ᐅ Mechanical ventilation with heat recovery and still keeping windows open at night

Hi,

I would like to have a mechanical ventilation with heat recovery system in our newly built house, but we always sleep with the window open at night (mainly because of the cool, fresh-feeling air; warm air doesn’t feel fresh to me).

Here in the forum, I’ve read several times that many people just do this “without any issues.”

I’ve also often read that this could interfere with the mechanical ventilation system (it might "malfunction"). It was mentioned that this leads to increased wear and tear, but I couldn’t clearly identify exactly how and on which components this higher wear would occur. Apparently, this only happens if the system uses some kind of dynamic pressure control and doesn’t operate with a constant static pressure.

I have also read that this not only cools down the bedroom with the open window but, in the worst case, could cool the entire house because the ventilation system causes a temperature equalization. So either the heating has to compensate or the other rooms get colder.

For me as a layperson and reader, this means:

- If you have a mechanical ventilation system, make sure it does not have dynamic pressure control to avoid the “malfunction” problem.
- When planning the ventilation, ensure that at least the attic and the ground floor have separate circuits for the mechanical ventilation and are not connected “in series.”

Regarding the latter: As far as I understood correctly from a planner, the pipe layout would look like this: assuming you have four rooms in the attic (bathroom, 3 bedrooms), two rooms would get supply air ducts, and two rooms would get exhaust air ducts (one of those definitely the bathroom). The airflow then passes under the door.

1) If I open the window in an exhaust room, I would expect the following:
- At most, only my room cools significantly due to colder outside air coming in through the open window.
- It might be that little happens (almost no fresh air in the room), except that the outside air flows quite directly into the exhaust.
- Other rooms lose their air exhaust; the air might stagnate there, causing the air pressure to rise and the pressure increase to reach the supply air fan. This results in more resistance and could lead to higher wear (it’s like a freight train with locomotives at front and rear: if there’s no locomotive pulling at the front, the one at the back has it harder, although it won’t supply more power than set). In the extreme case, this would be like holding the supply air fan in place, which I believe is not good for the component in the long run.
- The question is: how serious is this or am I overthinking?

2) If I open the window in a supply air room, I would expect:
- In the worst case, the supply air flows directly outside, and I get nothing from the open window.
- The “pushing” locomotive has more load because the “pulling” locomotive is absent.

If the answer is: yes, opening windows is a bad idea with mechanical ventilation with heat recovery, then my follow-up question is: how do I prevent mold if I can’t regularly manage to open windows?

In summary, it seems to me there are only four possible options:
- Spend a lot of money on individual controls.
- Forget mechanical ventilation with heat recovery, ventilate manually and, if you ventilate too rarely, just skip the insulation and build a house like in the 1970s.
- Install mechanical ventilation with heat recovery and live without opening windows.
- Install mechanical ventilation with heat recovery, ventilate anyway, and accept the consequences (energy loss, system wear, disturbed indoor climate).

What do you think?
Which of my statements are correct, which are not?

Thanks a lot for your answers.

Have you understood the purpose of a mechanical ventilation system with heat recovery? Mold does not occur because of a mechanical ventilation system – you don’t need to ventilate by opening windows anymore.

Otherwise, just get one without constant volume flow control, so you can easily open windows.

In summer, without air conditioning, we do the same – window ventilation is more refreshing and more effective than any mechanical ventilation system.

From a moisture protection perspective, it’s no longer necessary then, but air quality is a completely subjective feeling, so everyone should do it the way they prefer.

But yes, I’m now on the side of MyCraft; with automatic control, you will constantly be adjusting the system – whether that significantly affects its lifespan? Nobody knows…

...no warm air comes from a fresh air duct – just air.
Whether you find this as comfortable as having air through a window while sleeping is a matter of getting used to.

You hardly notice a mechanical ventilation system in living spaces – some people appreciate the feeling of a gentle airflow brushing their skin.

This topic keeps coming up in the forum... I still don’t understand why an open window would interfere with a mechanical ventilation system with heat recovery. The system supplies the nominal airflow into the house and removes the same volume out. Would it really be affected if you occasionally let in outside air? In your position, to fully understand and be sure, I would carefully review how the preferred device actually works. Then just open the window whenever you feel like it.

Just read my post...

With constant volume flow control, it works against it, otherwise it doesn’t care about it.