Q: House is two stories around 2400 sq feet with one 4 ton unit with a 5 ton coil, two thermostats with zoned ducts. No issues cooling the house even on the hottest days.  The problems I am having are short run times and elevated humidty…around 56% on average.  I know that having one system with a two story house is not ideal and is the low cost option for most builders. Is there anything that can be done to lower the humidity?

Humidity in the home, on average should be around 55% or lower. The 5-ton coil on a 4-ton condenser is too large. Humidity control is only a by-product of heat removal out of the air during the conditioning process. An oversized evaporator coil will not get cold enough to absorb moisture out of the air. The coil size determines the major balance between dry and wet heat removal. The coils remove dry heat (temperature) and the wet heat (humidity). The coil size determines this ratio. The larger the coil is size, the lower the humidity removal. In many cases, a smaller coil can increase the humidity removal by 30% to 40% resulting in greater dehumidification. After extensive research, we have found that having a matched or oversized evaporator coil will not control humidity enough. Rather it increases the humidity level in the home, especially at night. The reason is that the evaporator cannot get cold enough to properly dehumidify the air, causing your air to feel sticky and humid.

We recommend a 4-ton condenser with a 3-4 ton evaporator coil in the horizontal application. We also recommend an evaporator coil with an adjustable thermal expansion valve to help adjust refrigerant charge for maximum performance.

You also need to make sure that the home’s envelope is correct. Make sure that the attic access, canned lighting , windows and doors are properly sealed and weather stripped. You may also need to get the home closer to neutral pressure to reduce moisture and dust infiltration, which can be achieved with a Fresh Air Intake