Why is A-coil freezing still an issue in 2021?
Is there a reasonable reason why HVAC manufacturers cannot keep an A-coil from freezing up? With temporarily putting MERV-16’s in our return in place of MERV-13’s during periods of high smoke a freezing A-coil is a concern. My first lament to our HVAC folks was why isn’t there a window in the duct so that I can see the coil. Then I wondered why the system can’t simply monitor and know when the coil is approaching freezing.
What is the best way(s) to monitor that?
– Increase in pressure drop across the coil? Perhaps good to measure but by the time it’s noticed it’d be too late? Or would this still be a good safety?
– Lower/Higher than expected temps in lines?
– Lower/Higher than expected temps in air flow?
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Depends on which unit you have. Most modulating units I've worked with have coil temperature sensors and will throttle back if the coil gets cold. I've abused this feature a fair bit on my bedroom wall mount to get the air flow lower and increase dehumidification.
As for checking, you can measure the discharge temperature from the coil. Above 50F, the coil won't freeze, bellow 40F looking at an eventual ice ball.
TXV (Thermal Expansion Valve) keeps the coils from freezing to a point.
Obviously you can't reduce your airflow too far.
Run the MERV 16 and you'll know when the coil freezes up.
You would need a temperature sensor located within the A coil assembly to do this. It wouldn't actually be very difficult to put a temperature sensor between the fins of the A coil and use it to operate a relay that would drop the 24v control voltage to the compressor when the A coil got to cold. This would serve to make the system stop cooling when the A coil got to cold, but would keep the blower running so only the cooling part -- the compressor -- would cycle. Data center air conditioners operate a little like this, since we run our blowers 24 hours a day and cycle only the compressors to control the air temperature. We run cooling all year, so our systems are different from regular HVAC systems.
Note that reduced airflow will cause the A coil to run colder. Low refrigerant charge can also be a cause of freezeup problems, even though that's counterintuitive.