Handle9:bazzer:sbiddle:alasta:Jaxson: Heat pumps can dehumidify, but only in cooling modes, which you don't tend to use so much when it's cold.
This interests me. I had always believed that heat pumps would be an effective way of tackling household dampness in winter, but I guess a ventilation system or portable dehumidifier is also a must?
I don't quite agree with this. One of the basic principals of refrigeration is that a heat pump running in warming mode will reduce the humidity as the temperature increases above the dew point. It's not acting as a dehumidifier / air conditioner per se while in heating mode, but the warm air will be dry(er) air. Warm and cold air both have different characteristics when it comes to holding moisture.
Where is the water going?
It's still there but the capacity of air to hold water changes as it heats up. This is expressed in terms of absolute humidity and relative humidity. For the same absolute humidity (grams water/kg of air) the relative humidity (%) is different at different air temperatures.
e.g at 30 degrees Celsius 100% rH(i.e. when condensation occurs) is approx 27g H2O/kg of air
at 20 degrees Celsius 100% rH is approx 15g H2O/kg of air.
Warmer air can hold more water.
How is that heat pump specific?
As Bazzer says, you haven't removed any water from the house, just heated the air so it can hold more.