The air conditioner's A coil* froze up yesterday. Discharge air had felt unusually cold recently. I'd worried the coil might freeze up, and changed the filter the day before yesterday, hoping to avoid it.
Air conditioners play an interesting game: expansion of the working fluid cools it, which cools the A coil, which cools the air passing through it -- but it also removes moisture from the air passing through the coil. Old fashioned window units just drip the water outside. Central air-conditioning coils have a drip pan and a drain.
To get the best results, the air flowing through the coil needs to move slowly enough that it doesn't carry away any water droplets, but fast enough to deliver a useful amount of cooled air to the house. The working fluid needs to be cold enough to cool down the air sufficiently as it passes through -- but not cold enough to freeze the water even a little. Slush, frost or ice on the A coil will slow and then stop the airflow, and without warm air passing through it, the coil gets colder. Your house gets warmer, so the system keeps running, and that keeps the coil frozen solid. At that point, you're out of the air-conditioning business until the coil defrosts. In a properly-designed system, this should never happen The catch is in that "expansion of the working fluid cools it" thing. Get low on refrigerant, and you have less stuff expanding into more volume: it gets colder. If the system was already on the edge of getting cold enough to freeze the condensed water on the A coil, trouble's coming. In really hot weather, a small central air conditioning system will run all the time: the coil never gets a chance to defrost if it does freeze up, so even that last hope is gone.
The short-term fix is to raise the thermostat so that the system runs less of the time, hoping to find a point that keeps the humidity down while keeping the coil from freezing up too badly to defrost itself between periods of operation.
The long-term fix is arriving Friday afternoon in the person of a service tech, and I just hope it's not too expensive.
In the meantime, 79 degrees and very low humidity is tolerable. It's certainly much better than the 90-plus degrees and 80 percent relative humidity outside.
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* Why is the inside coil called the A coil, and is the outside one the B coil? Nope. There's less here than you might think. Both coils are close kin to a car radiator and the inside one is very often in the form of an inverted V in order to get the most amount of heat-exchanger surface in the airflow while taking up the least length. Large commercial units will just angle the whole thing, but the cooling unit bolted on to most furnaces has a coil that looks like a capital letter A when viewed end-on
Update
1 year ago
