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Question DetailsAsked on 6/9/2016

My a/c will blow out cold air for about 20-30 minutes then just blow out air for about 40-50 minutes then repeat.

My a/c will not cool the house to the set temp. (73-75) and it runs all day and night. It will blow out cold air for about 20=30 minutes the just blow out air for 40-50 minutes then repeat! The tech put in about a pound and half of freon but this did not solve the problem. It still doing the same thing over and over?

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1 Answer


During the 40-50 minutes is the outside A/C unit running - or shutting off ? Sounds to me like it is maybe overheating and the high-temp or high-pressure switch is shutting it down until it cools down, but the thermostat is still calling for cold air so the air handler/furnace fan keeps blowing thinking the A/C is running. Could be due to a compressor problem, fan failure, low gas pressure, faulty TXV valve, etc.

Another possibility is the evaporator coil is icing up because it is running too cold, and takes a long time to defrost and let air flow through it again, which would normally be due to a defective or mis-set TXV valve, or in the case of substantial leaks can be due to a leask in the evaporator itself. Outside unit might or might not keep running in that case depending on the design and sensors on it. In this case the air coming out would be slightly cool but nowhere as cold as normal, and airflow will likely be less than usual.

Since you say it keeps running day and night, could be a thermostat or control problem IF the inside temp drops below the 73-75 degrees so the thermostat should not be calling for cooling at night, but the unit keeps running.

Otherwise, if calling for cooling at all hours because it is not cooling the house down enough even at night, most common cause is a leak causing low gas pressure, so the A/C is not cooling down the evaporator coil asmuch as it should be. However, since he topped it up, it should have worked right at least for awhile (probably days at least if only 1.5 lbs low, because that likely indicates a slow leak), so you have two choices - pay for a full diagnosis (presumably from a different tech if he did not find and fix a leak, as he probably should have done before refilling the unit unless it has gone many years without a refill) and then also pay for whatever repair is needed, to keep an older unit going. If over 10-15 years old it is probably at or near its service life, so that could mean putting money into a unit which may or may not have long to go before another significant failure. Or if unit is pretty old, other alternative is to look at possibly replacing it with a new R-410a unit with higher energy efficiency (so commonly 20-40% savings on air conditioner electricity usage to the tune of typically a couple hundred to $1000/yr) and a new warranty.

Fair number of previous questions about replacing A/C units, including discussions on the replacement and R-22 (FReon, being discontinued) gas costs, energy efficiency, cost savings, and life-cycle cost effects of repairing versus replacing older air conditioners can be found in the Home > HVAC link in Browse Projects, at lower left.

Answered 3 years ago by LCD

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