I have a new home, insulated fairly well (6" walls, etc - not overdone, but good enough).
My HRV has a de-humidtistat, and I have it set for approx 55: relative humidity.

what would be the correct setting for approx -25° Celcius outside ambient temperature.

20% relative humidity (for -25°C) according to the table on my Honeywell Humidity Control. I set mine to 30% regardless.

I'm quite sure that if I set mine for 35%, it would be running on high all the time...

maybe that's how it has to be though.

I installed a humidifier in our home when I renovated (6 mil VB and R40 in the ceiling, R15+ in the walls.) I have never needed to use it and disconnected the line to it.

Our relative humidity is always around 50%. On the coldest days, we may go down to 35-40% and at its most humid in the summer perhaps 60%.

I don't believe that 20% in winter is adequate for good health and you must also get a lot of "zaps" touching metal.

Even when I lived in Calgary at -35C, our humidity was over 30%. Good windows prevent frost and condensation issues.

The relative humidity in my 2-year-old home has been between 46% and 50% all Winter so far. I'm not sure if this is a good thing or not. I've done a little be of research online and have seen it mention that anything over 55% isn't good and some sites say 50% or more isn't good either. I would really hate to start experiencing any sort of mold problems... :o

If you have excessive condensation on the windows, take steps to reduce humidity. (Turn down the humidifier, run exhaust fans or open windows slightly.) Slight condensation on cold nights is Ok.

Mold anywhere, water accumulating on window sills or water on the walls is a sign of excessive humidity and can be bad for most peoples' health.

A humidistat setting of 20%-30% is good for most areas of Canada. Slightly higher can be Ok in warmer regions or very well built homes.

I have little (very little) to no condensation on any windows (all triple pane), and never any mould such as Stachybotrys (http://gcrc.meds.cwru.edu/stachy/default.htm), or anything at all

just wondering where the break even point would be w.r.t. putting excess energy into heating humid air, or exchanging humid air through the HRV for dryer outside air - but losing some heat via the HRV in the process.

Anything over that required for personal health is unnecessary. Some people experience problems from overly dry air but most people can adapt. I experienced skin and sinus problems in an office due to the temperature being too high and the air too dry from constant air exchange. Turning the temperature down slightly can actually reduce the problems caused by dry air and increase the relative humidity (relative to temperature, that is.) Creating humidity consumes energy and can cause problems if it condenses in a poorly built home. Just because you cannot see condensation or mold doesn't mean that it doesn't exist behind the walls or elsewhere.

Before you turn off your humidifiers because you don't feel you need it check the warranty on your hardwood floors first.

Also, don't rely on the cheap non-digital humidistats that come with humudifiers such as a General Aire 1042 or 570 drip. They can vary by as much as 10 to 15%.

The HRV dehumidifies so be careful to not run the HRV all the time with the humidistat turned up. Defeats the purpose.

If the HRV is not install correctly it won't dehumudify properly either. Check to see if it is extracting stale air from a portion of the return duct further away from the 'drop' and supplying fresh air to the drop it self or very close to it.

We just had hardwood installed and the installer said that the relative humidity in our house should be between 40 to 50%. I have my humidifier set to 55 and the relative humidity of the house is around 37%. I know when we get colder weather, below -25C, my windows are going to ice up on me. This happened during the first winter in our house (new house). I forgot to turn down the humidity setting when the temps fell and we had thick ice on our windows. Before we had hardwood, I would just go with the temp chart that is on our humidity control unit.