Perfection is mandatory

When it comes to certification you can’t afford for anything to fail, especially the MVHR that’s fundamental for a passive house or anything close. And of all the MVHR requirements, running below 25 decibels on boost is the most difficult to achieve. That’s almost half as quiet as a regular MVHR compliant with Part-F regulations.

It takes a lot more than a ‘Passivhaus certified’ badge on the machine to qualify for certification, that’s only the starting point. Not only must the MVHR designer create a system that fits, they must accurately predict the system pressure, or air resistance, on the whole system, from the exterior terminals through to the air valves in the rooms. Then they must calculate the energy consumption and the acoustic performance of the proposed design. And they better be accurate calculations because someone will be testing the finished system.

There’s no such thing as an Energy Performance Gap with a Passivhaus.

Before you begin

Every component used in the system must have published air pressure performance data; the exterior grilles or roof terminals, the mass flow ducting, the machine, distribution manifolds and radial pipes, or the branch pipes and attenuators. Most exterior terminals are designed for extract-only ventilation, so only have data for air movement in one direction. But MVHR has an intake and exhaust, so you need data for air movement in and out of the building, which severely limits your options. On the ducting itself, every single bend adds more pressure, so the last thing you need is an installer forced to add more bends because the ‘indicative’ design they’re following doesn’t fit.

When you know the total air resistance on the system you can work out the Specific Fan Power needed to deliver the required ventilation rates. The maximum figure, the power used to deliver boost ventilation, must comply with Approved Document F as well as Passivhaus. And when you’ve accurately predicted the pressure and power, and supplied supporting evidence, you can work out the worst-case acoustic performance.

Passivhaus certifiers should ask for all this before you start.

After you finish

Trusting the system goes in exactly as designed and then runs exactly as predicted, i.e. very quietly, be prepared for the Passivhaus certifier to ask for evidence the system works as planned. That means the MVHR commissioner must not only balance the system and set the flow rates, they’ll also need to measure the ‘in use’ energy consumption and potentially the acoustic performance using a calibrated sound meter.

Knowing this at the outset should help you find the right designer/supplier/installer.

Still want certification?

Here’s our advice

Choose an MVHR unit with ‘Passivhaus certified component’ status.
Ask the manufacturer to recommend a designer capable of meeting Passivhaus requirements.
Check the designer knows the difference between ADF and Passivhaus ventilation requirements, understands acoustics, provides installable designs rather than indicative designs, and is ideally familiar with PHPP. It helps to speak the same language as the Passivhaus certifier.
Design the system very early in the construction drawings phase, before choosing floor joists, plantroom size and location, and planning the rest of plumbing and M&E equipment.
Find an installer with Passivhaus experience. It’ll pay dividends to find a site manager with Passivhaus experience too. Someone who’s already been through the process and knows the importance of airtightness at every step of the build.
Find an MVHR commissioner with Passivhaus experience too, preferably not the installer. Passivhaus systems need 3rd party oversight more than ever.
Expect to pay 25% more for a ‘certifiable’ system compared to a regular MVHR. Everything needs over-engineering to guarantee it’ll run quietly enough to satisfy your certifier. And that means a bigger machine running at a lower rate, anti-vibration products, more ducting, more attenuation, larger/quieter exterior terminals etc.