What size was the old A/C, and what size is the new A/C.
Were you using the attic fan last year? It could be drawing conditioned air from teh house into the attic. If it is, then the conditioned air is being replaced with high humidity air from the outside.

Is your duct work in the attic?

May have a leaky return or supply if it is.

Does your furnace have a VS blower.

 

I don't remember what size the old one was. It was an 18 yr old Carrier that came with the house. The model number is no longer listed on the Carrier web site. (38TG036310)

The new one is a 13-seer, American Standard Allegiance 3-ton unit. Our house is a two-story Georgian with the ductwork in the unfinished basement.

We've had an attic fan for a long time, but we just replaced the old unit with a GAC model with a humidistat. We also removed all the mushroom vents in the roof so the attic fan would be pulling the hot air out of the attic rather than the nearest mushroom vent. The humidity issues started, though, before the new roof/attic fan were installed.

The furnace does have a VS blower and the fan is set to on. (per the recommendation of the contractor) We used to run the old furnace/ac system on auto.

We did try to change the fan setting, so the condenser would kick on more often, and it did lower the humidity a little, but not much.

 

I don't

We've had an attic fan for a long time, but we just replaced the old unit with a GAC model with a humidistat. We also removed all the mushroom vents in the roof so the attic fan would be pulling the hot air out of the attic rather than the nearest mushroom vent. The humidity issues started, though, before the new roof/attic fan were installed.

The furnace does have a VS blower and the fan is set to on. (per the recommendation of the contractor) We used to run the old furnace/ac system on auto.

We did try to change the fan setting, so the condenser would kick on more often, and it did lower the humidity a little, but not much.

Removing attic vents increases the negative pressure from an attic fan exhausting. Increase negative attic pressure sucks more air from the home and therefore more outside air into the home. 50 cfm of fresh air into home when occupied will purge indoor pollutants and renew oxygen. But your a/c can not remove the humidity with low cooling loads typical of wet cool weather. Turn off the attic exhaust until you resove your humidity issue. With <85 outside temps and fresh air infiltrating your home, there is not enough cooling load to remove the 30-50 lbs. of moisture from fresh air infiltration and occupants. I agree with fan "auto" mode. Regards TB

 

Set the fan to auto.

Set to on, the humidity raises once the outdoor unit shuts off.
Next, most likely your old air handler was not moving 400 CFM per ton. And your contractor probably has the VS set for 400 CFM per ton.
Have him set it to 350 CFM per ton.
And enable comfort r.

 

I think you shuld listen to beenthere his opinion is correct in my opinion call your contractor re set your fan speed you will notice the diff, if your contractor still says your equipment is fine remeber there more than one contractor out thier get a second opinion good luck ..

 

American Standard units have a good sensible to total ratio. That tells how much of the unit's capacity is lowering temp and how much is removing humidity. They are among the lowest made meaning more capacity for moisture removing and rank with some of the oldies.

been's advise is right on as usual. Get the fan to auto ASAP.

 

auto fan will drop the rh

agree that the attic fan should be shut off

 

But if we turn off the attic fan, there will be *no* attic ventilation other than the intake vents (the eaves/soffits). According to the roofing guys, this would also not be good.

We turned the fan to auto early this afternoon per all of your advice. So far, the rh has dropped to 60%. I turned the thermostat down another degree to see how much more it will help. I will call the contractor in the morning.

Thanks for your suggestions.

ps. we should turn the fan back to "on" during the winter months??

 

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if using auto fan does not help enough, have your contractor make sure the ramping profile is being used

there is another function that your airhandler can utilize to lower humidity even further by lowering cfm

with the assistance of the proper t-stat there is even an additional dehumidification mode

that can be taken advantage of with your variable speed a/h



.

 

attic fans scare me

without proper fresh air intake you can pull enough negitive pressure in the house to reverse the draft on a natural draft water heater or funrnace if its old enough to have a drafthood.

 

Leaving the fan on in the winter months can gift the home a drafty feeling, though at the lower speed of the variable-speed blower, it may not be as noticeable. If you get a VisionPRO thermostat from Honeywell, (or the ACONT802 or ACONT803 from American Standard) there is a CIRC feature to run the fan only a percentage of the time (I think around 30%) when there is no call for heating or cooling. This might be an alternative for you. The VisionPRO IAQ can be set up to slow the blower down to dehumidify on demand. The ACONT803/VisionPRO 8321 can overcool up to 3 degrees to control humidity.