hi humidity when compressor is in off cycle

[hlwd2012]

This is what im dealing with if anyone has a solution it would be greatly appreciated! basically i have a 600sqft office area that is becoming very humid when compressor is not running and fan stays on! it is a mcquay ac that has a belt driven fan motor. while the ac is running the humidity is at about 50% but after the compressor shuts off and fan keeps running the humity rises to near 80% I know an easy solution would be to have the fan shut off with the compressor but does anyone have any sugestions with keeping the fan on? also the unit is oversized and im sure that cant help!

[TwincamDave]

look for a fresh air damper, it is probably open. It needs to be closed more or completely. If it is an electric damper change the connections to only open when the compressor runs.

[Six]

Being oversized is your primary problem. You could look into adding a dehumidifier to control humidity.

I dont imagine it runs for very long.

[hlwd2012]

I set the fresh air damper to a min 0% open and max 20% plus the unit has a fresh air enthalpy control. I guess at this point i should just start looking into a dehumidification system to add to existing equipment! I was just hoping there was a trick to fix this problem that i may have over looked

[ADillon]

You hit the nail on the head. Your unit is oversized

[Six]

I set the fresh air damper to a min 0% open and max 20% plus the unit has a fresh air enthalpy control. I guess at this point i should just start looking into a dehumidification system to add to existing equipment! I was just hoping there was a trick to fix this problem that i may have over looked

We have a very knowledgable member who sells dehumidifiers here. He might chime in and offer up some advice.

[teddy bear]

We have a very knowledgable member who sells dehumidifiers here. He might chime in and offer up some advice.

I was off to summer camp for a few days.
Do not shutt of the fresh air. Make sure you limiting the fresh air to local code or ASHRAE. This is usually an air change for the space in 4-5 hours. During long runs, the a/c should get the space down to 45-50%RH. If not slow the air flow through the cooling coil.
For times when the a/c is not running and you have outdoor dew points +50^F, you will need a good dehu to maintain <50%RH. Lood at the Ultra-Aire line of dehus. You can provide/control fresh air via the dehumidifier. CO2 controllers will also limit the excess fresh air when unoccupied, during windy weather, and if the ducts leak excessesively. This is all available with the Ultra-Air line.
Sq ft. of the space and number of occupants would also help is sizing.

[hlwd2012]

Well i slowed the fan speed down as much as i could without having the coil freeze over, adjusted fresh air damper to 15% per code, even increased the temp setpoint diff up to allow the unit to run longer and not short cycle. No luck! As the compressor off cycles the fan runs and reintroduces the moisture back into the space! guess the unit it just way too oversize to try and do anything with. I guess i will start looking into dehumidifiers! thanks for everyones help!

[just_opinion]

Well i slowed the fan speed down as much as i could without having the coil freeze over, adjusted fresh air damper to 15% per code, even increased the temp setpoint diff up to allow the unit to run longer and not short cycle. No luck! As the compressor off cycles the fan runs and reintroduces the moisture back into the space! guess the unit it just way too oversize to try and do anything with. I guess i will start looking into dehumidifiers! thanks for everyones help!

What does it mean 15% ? I hope it is not "15% open means 15% the total amount of system CFM"

The best method for fresh air is CO2 sensor. It is cheap comparing to all the headaches and guess works. Not counting economical too.

[R123]

As others have said, your oversized unit is the main problem. Also, make sure your drain pan is draining off good. If the pan stays full of water, the blower running continuously will re-evaporate the water back into the air. You may be able to tilt the air handler in such a way that there isn't much water in the pan.

[Failure2Comply]

Is this a multi-stage unit?

[terryhill]

Sounds like you didn't size or install the unit.
Building renovations without HVAC engineering typically
causes these problems.
Even though we don't like to do it sometimes you have to duct the excess capacity
to another location. We all know as soon as we touch the unit we own it.
Good luck

[socotech]

Is this a chill water system? If it is, you could have a frequency drive installed on blower to slow fan speed down.