Three things you need to pay attention to if you are specifying a non-certified MVHR system for a Passive House project: the ductwork system can becomes a real risk to the installed performance, the MVHR box itself needs a data package for review, and the certification booklet will note that the values used are estimates. This article walks through the full-system requirement, the three data pathways for small units, and the cost of sorting the MVHR box performance data.

The full system, not just the box

A Passive House MVHR is a full system, NOT just the box on the wall. The image below shows a certified system from a single supplier: bright green semi-rigid ductwork, splitter attenuators, and the MVHR box itself. The splitter attenuators and ductwork routinely account for over half the component cost, and for good reason they absorb sound, last 50+ years, are able to be cleaned without tearing out the gypsum board ceiling, and, when properly installed, reduce the odds of air leaks so low no testing is required.

Without these integrated semi-rigid ductwork and splitter-attenuators, the ductwork tends to be a cheap flex duct (insulated or not). Industry experience here and overseas notes that flex duct installations often have significant leaks, lack durability, and deliver poor performance due to airflow resistance and thermal losses. BRANZ-measured data for a flex duct routed above the insulation in the truss roof space put installed system performance at only 40% quite low. After BRANZ tuned up the ductwork removing excess flex length, adjusting values, and the rest efficiency rose to almost 60%. That is still well below the 70% to 90% installed efficiency of a Passive House full system but shows the impact and value of a good quality install.

Allow additional time for balancing these flex duct systems during commissioning. At least one commissioning experience in Australia required an entire additional day of engineering time, which is not cheap. Leakage testing the ducts is also strongly recommended: leaks are a common installation defect, and the resulting efficiency can be literally negative if cold air is being pulled into the home through the ductwork.

What the box actually delivers

Quoted MVHR performance for non-certified MVHR boxes can refer to the heat recovery core performance alone under perfect lab conditions. It does not include heat loss through the box itself, internal air leakage, or the real-world penalties that show up when the full system is tested. That distinction matters for the energy balance, and it is the reason the certifier asks for the data behind the box before any numbers go into PHPP.

Three data pathways for non-certified units (≤ 600 m³/h)

For a non-certified small ventilation unit, the data falls into three categories, each with a different penalty applied to the recovery value. For all of them the manufacturer must provide data or characteristic curves on electrical power consumption that covers the range of air volume flows in this project 100Pa pressure drop.

  1. German DIBt test (no condensation during test). The recovery value is reduced by 12 percentage points. A 90% core becomes 78% in the energy balance.
  2. EN 13141-7 at 7°C outdoor air temperature. If the data is reported on the supply air side, the same 12 percentage point reduction applies. If the data is reported on the exhaust air side, the measured value is used as-is.
  3. Other national standard or energy label. The manufacturer provides air temperatures at all four unit connections (fresh air, exhaust air, supply air, extract air), the approximate air temperature in the test room, the mechanical layout, HEX core information, and casing details. Theoretically, if the unit has a counterflow heat exchanger, a continuously insulated casing (minimum 20 mm) without significant thermal bridges, and tested internal and external air tightness, a maximum heat recovery of 75% may be used. In practice, this requires assembling a data package and submitting it to PHI for assessment.

For ventilation units above 600 m³/h, the data package goes to PHI regardless of the test standard.

What the review costs

Reviewing a DIBt or EN 13141-7 data sheet and calculation of performance is charged hourly and typically costs $330 + GST for the certifier to calculate the adjusted value. Preparing a data package and the PHI fee for PHI assessment typically costs $1,450 + GST, though this can run higher if the standard used is unfamiliar to PHI or the data is shaped differently than expected.

The data is only valid for the flow rates reviewed. A project running different flow rates can trigger a fresh review, at additional cost, for each project.

The certifier’s note

The certification booklet will record that a non-PHI certified ventilation unit was used, and that the values for MVHR heat or energy recovery and electrical efficiency in the energy balance are an estimate based on the manufacturer’s data. The certifier assumes no liability for these values.

This is the practical reason that a certified MVHR together with the splitter-attenuator and ductwork system that comes with it remains the recommended specification. The non-certified route is not closed, but it is more expensive to enter than many specifiers expect, and the values that come out of it carry an asterisk in the certification booklet.