Thursday, June 16, 2022

Climate Control On The Edge

      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.
* 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


Cop Car said...

Each of us is different in sensing heat. 79 degrees isn't too bad, to me, so hope you can stand it until your A/C is fixed. (And thanks for the tutorial. I'm happy that you are willing to provide the rest of us such thoughtful input on a wide variety of subjects.)

When we had our own A/C-Heat Pump replaced several years ago, I told "them" to increase capacity by 1/2 ton over the original equipment. "But you don't need it." I told them that, for the extra money, they should indulge my whim. Good decision. We have loved the new (high efficiency) system - very quiet even when cooling (I've set it so that the fan is always circulating the air on "low" speed and, when heating or cooling is active the system rarely has to kick into a higher speed than that) plus, the system actually gets to idle a reasonable amount of the time.

Anonymous said...

It's cottonwood seed season here. Is the condenser coil clogged up with cottonwood fuzz? Try to hose off the coil, do it with the water stream angled to minimize forcing the fuzz deeper in the fins. My boss made a wand for the transmitter A/C that can be shoved through the fan opening and spray water from the inside out to clean the coil without disassembling the condensing unit. Still need a full cleaning every couple of years, but the wand buys time.
At the studio, we had some window screens custom-made that fit over the cooling tower intakes. They catch all of the fuzz and are easily removed for cleaning.

Bruce said...

Dirty outside coil is most likely and not horribly expensive. Low on charge is a possibility and probably as bad as it should get. Never fear, they'll try to sell you a new one. It's rarely justified.