wrote:

I live in an air-conditioned mobile home.

There are no return ducts.
Air is returned to the furnace in the utility room.

Some rooms are quite cooler than others,
so I've closed the outlets in those rooms.

Friends tell me that if you close the outlet ducts,
theres a chance of freezing up the cooler coils.

Possible ??

Any rule-of-thumb for measuring/avoiding this ?

As a rule of thumb, if it freezes up then you've closed too many vents.

hvacrmedic