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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? Sep 29 '19 at 10:19
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    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? Sep 29 '19 at 14:42
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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.

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The OP says this is occurring in one room, but not in another. Possibly water is collecting in one run of ductwork due to a roof leak or a plumbing leak.

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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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  • Temperature increase doesn't change dew point, and it results in less condensation. You have a flaw in your thinking there. This would only apply if the outside temperature dropped.
    – isherwood
    Jul 21 at 16:04
  • Air Temp of 65°F. RH of 50%. Window temp: 48°. Dew point = 45°. No condensation. Air Temp of 70°F. RH of 50%. Window temp: 48°. Dew point = 51°. Condensation. Jul 21 at 18:41
  • RH goes down as air temp rises. That's fundamental.
    – isherwood
    Jul 21 at 18:52

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