My central heat runs for about 8 hours and thin freezes up with about 3 inches of ice around the whole thing

Sounds like to me that you have a heat pump and that it is not going in to the defrost mode. To get rid of the ice that is there urn the unit off or put it on emergency heat. You may need a Tech to check it out. Several things can cause this.

Is it Ice or frost? Frost is normal, ice is not.

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Originally Posted by NorthernHeat

Is it Ice or frost? Frost is normal, ice is not.

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Mostly frost but it gets about 3 inches thick

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Originally Posted by letmetellu

Sounds like to me that you have a heat pump and that it is not going in to the defrost mode. To get rid of the ice that is there urn the unit off or put it on emergency heat. You may need a Tech to check it out. Several things can cause this.

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What is the emergency heat

There should be a switch on the thermostat for emergency heat, it is a back up heat source for those very cold days when the heat pump can no longer keep up.