Retrofitting air-to-air cooling in a Passivhaus – maintaining airtightness

[Novice]

We have been living in our certified Passivhaus since it was completed in August 2017. It's a relatively large house (approx. 400 m²) and overall has proved extremely comfortable and energy efficient.

However, in recent summers we have increasingly experienced overheating in the main living areas. Despite using night purging and running the MVHR in summer/bypass mode, there are periods when we struggle to bring the internal temperature back down.

We are therefore considering installing a couple of air-to-air heat pump/split AC units – probably one serving the main living area and another on the top floor.

My main concern is maintaining the integrity of the Passivhaus airtight layer when installing them.

The wall construction is shown in the attached drawing. The airtight layer is 12 mm SmartPly/OSB on the inside of the 300 mm twin-stud timber frame, with a 38 mm internal service cavity before the plasterboard.

Obviously the installation will require refrigerant pipework, condensate drain and probably cabling to pass from inside to outside. I don't want an installer simply drilling through the wall and filling around the services with expanding foam.

Has anyone installed split AC/air-to-air heat pumps retrospectively in a Passivhaus or similarly airtight timber-frame building?

In particular, I'd be interested to know:

• How did you detail the penetration through the OSB airtight layer?
• Did you use proprietary airtight service grommets/collars or tapes around the pipe bundle?
• Is it better to install a larger airtight sleeve through the wall and pass all the services through that?
• How was the sleeve/penetration dealt with at the breather membrane and external cement board?
• How did you deal with the condensate drain without creating an air path?
• Would you pressure-test the house again afterwards to confirm the airtightness hasn't been compromised?

I'd be very interested to hear from anyone who has done something similar, particularly on an existing Passivhaus.

[ProDave]

51 minutes ago, Novice said:

running the MVHR in summer/bypass mode

Bypass mode, as it's name suggests, bypasses the heat exchanger.

 

In a hot summer, that will result in HOT outside air entering via the mvhr with no cooling effect.

 

Leave it in normal mode.  Then outside air that is hotter than inside, will be cooled by the cooler extracted air.

 

I have struggled to find any time when it is actually advantageous to use bypass mode.

[JohnMo]

5 minutes ago, ProDave said:

struggled to find any time when it is actually advantageous to use bypass mode

When inside is hotter than outside. That is the only time.

[JohnMo]

@Novice what is you normal heat system - ASHP?