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what causes a central air conditioning unit to freeze up?

I have a central air conditioning unit that is probably 12 years old or so.I recently had a situation where my central air would run constantly without shutting off.The air temperature in my house was 80 degrees and it was still running.Well,I also recently found out that the coils in the ac unit were completely frozen solid along with the hoses that ran from outside at the unit,all the way inside to my furnace.Id like to know what causes these coils and pipes to freeze and how does one keep it from happening again?

Public Comments

1. "what causes a central air conditioning unit to freeze up?" Every once in a blue moon, cold temperatures lead to things freezing. Is there something that may be causing cold temperatures in or around your A/C? That may be the problem. "Id like to know what causes these coils and pipes to freeze and how does one keep it from happening again?" Again, sometimes it's that cold weather... maybe put it next to your heater and crank it up all the way?
2. you need to change the thermo cupler. call a reparman.
3. the first and most likely answer is low freon
4. Not enough air flow or the cold air is being recycled back into the intake. Check the filter, check the fan, check for any vents which might be blowing into the intake.
5. lo charge
6. I go with Chester, simple things first. Check the air filter, if the machine is labouring for air it can cause it to freeze up.
7. It has a off cycle defrost that means the unit has to turn off long enough to melt the coils. Your settings are to low bring them up to about 75 degrees and when it is stable you can lower to 70 degrees but no lower. Leave the fan only and thaw the coils completely off.
8. happens to me all the time, but it's because I always forget to change the filters !! Only once was it due to low freon ... check the filter .....