A/C freezing up
 

We've got a fairly old A/c unit in our mobile home. Got it serviced at the beginning of the summer. Learned we can't set it at 74 and expect the coils not to freeze up. The unit worked great all Texas summer long. Even through the 20+ days of triple-digit heat. Usually had it set at 78. Now, the weather has cooled down, mid-70s, lower 80s. But now it has started freezing up again. . We change the filters about every 2 months, even though we keep a clean house and no smoking inside. What could be the culprit?

Typically a low freon charge will cause the coils to freeze up but, being that you are able to run the unit at 78 degrees without a problem; I'd say that it sounds like the blower motor is not running on high speed. Furnace blower motor have different speeds depending whether it is running in heat or A/C mode. Another cause would be a dirty air filter but, you have eliminated that issue so another possibility would be block air vents (supply and/or return).

Doubtful, but there could be some moisture in the system.

The air across the evap coil is probably not sufficient.

What's a rule of thumb for setting the blower motor speed as far as sq. footage or, as you mentioned, in heat or A/c mode. Didn't realize there was supposed to be a difference.

Sailor86 - you don't set the speed, the control board for the unit changes the blower speeds depending on heat vs. A/C via a switching relay. The CFM for the various speeds will then be predetermined by the blower motor's RPM ratings and other manufacturer specs.

So I suppose I should leave it up to an A/C tech then. I love nothing more than tearing things apart to figure out how they work, but i've learned my lesson a time or two.

Either that or meter the different wires on the blower or the switching relay to see which set of motor windings is being powered in A/C mode.

When it was hotter the air going through the coil was 78 degrees so the coil would not freeze. There are points in the coil that get real close to the freeze
point. so if the air is slowed down in any way it can freeze the water that is running down the coil. If the home get a lot of humidity in it then you have more
water going down the coil and that slows the air flow down a little, and your are now running cooler air , so it freezes. You can take all the numbers off the furnace and blower and take this info to a furnace wholesale house and they can tell you if you have the right blower for the size of your air con..
another you should thats get over looked a lot is, take your blower out and see if the curved fins are getting a lot of dirt build up in them. That can really slow the air flow down. Check to see if you are using the black hi speed wire
when in the air con mode.. Later Paul

Well if the unit is apparently older, then how could blower speed be a problem? If it works for years before then something beside blower speed could be the problem. There are several other things I would check before cfm on a blower. 1. Even though you change filters doesn't always mean your evaporator coil is clean. It could be partially stopped up. Still the higher temp would sometimes not all the time eliminate this. 2. The unit could be low on charge and freezing up when it gets colder or at night (when it tends to get colder). If you don't have any refrigerant gauges you will need a tech. I hope this helps, take a look if you can at the indoor coil first and then check refrigerant.

Just because you charged and old unit in spring does not mean it is still charged properly.

Does the air velocity still feel the same?