ton unit to replace the old one. Since then, I've replaced all the doors, all but one of the windows with double or triple glazing, and added R-30 insulation to the attic. There's still one single-pane window (a 12' slider, unfortunately). The walls are the weak link now, 6" of brick, 1" of sprayed-on foam, and 1/2" drywall, but there's not much I can do about them. Nevertheless, I probably don't need a 3-ton unit any more, but this Carrier thing is variable-everything, and is pretty-much loafing all the time. I rarely notice it when it's running.

    The argument about the extra return duct goes as follows: assume the extreme case of a gigunda return duct and a tiny supply duct system. The AC fan will suck more air out of the house than it can put into it, resulting in makeup air infiltrating wherever it can -- in my case, 6- 6" non-AT can lights in the ceiling, exhaust fans in the kitchen and bathrooms ("sealed" only with their butterfly valves), and (I think) very few other leaks, either to the attic or outside. By properly balancing the return and supply impedance, I would think you could bring that pressure differential to zero.

    Maybe I'd better start looking for one of those high-priced engineers, but I'm going to start experimenting by blocking off the 2nd return duct, which is very obviously a later add-on, and see what happens. I did measure the total area of the supply and return registers once, but I've forgotten the numbers; I'll do that again today.
  8. Dana

    Dana In the trades

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    The the AC fan can't "suck more air out of the house that it can put into it", ever. It creates a differenial pressure between the supply & return ducts- period. The bigger the ducts, the lower impedance, which results in a higher volume of air moving, but lower differential pressure. Blocking off ducts (supply OR return) only increases the pressure differentials, and will drive MORE air infiltration. This has been studied excensively by utilities in CA along with the Lawrence Berkeley Nat'l Labs. The old-school practice of turning off registers to unused rooms has been thoroughly debunked- it usually INCREASES rather than decreases total cooling & heating costs due to the increased infiltration drive. Closing off returns will make your problem worse.

    There are commercially made boxes for sealing over recessed lighting cans, but DIY versions can work just as well. Make sure that you have at least 3" of clearance between the lighting fixture and the inside of the sealing-box, and foam-seal all of the edges with single-part foam (Dow Great Stuff or similar.) Cardboard boxes are fine, but use 2" FSK (aluminum) tape or housewrap tape to seal all of the seams, including the glued edge at the corner of the box. (Typical box adhesives aren't designed to be fully air tight or to last for decades.)

    Fix all of the big leaks that you know about, then run a pressure door test. If history is any guide you'll find several more large leaks as well as a myriad of medium sized leaks before the house will hold any pressure. It's pretty common in FL these days to just bite the bullet and put 3" of open cell foam under the roof deck bewteen the rafters sealing all exterior venting to form a pressure boundary that puts all air handler & duct systems fully inside that pressure boundary. The effect on latent load & condensation issues is HUGE once the house is reasonably sealed, with all mechanical systems inside the tent. (3" of o.c. foam would add about R10-12 to the total stackup, and put at least some insulation between the ducts and the hot roof deck.)
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