Most of the hours when comfort cooling systems operate are well below outdoor design conditions. The equipment may have been selected to maintain 75°F and 50% RH indoors when it is 95°F outdoors, but what happens at lower outdoor temperatures? There's less run time to satisfy the thermostat, but not enough run time to maintain desirable or comfortable humidity levels. Some modern equipment and components are designed to overcome this effect.
But what can be done to target humidity concerns with single stage equipment, PSC blower motors, and a standard heat-cool thermostat?
If the equipment is set up to operate at 400 cfm per ton, as it should be, then reducing cfm 50 to 75 cfm per ton under high humidity conditions will have the dual effect of taking longer to satisfy the thermostat and extracting more moisture from the air due to a colder evaporator surface temperature: a desirable net effect under part load conditions.
An alternative to changing the thermostat or adding a dehumidistat is to use a Delay-On-Make timer. Since we know the majority of cooling operating hours are at less than outdoor design conditions, we can operate the blower at a reduced cfm for the first 5 to 10 minutes of thermostat demand in anticipation of humidity complaints.
Rather than a standard fan relay, which is typical in air handlers, a furnace may use an integrated control board to control many functions including fan speed outputs. Typically, the blower operates on heating cfm with a "W" input, low continuous fan speed with a "G" input, or cooling speed with a "Y" input. With this control scheme, the blower motor speed tap that will deliver 325-350 cfm per ton can be selected to operate on a "G" demand, and the blower motor speed tap that will deliver 400 cfm per ton can be selected to operate on a "Y" demand. Now, instead of landing "Y" from the thermostat directly on the integrated control, break it through the dehumidistat so that an increase in RH above the setpoint will open the "Y" circuit between the thermostat and the integrated control "Y" input. This will cause the motor to operate on continuous fan speed (325-350 cfm per ton) under high humidity conditions, and operate at normal cfm (400 cfm per ton) when humidity is acceptable.
Each of the above schemes operates to sacrifice sensible Btu's (temperature change) for latent Btu's (moisture removal). Reducing cfm reduces the amount of heat being added to the evaporator, thus lowering the evaporator surface temperature. This results in the evaporator temperature being further below the return air dewpoint temperature than it was with higher airflow, so more moisture will be extracted from the air. Since more Btu's are being used to change the state of water vapor to water, fewer Btu's are left over to change the temperature of the air. Net result is more moisture removal and longer run time. At 75°F, it takes 0.24 Btu's to change the temperature of 1 pound of air (about 13 cubic feet) 1°F, but it takes roughly 1025 Btu's to condense a pound of water vapor out of the air (into about a pint of water).