High humidity on second floor of home
We recently installed an new HVAC system for second level heating and cooling of a 2 story home. It’s a heat pump w/ the air handler in an attic. There is 18″ of cellulose insulation between the well ventilated attic and living area. The ducts are insulated (w/ R-6 duct wrap). The problem is the humitity during the summer is around 70% inside. The house feels wet! Does anybody have any ideas as to what could be causing this? I have had the mechanical contractor out several times and he thinks it’s just fine. He has even gone as far as saying he doesn’t guarantee humidity level (guess he didn’t even read his own contract). I’m trying to find out what could be causing this. The load calculations seem to show the system is sized properly and there seem to be no large leaks but very little condesation is coming from the condensation drain (that has no blocks in it). Any ideas????
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There are people much more experienced than me here to answer your question, but since no one has answered yet, I'll get things started. By the way, you did not say where you are located.
A few possible causes come to mind. First, if the system were sized too large, it would short cycle. This would cause the air to cool quickly, but not allow the moisture time to condense onto the coils. Another is if the fan and coil were mismatched in size, so that you get too high of a velocity over the coils. Another is failing to provide a moisture trap (a loop in the drain line, like a sink trap) in the drain line. If the coil is on the low pressure side of the fan, and there is no trap, the condensate won't drain.
It sounds like the system is either too large or has a mechanical issue.
What are the run times for the system? Does it rarely run even on hot days? It can't de-humidify if it isn't running. Too large of a system will mean short run times, cool temperature in the conditioned space and high RH.
What is the temperature drop across the evaporator (indoor) coil? It should be about 20*. Higher is caused by insufficient load, check your filter, fan speed, diffusers open, you get the idea. If the temperature is less than 18* then the charge is either too low or too high. Too low is not fully utilizing the coil to evaporate the refrigerant, Too much charge is using part of the coil to carry liquid refrigerant that is not evaporating. The contractor needs to check the delta T, the superheat and the sub-cooling to determine if the system is working to design.
I think you're onto something. They did a load calculation before the system was installed. This contractor has done homes for me before and knows I build 'em tight. We put fresh air intake units in our homes for this reason. This project didn't have the budget to allow this upgrade but was nonetheless another "tight" home. I wonder if it is oversized? During the summer (very hot & humid summers here in the DC area) the unit runs very little. During the winter it runs almost non-stop. Further, I have variable speed fans in the air handlers and have tried to run the air handler constantly but this seems to make the humidity level even higher. When it's switched to "auto" the humidity comes down about 10%. We've also checked for leaky return ducts but they seem fine...I stumped, but only know enough to to simple troubleshooting. Do any of these clues put you in the same direction of a coil temperature problem?
During the summer (very hot & humid summers here in the DC area) the unit runs very little.
Unit is not running long enough toe dehumidify (remove latent heat). It satisfiies the T-stat setting (sensible heat) and shuts down.
Unit is oversized for the cooling load.
When the contractor thinks 70% humidity is just fine, it suggests to me that he is covering up for his own mistakes right from the start.
I'm thinking poor design. Are the loads calculated on typical or onyour well insulated place? too many guys have anolder standard typical insualtion in mind so that would make this too large of a system that could leave extra water in the air because it doesn't run lng enough.
But that's a half-azzed gues.
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This might be all wrong - parents had a large house that had 2 systems - one for heat (basement oil furnace with duct work just handled the Heat) and one in the attic with a dedicated duct system only to the seonc floor berooms that cooled the entire house because the cooler air fell.
What if you just run the second unit during the cooling season - distributes the air on the second floor and keep things comfortable. If you have a basement though, you would need a dehumidifier - pls let us know what you decide