Ah, I think I’ve found the answer:
Admitting cold outdoor air into a space will lower the indoor RH. When the lower temperature outdoor air is brought indoors and heated, it loses moisture and reduces the overall RH.
For example: if the outdoor temperature is 0°F and 50% RH and the air comes indoors and is heated to 70°F, the residual moisture after heating the outdoor air will only be about 3% RH. Even on a nice, sunny 35°F and 50% RH day, the residual indoor RH will only be about 14%.
So by heating the cold, high-RH outdoor air up, the RH content drops tremendously. By introducing this extremely low-RH air into the house, you can expect the indoor RH to plummet.
(1) Condensation forms on the inside of the cold windows in the winter because warm air in the house can hold more water vapor than cool air. Once warm air in contact with the cool window is cooled below the dew point, water falls out of the air and onto the surface of the glass. An ice cold drink left out on the counter sweats for exactly the same reason.
(2) All else being equal, condensation forms on the inside of the windows more rapidly at night because the temperature of the indoor window surface is colder at night. The colder the windows are, the more water will fall out of the air onto the surface of the glass. If you want to know the exact temperature that condensation will start to form, it can be looked up on a psychrometric chart like this one:
Find the intersection of dry bulb temperature and relative humidity, then follow the dashed diagonal enthalpy line intersecting that point left and up to the edge of the curved area on the chart (the line marked “saturation temp”). That is the temperature at which your air will start to deposit condensate on surfaces. As you can see on the chart, cold air with a high RH carries less water in it than warm air with a moderate RH and cold air requires even colder surfaces before the remaining water vapor will condense.
(3) Set the humidity in the house low enough that there is little condensation on the windows. Very low humidity (< 20% RH) can be hard on one’s skin and one’s wood furniture, but 30% RH is not that bad. When you witness large amounts of condensation on the windows there may also be condensation in cold locations concealed within the walls and roof, and that moisture can lead to water damage or mold growth. Water damage and mold growth are harder on one’s health and pocket book than dry air.
(4) (a) Check that the supply registers are not aimed directly at the windows. With a central humidifier running, the air from the registers can be much more humid than the room average. There will be less condensation on the windows if the registers are aimed to mix with room air before coming in direct contact with the glass. If the registers cannot be adjusted to mix supply air into the room without directly hitting the glass, then consider adding directional covers to the registers or replacing the registers. (b) Decrease the room temperature. At any given relative humidity, cool air carries less water than warm air, leading to less condensation on the glass.
Best Answer
You cannot actively increase humidity with an HRV, at least during the cold months. The dial control allows you to run the unit as needed to reduce humidity by exchanging more moist indoor air with dry outdoor air. If you'd like to raise humidity you need to actually introduce moisture into your home.
A standalone humidifier in an appropriate location has done well for me in the past if you wish to avoid the expense of a whole-house unit. Place it where the air movement won't create discomfort and where the HVAC system will help circulate and distribute moisture.
By the way, you should not settle on a fixed RH for the entire winter season. You want to vary it depending on the outdoor temperature to avoid excessive moisture buildup and to prevent discomfort. 35% is quite low if it's 50 degrees F. outside, and may be high if it's -30F.