Liebert mini mate

[blewbyu2]

Sorry no model or serial no. Unit serves 20x20 server room, very light load, only 2 small server racks. Sensor is located at about 7 feet elevation immediately to side of walk in door. I have replaced the sensor circuit board. This unit has been in service about 5 years and this problem just recently occured and it has been driving me nuts. I get a big swing in humidity. Set points are temp 70 w/3 diff, rh 45 w/4 diff. I'm even starting to get mold occasionally near the liebert. The wall the unit is mounted to is common to a large maintenance shop which is non conditioned, although it is insulated it does not have a vapor barrier. We are having 90 degree days and 80% rh quite frequently, also shop overhead door is typically open. I'll typically check unit stat a couple times a day. Sometimes I'll have as high as 85% rh . As soon as the unit starts to dehum rh will drop 5% a minute but by the time the rh drops to set point room temp will also drop to low 60s bringing on the heater. Unit's charge is good. Are my parameters way off? Yesterday I disabled the humidifer and set temp @ 69 degree with no diff when I left the site to see what would happen over the weekend. Any suggestions?

[The Doctor]

Sounds like the cooling/dehumidification is overwhelming the reheat. The cooling tonnage vs. the reheat capacity.
You could reduce the temp differential to 2 just to use reheat that much sooner, but the low 60s?? And to have humidity in the high 80s sounds like someone doesn't have a large concern for the design environment, whether that is due to infiltration or neglect.
Not enough reheat is happening

[whec720]

Mini-Mates often have two fan speeds, high and low. Looks like you got quite the load, so make sure it is in high or at the highest speed.
Doc hit right about the reheat. Give it a close check. I'm interested at what it is amping at. When is the last time it has been cleaned also?

[Domcort]

blewbyu2,

Sounds like the unit is preforming to the setpoints.
Liebert units and other speciality units for Data Centers work with 0% outside air, from what you are describing you do not have a great vapor barrier on the room and outside air from the maintenace garage area is causing the room swings per your description. The units control is based on temperature first and then humidity, you should try and seal the room a bit further. If the load is light as described you may want to try and run the unit with the low fan speed enabled so that it can cool and dehumidify at a slower rate.
Possible relocation of the sensor may be needed depending on the rooms seal.

Try and get the model and the heat load of the room so that we can help you further.

Domcort

[blewbyu2]

blewbyu2,

Sounds like the unit is preforming to the setpoints.
Liebert units and other speciality units for Data Centers work with 0% outside air, from what you are describing you do not have a great vapor barrier on the room and outside air from the maintenace garage area is causing the room swings per your description. The units control is based on temperature first and then humidity, you should try and seal the room a bit further. If the load is light as described you may want to try and run the unit with the low fan speed enabled so that it can cool and dehumidify at a slower rate.
Possible relocation of the sensor may be needed depending on the rooms seal.

Try and get the model and the heat load of the room so that we can help you further.

Domcort

Thanks for the suggestions. I'll check fan and amps tommorrow and follow up with a post to let you know what I've found out.