ᐅ Airflow specification of the mechanical ventilation system for heating load calculation

Hello everyone,

I think I need your expertise again.
Originally, I didn’t plan to install any ventilation during the house renovation, but after seeing how much heat loss the energy consultant calculated for the ventilation, I decided to install two decentralized ventilation units in the open kitchen-living area. I didn’t consult an expert for this.
Now, for the precise heating load calculation of the heat pump, the air volume flow rate of the ventilation is being requested, and before I give completely wrong information, I wanted to ask for your opinion on my calculation:

We’re talking about an approximately 57m² (614 ft²) open kitchen-living-dining space based on the floor plan. Ceiling height is 2.68m (8.8 ft). That results in a volume of about 153m³ (5,404 ft³). However, this space will have quite a bit of furniture such as a fitted kitchen, fireplace, large refrigerator, couch, sideboards, etc., which I’m subtracting from the room volume.
Let’s roughly subtract 10m³ (353 ft³), leaving 143m³ (5,048 ft³) of room air volume.

Now I come to the point where I’m unsure. I have two Blauberg Vento Expert A-50 units operating alternately—one blows air in while the other exhausts air out.
This should mean double the air moved, so everything times 2.
The fan has three speed levels with heat recovery:
Level 1: 8 m³/h (5 cfm)
Level 2: 15 m³/h (9 cfm)
Level 3: 25 m³/h (15 cfm)
Additionally, the datasheet states a maximum exhaust air volume flow of 50 m³/h (30 cfm).

Now I’m not sure which value to use. For noise reasons, most people seem to run the units at Level 2. Since I have two fans, that would be 2 x 15 m³/h = 30 m³/h (18 cfm). So that would be my volume flow rate in this room, right?
Or should I use the highest possible value of 2 x 25 m³/h, or a combination like two-thirds at Level 2 and one-third at Level 3?

I only need the room volume for the air exchange rate calculation (which with 30 m³/h would be 143 m³ / 30 m³/h = 0.21 air changes per hour, which is below the recommended 0.5).

Is there anyone here who knows better than I do?

Do not double it. 15 go in, 15 go out. Then you have changed 15, not 30.

As you already notice yourself, the volume of your devices is far too low for the described room.

guckuck2 schrieb:

Do not double count. 15 go in, 15 go out. That means you have exchanged 15, not 30.

As you already noticed yourself, the capacity of your devices is far too low for the described room

and on setting three for a higher air exchange rate, it is too noisy, especially for the open living, kitchen, and dining area 😳

A decentralized mechanical ventilation with heat recovery is modest, at least everything I have seen or heard so far. However, I am open to being proven wrong.

guckuck2 schrieb:

Don’t double count. 15 go in, 15 go out. That means you have exchanged 15, not 30.

As you’ve already noticed yourself, the capacity of your units is far too low for the described space.

It somehow sounds logical yet confusing that two systems working against each other should not affect the airflow volume. The counterflow operation simply supports each fan to reduce the resistance it has to work against.

i_b_n_a_n schrieb:

And at setting three, it’s too noisy for a higher air exchange rate, especially in the open living, kitchen, and dining area 😳

A decentralized mechanical ventilation system with heat recovery is disappointing, at least everything I have seen or heard so far. But feel free to prove me wrong.

Yep, I totally agree with you. That’s why I originally decided against it. But then roughly 40% of the total heat loss was attributed to ventilation. Since energy costs aren’t getting any lower, I wanted to achieve a reduction in the room where people spend most of their time—and only during the heating months. In the summer, the patio door is almost always open.

But the decentralized mechanical ventilation with heat recovery is definitely not the holy grail.

It’s not just about spending time there, and even if it is, it’s usually the bedroom. Bathrooms are also important to prevent mold. In the large living area, there probably need to be either more or larger ones.

JaiBee07 schrieb:

It somehow sounds logical yet confusing that two systems shouldn’t affect the airflow volume compared to just one.
The opposing operation is simply a way for each fan to support the other by reducing the resistance they have to work against.

You need to consider the proportions. 15 cubic meters per hour at 150 cubic meters of room volume is 10%. However, the fan isn’t continuously blowing air in or out; it operates in a pendulum mode, meaning it fills this large room with about a bathtub full of air, then switches and blows that amount back out. If you only have one fan, it barely changes the pressure conditions in the room because the resistance is very low.
With two fans, each one compensates for the slight pressure change created by the other. Since they always operate in opposite directions, this will hardly affect the airflow volume.