Q&A System Oversizing – Short #291

In this short Q&A podcast episode, Bryan answers a question from Evan, a licensed mold assessment consultant, about system oversizing: What can be done to control humidity and keep occupants comfortable when an HVAC system is oversized?

The extent of the problem will depend on how badly the equipment is oversized, whether it has any turn-down or staging, and how the latent capacity is set up. To make the equipment work as best as it can, reducing the airflow to about 350 CFM per ton can help, as well as properly setting up dehumidification modes. The downside to slower airflow is a likely increase in cabinet and duct sweating.

In brand-new houses, it may be possible to swap the condenser (though it may be unlikely with the recent refrigerant change) and then drop airflow at the air handler. You can use AHRI's data to look for a possible match and then check the manufacturer's expanded performance data to verify whether a new condenser is a match.

It's also possible to decouple the latent and sensible loads by installing a dehumidifier. However, completely decoupling the loads is not always practical, as a properly-sized HVAC system is the main source of dehumidification (and the dehumidifier fills in the gaps). Dehumidifiers need to be installed properly (with the supply ducted into the supply duct).

Intentionally derating the equipment's cooling capacity, such as via reheat, will turn it into a better dehumidifier, but you're still adding sensible BTUs and need to be aware of the consequences of that, including higher power bills. You can use electric reheat, remove insulation, or remove shades from windows. Using a smaller compressor until the system can be replaced and downsized would achieve a similar effect but comes with other negative consequences.

Right-sizing equipment, load-matching, and managing air movement and heat transfer are ultimately the keys to preventing moisture problems.

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