Central AC Dehumidification ability (1 Viewer)

[Todd Hochard]

What determines a central AC system's (heat pump, actually) dehumidification ability, during the cooling season?

I'm guessing it's a function of the coil temperature (lower is better), coil surface area (more is better), and airflow speed across the coils (slower is better, or would there be a happy spot between too slow and too fast)?

Have I guessed right? I can't seem to turn anything up on a Google search.

The reason I bring this up, is that my friend's house is "uncomfortable." I keep my home at 78* in the summer, and his stays at 77*. Yet, my home (1500 sq.ft with high ceilings) stays at 40-43% RH during the hot times, and his (enormous McMansion, 3000 sq ft with 12+ ft ceilings in most places) has trouble staying below 55%. Even when he sets his AC to 74* (which guarantees near 100% compressor run time during a hot day), the humidity struggles to get to 50%. If I do that, my compressor run time is near 100% also, but humidity gets down to 27-29% (too dry, really).

These temps and humidities were measured with the same digital unit in both homes. Anyone have a clue?

Todd

 

[Eric Kahn]

he needs to have his unit or units checked, sounds like the evaporator is not getting cold enough, usually means a miss charge, can't remember if this is overcharged or undercharged
the other possibility is that the unit is just the wrong size or the house leaks a tremendious amount of air

 

[Philip_G]

it can be a lot of things, capacity, charge, airflow, has he checked his filter lately? ever?
the dew point is kind of like a brick wall, it'll cool right down to it and then stop for awhile I think it's a combination of all the things you mentioned, larger coil, more BTUs of cooling, but if his house is newer I'd put my dollar on terrible duct work and bad or no air flow.

 

[SteveA]

The humidity issue is one reason that most heating/AC experts say it's better to have a smaller unit that works really hard instead of a large, under-utilized unit. An AC unit that is too large for your house will cool it very rapidly, but won't run long enough at one time to remove much humidity from the air.

 

[Frank Zimkas]

It may be that he is getting more infiltration of outside air into his home. Another thing to consider is whether or not he has a make-up air duct that is bringing in outside air everytime the fan in his furnace/airhandler is on.