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The Mysterious Ventilator - Condensate on Windows

Since you read our last post and now know how your air exchanger works, you can begin to understand the debate and differing viewpoints/advice on how to set them. Let’s begin by talking about Relative Humidity. Basically the colder something is, the less moisture it can hold before it becomes condensation or basically, water. The hotter something is, the more moisture it can hold before it becomes water. There’s way more water in the air on a hot 30 degree C day at 90% RH than on a cold -30 degree C day at 90% RH humidity in winter. Humans like around 40% RH (relative humidity) at room temperature as do our wood floors and furniture, but, we don’t like mold. On those awful -40C days, our not so efficient windows become really stinking cold. So as soon as our normal temperature and normal humidity air touches the freezing cold window, the air drops in temperature drastically. Our 20 degree 40% relative humidity air just dipped super low and became much colder, 100% relative humidity air. Now there’s water on our windows. Standing water = moisture = mold which is also not great for our breathing or our windows.

We can try to get our indoor humidity lower by turning on our ventilator to a lower RH setting to make the windows happy, but then our indoor air will feel dry to us, leading to breathing issues, bloody noses and general discomfort. Or we can keep it where it is and get condensation on our windows. So for most homes with builder grade windows, we can’t have it both ways, and that’s where the debate really lies. The colder it is outside, the harder it is to keep humans and windows happy from a humidity perspective. But now you know, and can make a decision that works for you!


Ok, fine. I’ve been yammering on like some kind of baking blogger when all you really want is the recipe…


- Summer: off/summer

- Winter: 35-45% when the weather isn’t too bad outside. The colder it is outside the lower you should set it to preserve your windows and prevent mold. Some fancy controls will do this automatically for you in “smart” mode. I disobey this though and keep it at no lower than 35% because of my allergies and asthma etc. I then periodically wipe the windows down when needed.

To mitigate condensation on windows, keep blinds and curtains open and run the furnace fan by turning the thermostat setting from “auto” to “on”. It won’t completely remove it by any means, only lowering the RH% will, but it will help.


- Summer: Off or on periodically, on minimum settings if the air feels stale. If your control has timer setting, this may be a good option on days that aren't too humid. 20 minutes on every hour is more than enough.

- Winter: Same advice as the HRV.

Final advice: Always remember what your air exchanger is doing. When it's on it's always replacing indoor air with outdoor air. If it's hot and humid outside, that's what it's bringing in the home. If it's cold and dry, that's what it's bringing in the home. Your hrv and erv is just trying to do this more efficiently than opening a window for fresh air. Also remember that if you have a humidifier in your home, in wintertime the air exchanger and the humidifier are fighting each other a bit. So if you want to avoid that, set the RH on the hrv control about 5% higher than the setting on the humidistat. Humidistats are often not accurate, so there may be some more adjusting to do there as well.

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