Q&A – System Won’t Dehumidify? – Short #214

In this short podcast episode, Bryan answers a listener-submitted question about an HVAC system that won’t dehumidify. This question was submitted by a homeowner with a Carrier heat pump with an attic air handler that cools but won’t dehumidify on the hottest days of summer. One of the past companies that serviced the system added charge, but the problem wasn’t resolved.

This case doesn’t seem to be a sizing issue, as the insufficient dehumidification is happening on the hottest days. Normally, sizing issues happen during the edge seasons when temperatures aren’t as high as in the summer, but the dew points are still high.

Bryan’s first recommendation is to use measureQuick to evaluate the delivered capacity and determine the sensible heat ratio (SHR) by comparing sensible BTUs to latent BTUs. High SHRs are associated with less effective dehumidification and more effective sensible BTU removal.

Then, we have to determine that airflow is properly dialed in. Typically, lower fan speeds are associated with better dehumidification; 350 CFM per ton is typical of humid climates. In the case of two-speed or variable-speed equipment, we should verify that it is staging up and down properly.

On the ventilation side, a duct leakage test is recommended. Proper use of exhaust ventilation, like bath fans and kitchen exhaust, is also critical. Structural leakage and MAD-AIR are also worth checking if a company can measure those, though those are premium services that are usually more expensive.

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