Plumbing and heating expert Richard Trethewey shows how to prevent ice buildup from shutting down an air-conditioning system. (See below for steps.) SUBSCRIBE to This Old House: http://bit.ly/SubscribeThisOldHouse 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…