Wednesday, June 7, 2017

Latent Cooling and Variable Capacity Systems

If you live anywhere other than the southwestern part of the US, you probably need latent cooling in the summer. The word latent means hidden. Cooling capacity is required to condenses water on the evaporator coil. This is referred to as latent cooling because there is no temperature change involved, you can’t sense, or measure the heat change using temperature, but cooling capacity is required. Two things help increase latent cooling: long run times and reduced airflow across the evaporator. These increase the percentage of system capacity used for latent cooling.

I have a brand new communicating, 20 SEER system with a variable speed scroll and an ECM indoor blower motor. One fun thing about the thermostat is that it reports the compressor speed and the furnace reports the blower CFM. I have been watching both.

The compressor is most often operating less than 50% capacity, but stays running most of the day once it starts. This ability to match system capacity to the load makes for long run times, which helps control humidity. It does not use more power, even though it is running a lot because it is using much less electricity while it is operating.

I have noticed that the fan almost never runs at the traditional 400 CFM per ton. For example, on one occasion I found the compressor running at 96% while the fan was moving 1230 CFM. It is a 4 ton system, so traditional CFM math would place the “normal” airflow at 1536  (4 x 0.96 x 400 = 1536). However, the system was operating at only 320 CFM per ton ( 1230 / (4 x 0.96)).

The thermostat also lets you set the indoor relative humidity and reports the indoor relative humidity. I set it at a fairly low 45% and the system has kept it between 45% and 50%. During the day when the system is running, it keeps it right at 45%. It accomplishes this by using long run times at reduced capacity with lower than normal airflow. Summer comfort in the southeast involves more than controlling temperature, it also involves controlling humidity. One bonus of the variable capacity systems is that they do a better job of controlling humidity than fixed capacity systems.


  1. Very common for people to have it so cold it will "take your head off" on a hot, sunny day and then sit around tolerating humidity so high that paper won't rattle on a 75° rainy day "letting the thermostat do it's job".