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.
- Two stage equipment can operate at reduced capacity under reduced loads
- Variable speed motors can reduce cfm under reduced sensible loads or under increased humidity levels
- Some thermostats can respond to humidity levels by changing a digital output state such as turning on or turning off a 24 vac output, or simply changing the state of dry contacts.
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.
- Either change the thermostat to one that will provide a dehumidification output in the cooling mode, or add a wall or return duct mounted dehumidistat.
- Program the thermostat or set the dehumidistat to activate dehumidification mode on a humidity increase. For instance, energize a dehumidification relay at 53% RH and de-energize the relay at 50% RH.
- Break the cooling speed to the evaporator motor through normally closed contacts on the dehumidification relay.
- To the normally open contacts, wire a lower blower speed that will provide 325-350 cfm per ton. (Make sure the wiring scheme will not allow more than one motor speed at a time.)
- In cooling mode when the RH is acceptable, the blower will operate at a speed to deliver 400 cfm per ton (normal cooling blower speed).
- In cooling mode when the RH is high, the dehumidification relay will energize the relay to operate the blower at a reduced cfm (dehumidification cooling blower speed).
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.
- Install a Delay-On-Make timer in series with "Y" and the dehumidification relay coil. So a "Y" output from the thermostat will energize the outdoor unit contactor and the added timer, which will energize the added dehumidification relay after the selected time delay period.
- Connect the normally open blower relay contact to the dehumidification relay SPDT common terminal.
- Connect the slower blower motor speed tap to the dehumidification relay normally closed contact.
- Connect the faster blower motor speed tap to the dehumidification relay normally open contact.
- On each cooling demand, the blower operates at reduced cfm until the timer period has ended, at which time the blower will operate at higher cfm to finish out the cooling cycle.
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).