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

[jack]

19 hours ago, Mike said:

Here are Zehnder's criteria for triggering the bypass - all 4 must apply:

- The ventilation system must be operating in summer (based on long-term temperature data)
- The outside temperature must be 2°C lower than the exhaust air temperature
- The comfort temperature must be set lower than the exhaust air temperature
- The outside temperature must be greater than 13°C (below this the bypass is always deactivated)

 

Other brands may use simpler criteria.

 

We have an 11 year old Brink Excellent 400, and the criteria are similar (with the possible exception of the first one - I don't recall seeing anything like that in the manual).

 

I seem to recall that some of the settings are adjustable.

[jack]

On 08/09/2026 at 21:42, Novice said:

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

 

I just noticed no-one actually tried to answer any of your questions. I'll give them a go.

 

I haven't quite done exactly what you plan, but I did handle the MVHR penetrations during the build and have run various additional cables etc over the years, through a very similar structure.

 

How did you detail the penetration through the OSB airtight layer?

Cut a slightly undersize hole in a piece of membrane (thick plastic sheet or film is fine) and push the conduit or whatever through it. Seal the edges of the membrane to the OSB with airtightness tape. Cut the membrane square ven for round conduits - much easier to apply airtightness tape in a straight line! 

 

Did you use proprietary airtight service grommets/collars or tapes around the pipe bundle?

Generally not. They're expensive for what they are.

 

At different times I've used offcuts of EPDM and DPM I had lying around. I've also used neoprene insulation a couple of times for cables. For example, I used a disc of thick neoprene insulation to run satellite cables through a snorkel on the roof, where I didn't want to disconnect the cables. I cut the disc a little oversized for the inside diameter of the snorkel, drilled some undersized holes through it for the cables, then cut from the edge to each hole. Slid the cables into the holes through the cuts, then slid the disc along the cable into the end of the snorkel. Neoprene is closed cell, so it was perfectly airtight. It's not a bad approach for running cables.

   

Is it better to install a larger airtight sleeve through the wall and pass all the services through that?

If I understand what you're saying, probably not. It's straightforward to seal a single cable, conduit, whatever, passing through a penetration. It's really hard to seal multiple cable etc passing through the same hole. We have one penetration like that, from the house into the attached garage, and it's a hideous nightmare that's probably responsible for half of the air leakage in the house. I suppose you could have one larger airtight seal over multiple holes in the OSB - if that's what you're saying then I think it'd work fine.

 

How was the sleeve/penetration dealt with at the breather membrane and external cement board?

Same as on the inside, but use membrane similar to whatever is currently in place. You could use non-breathable if that's all you have (e.g., just repeat what you did inside), as long as it's resistant to damp and whatever UV it might be exposed to.

 

How did you deal with the condensate drain without creating an air path?

We didn't have to do this, but consider whether you can run the copndensate drain to an internal drain. Alternatively, you could use a dry trap. We have one of those under our MVHR and it's worked fine for 11 years. 

 

Would you pressure-test the house again afterwards to confirm the airtightness hasn't been compromised?

I wouldn't. Just pay attention to the detailing as you go and it'll be fine.

 

On the underfloor cooling, we have a very similar Panasonic ASHP, probably an earlier model of what you have. It's very effective downstairs, but makes only a modest difference upstairs. Personally I'd consider installing aircon upstairs first, and seeing whether you can get away with mild underfloor cooling downstairs. It helps a lot if you have hard floors - tiles and concrete (we have the latter) are ideal. Not sure whether I'd bother if I had a lot of carpet.

 

With wood floors, I'd be extra careful about making sure the temp is below the dewpoint. We had an incredibly humid couple of days in one of the hotter periods earlier this year, and we had literal wet patches appear on the surface of our concrete floors over the course of one day. They disappear overnight. I think the floors were probably only a couple of degrees below the dewpoint. I'm not sure I'd have been happy having that with wood floors.

[Novice]

On 10/09/2026 at 22:23, Nickfromwales said:

 

What are the exact difficulties after getting from inside to out?

After the advice you gave I believe I have a clearer idea of what is required.

 

On 09/09/2026 at 10:20, Nickfromwales said:

I’m home today but back on site tomorrow, so I’ll post some pics for you. 
 

Are you able to post some pics of the preferred method you would use in my wall type?

 

On 10/09/2026 at 22:23, Nickfromwales said:

 

This is where the PH ideology is a bit 'over-sold' imho.....  You are where 'you are', so let's get this moving   

 

We need to know how much heat (energy) you need to move from indoors to out, and in particular which areas are worst affected so you get the balance of problem vs solution as close to perfect as is practicable.

 

Agreed!

 

The areas most affected areas are the ground floor (main living area) and the top floor (3rd floor, single large craft room). How would I calculate the amount of heat that needs to be removed? Or a resource I can use to help with this?

I have historic data of internal/external temp and humidity.

[Novice]

@jack Thanks for answering my questions.

 

With the help of everyone on here, I feel happier moving forward with an A2A unit

[G and J]

6 minutes ago, Novice said:

The areas most affected areas are the ground floor (main living area) and the top floor (3rd floor, single large craft room). How would I calculate the amount of heat that needs to be removed? Or a resource I can use to help with this?

I have historic data of internal/external temp and humidity.

I’ve wrestled with this one, but I calmed down when I thought about the fact that the power of the smallest unit was similar to the heating power needed to maintain 21°C internal when it is -3°C outside.   At that point i concluded that the smallest unit will more than do.  Bit like our ASHP actually.

[JohnMo]

1 hour ago, Novice said:

How would I calculate the amount of heat that needs to be removed?

Plenty on here have calculated and ended with huge numbers.  The period of time when you get the heat gains is pretty short, and generally solar gain. Just get a small unit and run it longer. If you are wanting to cool with limited solar gain (it's just hot outside) you are only cooling less than 10 degs, so listen to @G and J about realistically sizing.