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water in cold air return

I need your help. We just moved into a home in AZ which has 2 furnaces and heat pumps. When the A/C for the lower level of the house turns on, I get some water dripping out of one of my cold air returns. It doesn't last the entire time the A/C is running, but at least for 15 minutes. I went outside and took the shop vac and tried vaccuuming the over flow drains (which are always dripping, both upper and lower ones)but that did not solve the problem. Should I repeat this process or snake the lines? or is it something else?


  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    Water from return duct

    If water is dripping from a return duct, there are a number of things that you should consider.

    #1. If the return duct is uninsulated and run through an unconditioned space, the return duct may sweat (drip). For example, if the return duct passes through a 100-degree space at 50% relative humidity, moisture from the air will begin to condense if it comes in contact with a temperature lower than 78-degrees. This is referred to as the dew-point temperature. So, if the space is cooled to a temperature of, let's say 70-degrees, the ductwork, being uninisulated, will cool to a temperature lower than the dew-point temperature of 78-degrees. If such is the case, you will get dripping.

    #2. If the condensate line is clogged, you will get dripping. You will need to check the air handler and see if there is water accumulating in the base of the unit. Quite often there are two drains. One is the primary and intended to carry the condensate from the unit. This drain is often piped to an internal waste or drain line in the home. The second drain line is piped from an auxiliary drain pan that is located underneath the unit and should be perfrectly dry unless the main drain fails to carry the condensate away. This secondary drin line should be piped to a conspicuous location such as over a door. In the event water drips from that pipe, you know that the primary drain has failed to function correctly.

    Please keep us posted about what you found and how you proceeded.

    Hope this helps.
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