I have a heat pump system that is increasing the humidity in the house when it is in heating mode. So much so that when the heat pump comes on condensation starts to form on the windows.

It only affects the rooms where the heat pump is heating and even then it is not equal. For example, I have one zone that has two rooms. When the heat pump comes on the windows in one room fog up, but in the other room which is directly adjacent they do not, so the moisture might just be coming from one supply--not sure how that could happen.

How can I further diagnose and fix this problem?

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    There is no process in a heat pump system that would generate or create water vapor in the heating cycle. Question: Are you sure that there isn't a HUMIDIFIER installed that is setup to run when heating? – jwh20 Sep 29 '19 at 10:12
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    @jwh20 I am not sure. How can I determine if there is such a humidifier? – Tyler Durden Sep 29 '19 at 10:19
  • Can you take some photos of the air handler? If there is a humidifier it will be installed on the OUTLET side of the handler. – jwh20 Sep 29 '19 at 12:03
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    To find a humidifier, look for water lines running to your furnace. Often I've seen it installed as a 1/4" copper water line running from an additional valve on top of the hot water heater which runs to the humidifier installed on a bypass duct between the supply and return near the furnace. Sometimes there also is a duct shutoff damper on the bypass duct that people label "Winter/Summer" or something along those lines for open/closed. – Dotes Sep 29 '19 at 14:21
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    is this thing running in A/C mode in the hours before this happens? How soon before? – Harper - Reinstate Monica Sep 29 '19 at 14:42

If your drip tray in the inside unit has standing water this will cause high humidity when heating. Other than this there is no way for a heat pump to generate moisture in heat mode.


This is the natural phenomenon of condensation. The increase in temperature changes the dew point of the air, and the cold windows start to form condensation since they are below the ambient dew point. This is the same reason why the part on your fridge between the freezer and refrigerator is heated. The cure is either to warm the windows, or cool the air next to them. You can use curtains or sheer to keep a layer of cool air trapped next to the glass. An exterior insulating film could help keep them a little warmer, but I'm not aware of any such product.

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