ᐅ Integrating Cooling into Mechanical Ventilation with Heat Recovery or Using a Separate Air Conditioning System?

Hello everyone,

In our future house, the two children will have their rooms under the roof. From experience, I know that it gets extremely hot there in the summer.

We want to integrate a possibility for active cooling into our house. We have already had a conversation with an HVAC technician who, based on the size of the rooms, proposed an air conditioning system with a cooling capacity of about 13 kW. The cost would be around 15,000 €.

There is also a cooling element from Zehnder (ComfoCool Q600 ST), which is significantly cheaper but only has a cooling capacity of 2.3 kW. The technician estimated that his system could lower the temperature by up to 10°C (18°F). The Zehnder unit, however, has only about 20% of that capacity. Would it even make a difference? If 20% cooling capacity means it can only cool about 2°C (4°F) down, isn’t it basically useless?

• Has anyone here used this unit?
• Should we install an air conditioner in addition to the mechanical ventilation system with heat recovery?

Honestly, I find it quite strange that mechanical ventilation systems with heat recovery usually don’t include proper cooling units. Until recently, I thought nearly all these systems could also cool.

Thanks for your feedback,
junge_familie

Mechanical ventilation with heat recovery has nothing to do with cooling. For cooling, either a chilled ceiling or a separate air conditioning system is needed.

Good roof insulation and shading would help.

Cooling with a controlled mechanical ventilation system is far too weak for proper air conditioning. It might provide some buffering but nothing more.

For it to work effectively, you would need at least a 4-fold air exchange rate as well as a significantly higher cooling capacity (an additional cooling unit).

How large are the rooms that need to be air-conditioned?
From experience, I would say that the quote from your HVAC technician is oversized. Maximum $2,500 plus installation. The rooms are probably of standard size. A multi-split system with two indoor units should be sufficient.

As already mentioned, the most important thing is to keep the heat out. Heat that doesn’t get in doesn’t have to be removed again.

Then, a cooling load calculation is performed. This is used to size the system.

I find 13 kW very high. If I remember the figures correctly, we cool about 150 m² (1,615 sq ft) of office space with 13 kW.

The 13 kW system is probably designed for the entire house. You should ask him about a system specifically for the two children's bedrooms.

The Zehnder cooling unit you mentioned... besides having far too little capacity, you cannot target cooling with a ventilation system. This means the supply air is pre-cooled centrally and distributed to all rooms. In my opinion, this doesn't help much for the "hotspot" children’s bedrooms.
It’s not exactly cheap either, and it seems to be compatible only with the Q600, which would be quite a large size for a single-family house.

I wouldn’t invest in either of those options. It’s better to have more thermal mass in the roof along with proper exterior shading, such as shutters.

We insulated our roof with standard mineral wool, and the attic temperature never exceeded 23°C (73°F), even during this prolonged warm summer. New houses don’t heat up as much if you provide effective shading. Once the heat gets inside, for example through improper ventilation, it’s naturally hard to get it out again quickly.

Blown-in insulation is said to be even more effective than mineral wool against heat...