Air Conditioner Freezing Up

[Edwin Pawlowski]

OK so we put in a larger motor and now have 1125 rpm. There is a threshold for everything and this may be enough to cause the freezup, but is it the root cause? Given the design parameters and operating conditions of most home ACs, do you think the oversized motor (additional cfm) is the problem or it is just showing that there are other problems in the system? What is the increase of cfm of the 75 rpm when compared to a very strong wind over the condenser?

[RP]

There is a tolerance, or should be. That doesn't mean that you should purposely rig your system so that it's operating at one or the other extreme. The root cause in the hypothetical set of conditions that I referred to is that it's

1) cool outside. 2) too much cfm. This is by definition of the conditions. Now if I had said the unit is undercharged and running at a higher ambient, and you then increase rpm, then the root cause would be the low charge at a higher ambient, but not the motor, because in this case correcting the charge will stop the freezing. OTOH, the system will now freeze up at a higher ambient than before, so when the ambient falls well be right back to no.1 above. A wind will typically not affect head pressure significantly. Although it assists draw-through on the side that it hits, it retards it on the back side with a negative pressure. Very strong winds are a different story. But if they cause freeze up, and they can at lower ambients, then they would be the cause of the freeze up.

Just let me summarize the entire argument again: Virtually anything that reduces head pressure below what it would run with all components operating optimally, will also reduces SST, and thus evaporator temperature. Thus any such change can lead to immediate evaporator freezing if conditions are such that it was close to freezing before that change was made to the system. Simply installing a more restrictive filter can do it. Setting the t-stat down one more degree can do it. Shutting a bedroom door can do it. The list is endless, but all have in common that the coil skin temp must have been just above freezing beforehand. No problems with the system are required for this situation, since lowering either the indoor or outdoor temp tends to shove the evaporator closer to the freezing mark. You wouldn't for instance want to go off and leave the t-stat set to cool and 65F on a night when the ambient is supposed to dip to 60. That is just asking for a freeze up. HTH.

hvacrmedic

[Edwin Pawlowski]

Thanks