ᐅ Does the ventilation system really pump humid outside air into the house?

Created on: 13 Aug 2023 22:37

Good evening,

I need the help of this great forum again. I searched online beforehand but couldn’t find anything. So, I’m trying here:

We have a single-family house built in 2021. The ventilation system provides a very comfortable indoor climate. We feel good. However, my hygrometers show quite high humidity levels (in the basement 70% / in the living area just over 60%). On one hand, I find this worrying, but on the other hand: the ventilation system brings outside air inside, and currently, the outside humidity is over 70%. So, you could say: no wonder! If that’s true, I don’t understand the purpose of the ventilation system if it’s blowing humid outside air into the house.

Thanks for clarifying where my thinking is wrong…

Have a nice evening
Daniel

RotorMotor14 Aug 2023 08:55

xMisterDx schrieb:

But you should only run your ventilation system at a minimal level during the day in summer, especially in the peak summer months. Because besides moisture, you also bring warm air into the house. The heat exchanger can hardly prevent this.

This is incorrect. Modern ventilation systems can recover up to 90% of the heat.
This works at -10°C (14°F) outside and 21°C (70°F) inside.
It also works just as well at 30°C (86°F) outside and 21°C (70°F) indoors.
In fact, it is noticeable that the temperature difference (delta T) in summer is often much smaller.

I have already done several calculations here in the forum showing how minimal the heat gain from running ventilation in summer actually is.

So keep the ventilation running in both summer and winter to maintain a comfortable indoor climate.
A control system based on humidity and CO2 levels can make sense.
Temperature should not be a deciding factor due to the heat recovery.

HoisleBauer2214 Aug 2023 13:44

RotorMotor schrieb:

I have already calculated several times in this forum how minimal the heat gain from running ventilation is during summer.

Links would be great...

RotorMotor schrieb:

Wrong, modern ventilation systems recover up to 90% of the heat.

But what do you do if it’s 26 degrees Celsius (79°F) inside and 30 degrees Celsius (86°F) outside? Then the ventilation system brings in humid air (assuming there was a thunderstorm or heavy rain 30 minutes ago) at about 28 degrees Celsius (82°F)—a mixture of 26 and 30—which still leads to an increase in temperature and humidity inside the house. In that case, running the mechanical ventilation with heat recovery is not advisable, right?

RotorMotor14 Aug 2023 14:07

HoisleBauer22 schrieb:

Links would be great...

For example here: https://www.hausbau-forum.de/threads/haus-bau-2019-immer-zu-warm-april-oktober.45562/post-630231
Feel free to ask if you have any questions.

HoisleBauer22 schrieb:

But what do you do when it’s 26 degrees inside and 30 degrees outside?

Even better. That’s only a 4-degree difference. With active heat recovery, it’s at most 1 degree.
So, hardly any heat gain occurs.

HoisleBauer22 schrieb:

Then the ventilation brings in humid air (assuming there was a thunderstorm or heavy rain 30 minutes ago) at 28 degrees (a mix of 26 and 30), which still leads to increased temperature and humidity indoors. So running the controlled ventilation system is not advisable, right?

Here you’re a bit unclear and are mixing up temperature and humidity.
I mentioned that adjusting the control settings can make sense when humidity is high, as long as CO2 levels remain low.
However, this has nothing to do with temperature (except for its effect on relative humidity).

KarstenausNRW14 Aug 2023 14:09

#10

HoisleBauer22 schrieb:

But what do you do when it’s 26°C (79°F) inside and 30°C (86°F) outside? Then the ventilation system sends in humid air (assuming there was a thunderstorm or heavy rain 30 minutes ago) at 28°C (82°F) (a mix of 26 and 30), which still leads to a temperature rise and increased humidity inside the house. So running the mechanical ventilation system isn’t advisable, right?

You don’t need to do anything because, in practice, nothing really happens. You already had the humid air both outside and inside before. Turning off the ventilation system for half an hour isn’t worth it.

And the temperature increase caused by a mechanical ventilation system can be safely ignored. Last month, during constant outdoor temperatures over 30°C (86°F), I never got the indoor temperature above 24.5°C (76°F) (there was always some sunlight coming through the windows, I don’t like completely darkened rooms).

Reaching 26°C (79°F) indoors requires several weeks of Sahara-like summer with nighttime temperatures still around 25-30°C (77-86°F).

HeimatBauer14 Aug 2023 14:45

#11

And even if some moisture gets in, it is still less than the uncontrolled ventilation through open windows.

I have a humidity control system that I built myself in the garage. Maybe someday my ventilation control will become a bit smarter in this regard – it’s nice when you already have the ductwork in the house and just need to install a more modern control unit.

HoisleBauer2214 Aug 2023 15:52

#12

KarstenausNRW schrieb:

26 degrees Celsius (79°F) indoors requires several weeks of Sahara-like summer with nighttime temperatures still between 25-30 degrees Celsius (77-86°F).

That’s true for a well-insulated house. Otherwise, indoor temperatures often rise to 28-29 degrees Celsius (82-84°F) (house built in 2012, meeting the minimum standards at the time).

ᐅ Does the ventilation system really pump humid outside air into the house?

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