High humidity

[jayjayjayjay]

We have a new duct work system total of 4200cfm. One existing roof top unit has 2900cfm output. We step up the evaporator fan RPM to match 4200cfm. The room temperature set to 75-76 degree with no outside air. The inside room feel very sticky. We did some data of the existing unit. When the unit is on, Circuit 1 the head pressure-280 psi, low pressure 77psi. Circuit 2 the head pressure-270 psi, low pressure 80 psi. Return temperature is 68 F and supply temperature is 56 F. Why the unit can't property dehumidify the room?

[54regcab]

This is in a residence or commercial?

[teddy bear]

With a 68^F return, you need a <45^F coil temp. to deliver 50%RH. Slow the air flow until the cooling coil is cold enough to remove the moisture to the desired level. Over cooling to control the indoor %RH has its problems. Certainly at 75^F, 50%RH requires a <55^F dew point. A 50^F cooling coil is needed minimum. Lowering %RH with low/no cooling loads requires supplemental dehumidification. There are units available for light commercial applications. Suggest checking on the large Ultra-Aire 200H dehus. They connect to the a/c ducts.
Regards TB

[jtrammel]

You have almost twice as much airflow as the unit is designed for? 2900cfm changed to 4200cfm?That would cause humidity

[jayjayjayjay]

Residence.

[mofotech]

You have almost twice as much airflow as the unit is designed for? 2900cfm changed to 4200cfm?That would cause humidity

[BaldLoonie]

What size unit? 2900 CFM sounds like about a 7.5 ton. Big house!

[jayjayjayjay]

Yes, 7.5ton. Is a penthouse apartment.

[jtrammel]

Airflow is way to high. Moisture laden air does not have enough time to hit cold coil to release moisture