Dana1 I stand corrected. I mentioned HRV to control humidity when I should have said ERV. Doesn't the ERV remove some humidity? I questioned the validity of the comments in the first place as I know the minute the inside humidity reaches same as outside you are fighting a losing battle without a de-humidifier so at best this system may give you a few days of relief from the super hot humid weather. In Ottawa, we run into some hot humid spells that can last several days or more but usually there are cool dry nights where you could ventilate the ERV or HRV on high speed to change the air. Do any of these units have an option that will exchage air without moderating the temperature? I would think it would be more useful to be able to bring in cool night air and exhaust the warmer house air but NOT changing the incoming air temperature. This is useful if your wife won't let you open windows at night.

ERVs do not remove humidity. they help retain a humidity differential, if one exists between the inside and the outside, but it cannot create a differential.

Rob has it right- an ERV may "preserve" something like 40-50% of the humidity differential between the incoming and outgoing air streams, but it can't by-itself lower the humidity level in the house to lower than he outdoor dew point. The higher the ventilation rate, the more rapidly the interior air's dew point reaches that of the outdoor air.

But at the same ventilation rates ERV does lower the latent load for the dehumidifier as compared to HRV during high outdoor humidity periods.

Temperatures, drop over night, but dewpoints move more slowly. Flinging the windows open at night may reduce the sensible load (temperature), but when dew points are ~17C or higher it'll be noticeably less comfortable due to the higher latent load (humidity) that gets brought in. I haven't summered in Ontario, but in New England the higher humidity periods tend to coincide with higher sensible loads, making nighttime ventilation strategies less useful- the latent loads introduced exceed the value of the reduced sensible load. The sensible loads in my current place are quite modest, but the dehumidifier's duty cycle goes way up in July/August, and the total annual power consumed by the dehumidifier is well in excess of that used by the central AC. But in the much drier air of the Pacific Northwest (where I've spent decades) night time ventilation strategies work GREAT.