fundamentals vs fundamental belief

[Rob White]

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I agree with Mad Fridgie, different things cause the AC evaporator
to freeze over but the term freeze over is a broad statement and
not all ice is the same.

When an evap ices up due to poor air flow the ice formed tends to
be frozen water ice and very hard, very solid. The ice that is formed
from a low refrigerant level is more frost or snow and as the snowy ice
insulates the evap it prevents the heat exchange so the evaporation
process slowly moves out of the evap and can go into the suction pipe.

That is why frost forms on the suction valve (on the outside unit) when
the system is short of refrigerant.

Also as it has been explained earlier if the system looses refrigerant the
evaporating pressure tends to drop. With a room temperature at +21 degsC
the air off the evap coil is going to be about 15 ish degsC so the evaporating
temp will be about 5 ish degsC.

5 degsC is above freezing so ice is very slow to form, when the evap pressure drops
the evap temp drops and instead of evaporating at a temp above freezing it evaps
at a temp below freezing.

Regards

Rob

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