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Question DetailsAsked on 7/13/2016

If one of the outside pipes leading into the ac unit freezes what does that indicate?

When trying to lower ac, especially at night the pipe will freeze and the unit will not put out cold air. What would this indicate as the problem. We've been told many different things

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1 Answer


Rarely a leak causing freezing at the point of leak - which would normally, unless just a pinpoint freezing spot, leak out all the gas within a few days. Generally, if not just at a small spot this indicates that there has been a leak so the pressure in the system is low (which would normally cause freezing at the evaporator coil ("indoor coil") and on the return (low pressure) line from the coil, which usually has the filter/dryer canister installed on it, that there is a flow restriction or control valve failure so the system is running at the wrong pressure, or that the system is being run at too low an ambient termperature.

Since you say at night but not only at night, and when trying to lower the A/C a couple of most likely causes:

1) if occurs only when there are low inside OR outside temps - typically below about 60-65 (check owner's manual), then the unit is probably just not designed to run at outdoor temps that low. It is possible with some systems to (if it does not have it already) retrofit a control unit with thermostat on the condensor unit and fan so the fan operates only when it is actually needed to cool the compressed gas before circulating it through the evaporator - many fans run whenever the compressor is running. There are also control valves (which however add to the system complexity) which can monitor the evaporator temp and shut the unit off before it starts freezing up - all units should in theory be designed with this as well as the normal overtemp control but many or most are not, resulting in damage to the units in some cases.

2) if the evaporator unit and maybe return line is freezing up (usually at and near the evaporator only if outside unit is in temps needing A/C), then could be from reduced airflow across the evaporator - mis-set duct damper or duct registers closed too much, or more commonly a clogged air filter.

3) could be low refrigerant pressure in the system causing excessive cooling at the evaporator due to too low a return pressure in the lines - almsot always caused by a leak in the system (unless system was improperly charged with gas, like if the tech did not check the temperature tables to be sure the correct amount of gas was put into the system for that ambient temp)

4) if lowering household temp, do it in small stages - a couple of degrees at a time then wait several hours before lowering it again to give the unit a rest - because lower it a lot at one time and the A/C will have to run a LONG time to draw the indoor temp down that much before it shuts off, resulting in possible overcooling and excess condensate formation and evaporation at the evaporator coil followed by freezing up of the coil. Setting it back just a couple of degrees at a time till it stabilizes gives the coil a chance to dry off and warm up, melting any incipient frost that formed.

5) few other possible causes like a malfunctioning TEV/TXV control valve, fauylthy control board (if digital controls), thermostat using too big a setback, etc - but they are relatively rare in comparison to the above causes

Heating and A/C is the Search the List category to find a well-rated and reviewed vendor to solve this problem.

Answered 3 years ago by LCD

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