ᐅ 20% Relative Humidity in a KfW 40+ New Build

Tolentino schrieb:

Hmm, there seems to be a misunderstanding here.
If you ventilate manually the right way, you have the same dry air issue as with a mechanical ventilation system without an enthalpy heat exchanger, but on top of that, more effort and more heat loss.
The dry air is not caused by any technical conversion in mechanical ventilation but by exchanging moist indoor air with dry (because it’s cold) outside air. In mechanical ventilation, at least 80-90% of the heat is recovered.

If you want to achieve the same air exchange manually, you also have the heat loss and have to run through the whole house at least three times twice a day (opening and closing).
Most people don’t do that and therefore have better indoor humidity. However, there is still some risk of mold in doubt.

With manual ventilation, you can control it better. You simply close the window when the desired humidity is reached.
You don’t need to run through the house—you can just walk normally 😉.
You only face a mold risk if you don’t monitor things properly and adjust accordingly.

WilderSueden schrieb:

Depending on the figures, with 3 air changes per hour (= ventilating 3 times), you get 6-10 kWh per square meter per winter

In the heating load/energy rating calculation of our house without mechanical ventilation, heat loss due to ventilation accounts for about one-quarter of the total losses.
The remaining three-quarters come from all other building components, such as the slab, walls, windows, roof, etc.

Nida35a schrieb:

In the calculation of heating load/energy rating for our house without mechanical ventilation,
heat losses due to ventilation account for roughly 1/4 of the total losses.
The other 3/4 come from all other components such as the foundation slab, walls, windows, roof, etc.

That amounts to about 500 to 600 kWh per year.
How much electricity does the mechanical ventilation use annually? And what was the initial cost?

For us, airing out twice a day (morning and evening), plus a bit more when lighting the fireplace in the open-plan living area, is sufficient to maintain comfortable humidity levels. You should also remember that the front door is opened several times during the day, letting in fresh air as well.

motorradsilke schrieb:

With manual ventilation, you can control it better. You simply close the window once the desired humidity level is reached.
You don’t have to run through the house either; you can move around normally 😉.
You only risk mold if you don’t monitor the situation and adjust accordingly.

Oh man, now windows are supposedly better for “controlling” ventilation than a device designed specifically for that purpose, equipped with sensors and control technology.

motorradsilke schrieb:

For us, ventilating twice a day (morning and evening), plus an extra time when lighting the fireplace in the open-plan area, is more than enough to maintain comfortable humidity levels. You also shouldn’t forget that the front door is opened several times a day, which lets in fresh air too.

Is it only humidity you care about?
Sure, that fits this thread well, but:
CO2, VOCs, and other pollutants—don’t they matter to you?

RotorMotor schrieb:

I’m always a fan of calculations, but those without figures don’t make much sense.
The first search result shows 70 kWh/m²/a (7,100 BTU/ft²/year), which is 10 to 7 times higher than your value.

A main issue is probably that you assume only air is exchanged. While that might be true for a mechanical ventilation system with heat recovery, floors, furniture, walls, etc., cool down when windows are open and need to be reheated.

I’m not familiar with that calculation, but I’d be interested to know the assumptions behind that figure.
For me, the assumptions are roughly as follows:
100 days × 20°C (36°F) temperature difference × 3 air changes × 2.5 m (8 ft) ceiling height × 0.33 Wh → 5 kWh (winter)
90 days × 10°C (18°F) × 3 air changes × 2.5 m × 0.33 Wh → 2.2 kWh (transition period)
That would result in about 7.2 kWh/m² (670 BTU/ft²) in this example. This should be quite close to the range officially calculated by @Nida35a. Of course, this is only an approximation. Cooling of furniture is not included; at the same time, furniture also reduces the air volume. With proper shock ventilation (when weather conditions allow), mostly only the air is exchanged. Of course, if someone showers in the morning, then opens the bathroom window and returns after breakfast half an hour later, this calculation does not apply. The same goes for tilted (ajar) windows, etc.
Whether 3 air changes are sufficient is debatable. More than three ventilations per day, however, is certainly unrealistic. For most people, I would expect even only twice daily. See Silke 😉

WilderSueden schrieb:

I am not familiar with the calculation but would be interested in the assumptions behind that figure.
For me, the assumptions are roughly as follows:
100 days x 20 degrees difference x 3 air changes x 2.5m (8.2 ft) ceiling height x 0.33 Wh -> 5 kWh (winter)
90 days x 10 degrees x 3 air changes x 2.5m x 0.33 Wh -> 2.2 kWh (transitional period)
That would amount to about 7.2 kWh per square meter. This should be pretty close to the area that @Nida35a officially calculated. Of course, this is only an approximation. Cooling of the furniture is not included, although at the same time furniture reduces the air volume. With proper shock ventilation (if the weather allows), mostly just air is exchanged. Of course, someone who showers in the morning, then opens the bathroom windows and returns after half an hour of breakfast is not reflected in this calculation. Similarly, tilted windows, etc.
Whether 3 air changes are enough is debatable. More than 3 air changes is certainly unrealistic. In most cases, I would even assume rather 2 times daily. See Silke 😉

But the counter-calculation is still missing: what the energy consumption of the mechanical ventilation system itself is.
I did some quick research and found values ranging from 200 to 500 kWh per year. So even assuming 200 to 500 kWh for ventilation energy use, I would still save around 300 kWh, which amounts to about 60 euros per year. So it would not pay off in my lifetime, even if electricity prices rise.