ac froze up when fan on low speed.

Posted By: TXiceman on 08/24/14 10:29pm

3PuttNut wrote:

Low refrigerant can also cause the coils to freeze up.


Actually this is the least likely cause of an evaporator coil icing.

The compressor on these small units are what is know as a positive displacement or PD compressor. This means that it pumps a relatively constant volume of refrigerant on each revolution of the shaft.

With the fan on low speed, there is a lesser amount of refrigerant vaporized in the evaporator. The compressor keeps trying to pump the same volume, so the pressure drops in the evaporator to the point that the specific volume (this is the reciprocal of the density of the vapor) of the refrigerant increases to the point that the volume will satisfy the compressor. As the pressure in the coil drops, the temperature drops due to the properties of the refrigerant.

You let the pressure drop to the point that the coil temperature drops below the freeze point of water and the coil is iced or frozen.

I wish that the manufacturers would do away with the low speed on the RV units.

Makes sense. Thanks.

3PuttNut wrote:
Low refrigerant can also cause the coils to freeze up.

Actually this is the least likely cause of an evaporator coil icing.

The compressor on these small units are what is know as a positive displacement or PD compressor. This means that it pumps a relatively constant volume of refrigerant on each revolution of the shaft.

With the fan on low speed, there is a lesser amount of refrigerant vaporized in the evaporator. The compressor keeps trying to pump the same volume, so the pressure drops in the evaporator to the point that the specific volume (this is the reciprocal of the density of the vapor) of the refrigerant increases to the point that the volume will satisfy the compressor. As the pressure in the coil drops, the temperature drops due to the properties of the refrigerant.

You let the pressure drop to the point that the coil temperature drops below the freeze point of water and the coil is iced or frozen.

I wish that the manufacturers would do away with the low speed on the RV units.

Ken

Yes it has a wall mounted thermostat- I will have to check on the freeze sensor apparently it isn't functioning properly.

Does it have a wall thermostat? If so it should have a freeze sensor. Sensor is accessed from the inside and often falls away from the evaporator rendering it useless. Or was poorly placed during OEM install.

Also check the air plenum inside. Often the IN is not well separated from the OUT causing cold air to recirculate and over chill through the evaporator.

Thanks for the replies- all helpful with exception of that last one.

Which is why it should ALWAYS BE SET TO HIGH...

Low refrigerant can also cause the coils to freeze up.

Wasn't trying to regulate the A/C so much......was trying mitigate the noise from the fan on high.

Can the evap coils be cleaned from inside- or is this done from the top?

My friend who is a mechanic, told me to never regulate the ac with the fan speed. Always keep the fan turned up high and adjust the temp control to regulate temp. He warned me if I turned the fan down it could freeze up the ac unit. Guess he was right.

Set fan to high.......

Low air flow, high humidity causes icing on evap coil.

Dirty air filter, low air flow......same

Dirty air filter, low air flow, dirty evap coil......same