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How to Repair a Frozen Air Conditioner | Ask This Old House

Plumbing and heating expert Richard Trethewey shows how to prevent ice buildup from shutting down an air-conditioning system. (See below for steps.)

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Richard troubleshoots an air conditioning coil that freezes up with use. Richard replaced a 4-ton air conditioner with a more appropriately sized 3-ton air conditioner.

Steps for How to Repair a Frozen Air Conditioner:
1. Clean or replace the return-air filter to ensure it’s not obstructing the flow of fresh air to the air conditioner.
2. Confirm that the size of the air-conditioning unit can accommodate the diameter and number of cool-air ducts.
3. Remove the metal access panel to expose the evaporator coil.
4. Check the size of the outdoor condenser unit. Typically one ton of cooling is needed for every 500 to 600 square feet of living space.
5. If the condenser is too big for the house, it can cause ice to form on the evaporator coils and shut down the system.
6. Pump out and collect the refrigerant from the existing too-large condenser.
7. Disconnect the old condenser and cart it away.
8. Install a new appropriate-size condenser.
9. Connect the new condenser to the existing electrical power supply and refrigerant lines.
10. Remove the old evaporator coil and replace it with a new coil that matches the tonnage rating of the new condenser.
11. Reconnect the ductwork and seal the new evaporator cabinet at top and bottom with sheet metal strips and foil tape.
12. Braze new copper connections to the refrigerant lines.
13. Insulate the suction line.
14. Connect PVC pipe to the condensate drain and then attach the pipe to a pump.
15. Run flexible tubing from the pump to a drain.
16. If necessary, add refrigerant to the new system.
17. Turn on the air conditioner to ensure it’s operating properly.

Expert assistance on this project was provided by Papalia Plumbing [https://papaliaplumbing.com/] and Air Purchases [https://www.airpurchases.com/].

About Ask This Old House TV:
Homeowners have a virtual truckload of questions for us on smaller projects, and we’re ready to answer. Ask This Old House solves the steady stream of home improvement problems faced by our viewers—and we make house calls! Ask This Old House features some familiar faces from This Old House, including Kevin O’Connor, general contractor Tom Silva, plumbing and heating expert Richard Trethewey, and landscape contractor Roger Cook.

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How to Repair a Frozen Air Conditioner | Ask This Old House


  1. The homeowner should go back to the company who sold him the totally wrong size A/C for his house and file a complaint.

  2. I would add that a unit low on charge will provide high superheat, like a faulty metering device; the evaporator is being starved and the refrigerant will boil off early in the coil; ergo high superheat. However ANYTHING that inhibits airflow—dirty filter, closed off registers, eliminated registers (remodeling) bad board, cap, motor, dirty coil etc etc—-means there will be LOW superheat. The refrigerant boils very late in the coil, or not at all. That COULD slug the compressor. The unit will be cold as hell and wet, and may, or not freeze. It could be well below 32 degrees, cold and wet as hell, and not freeze 100% of the time. Richard is smart as hell. But a smart tech looks at deltas, superheat, subcooling, static pressures, CFM before he concludes his diagnosis. I know it's a TV show, so editing for time is necessary. But jumping to oversized? That's flying blind, and a risky move.

  3. This is a “replace”, NOT REPAIR!

  4. I think it was just low on Freon

  5. Bro just replaced the whole thing

  6. So you had a 4ton condenser and a 4ton evaporator and you could have solved the problem with more space. Why not simply duct the basement space to take up the excess? Seems like that would have been cheaper.

  7. Purchased: August 2023 – still works GREAT!I https://www.youtube.com/post/UgkxxsUnXhGsSJLim_XnMHyQK0u3XVaW-CGn live in a studio and during the summer it gets scorching hot – really old building with no ac units. I can’t express how EASY it was to install. This unit has been a life savior during the summer and some days during other seasons where it can still be a bit warm at night. In this small place is my friend, a husky, poodle mix and myself. We need AC – lolI don’t use the dehumidifier option – I’m not sure if it will leak in my house, since I did not install the small draining hose that came with it. May look into it late but I don’t worry about much humidity in the apartment. I don’t understand why the negative reviews since all things mentioned, I personally did not find issues with. Definitely worth it!

  8. Wonder how much This Old House made out of this installation that wasn’t really necessary

  9. Under charge, over charge, dirty evaporator or lastly a clogged condensation drain. Check in reverse order listed.

  10. Lot of questions remain unanswered here. The return duct seems greatly undersized? Were the return and supply ducts checked for blockage? How old was the current unit and did it use the current refrigerant? Why did they cheap out from getting a new line set and install it where it is supposed to be?

  11. You just saved me a ton of money and sweat

  12. This guy is wrong. Too large a unit may under dehumidify, will not freeze the evaporator.

  13. So didn't actually repair it, replaced it.

  14. I am no expert but they just threw away a good unit.

  15. Title should say replace, not repair

  16. It's a wonder that he didn't also have a buddy ready to build an addition onto the house. I don't even remember seeing them check if the refrigerant level is low.. just replace everything because it cost the most to do!

  17. Didn’t bother to check the registers?
    Didn’t bother to check the TXV?
    Didn’t bother to ask how long the problem had been happening?
    Didn’t bother to get superheat/subcool?
    Didn’t bother to put gauges on?

    Good enough for This Old House, I guess.

  18. That thing is grossly oversized for that ductwork. I could tell just by the thumbnail. Bigger is not always better. 4tons for 1300 square feet. Yikes. Needs maybe half that

  19. I was having issues with mine freezing and come to find out my A coil in the furnace was half clogged with pet hair so not enough airflow was getting through.

    It’s not easy to clean so if you are not a handyman do not try and clean it yourself.

  20. Love how there's no mention of the system being short on return air, that single 12" pipe isn't enough for that that 100k 80%. They're still short on return for a 3ton so they really didn't do this guy any good. And it's zoned, that whole system is a mess.

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