HVAC School - For Techs, By Techs
Q&A - System Won't Dehumidify? - Short #214
· Summarized
Key takeaways
- High humidity in HVAC systems often relates to improper airflow and equipment functioning rather than mere sizing issues.
- Using tools like MeasureQuick is vital in assessing an HVAC system’s capacity and heat ratios.
- Finding a competent HVAC contractor who addresses all facets of humidity control is essential for troubleshooting persistent issues.
Summary
Humidity Issues in HVAC Systems
In this episode, Brian addresses a listener question about an HVAC system, specifically a Carrier heat pump located in an attic, that struggles to dehumidify despite the air conditioning functioning properly during hot weather. The homeowner reports persistent high humidity levels, sometimes reaching 80-90%, after multiple service calls from different HVAC companies yielded no solutions. Brian emphasizes the importance of understanding humidity issues specifically, noting that they often relate to improper airflow and equipment functioning rather than being a sizing problem.
Troubleshooting Steps for HVAC Problems
Brian provides a step-by-step troubleshooting guide for dealing with humidity issues. He advises starting with a MeasureQuick report to evaluate the system’s actual delivered capacity and check the sensible heat ratio (SHR), aiming for values below 0.7 in humid conditions. Proper airflow is also crucial; setting it to about 350 CFM per ton is recommended. Additionally, he stresses the importance of checking duct and structural leaks, as well as ensuring ventilation systems, like bath and kitchen fans, operate correctly. These steps are essential for rectifying the humidity control issues.
Finding the Right Contractor
The importance of selecting the proper HVAC contractor is highlighted throughout the episode. Brian explains that many contractors focus solely on swapping equipment and measuring temperature splits without addressing underlying issues like airflow, duct leakage, and home behavior. He encourages listeners to seek out contractors who are knowledgeable in comprehensive diagnostics and who can systematically analyze the problems, even if it leads to higher costs. Finding the right professional is critical to resolve complex HVAC system issues effectively.
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Show notes
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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