Local MVHR only 40% efficient, says BRANZ test

16 October 2020 by Jason Quinn

We go to great lengths to maintain the certified level of MVHR efficiency in the high-performance building projects we consult on. We recommend or specify cross-counter flow heat exchangers with well insulated boxes, short exterior air ducts (also well insulated) and semi-rigid or rigid distribution ducting inside the thermal envelope; with all that, projects often achieve 80-90% overall efficiency (ie 80-90% of the warmth in the exhaust air is retained within the building). This level of efficiency is critical to hitting the Passive House standard in colder New Zealand climates.

We’d love to use the lower cost local units (and some cheaper imported ones) but have avoided them thus far because we expected the efficiency to be quite low, even while the sales folks claim 90% efficiency. That figure is typically just for the heat exchange core and due to their design and the way they are typically installed, we expected performance below 60%, maybe as low as 50%.

So it was interesting to read of just 40% efficiency of a reasonably decent installation, as measured in a BRANZ test. A MVHR was retrofitted in the ceiling space of a 100 m² house and equipped with sensors to measure the performance in situ.

The BRANZ case study also provides some good advice on steps to take during design and install of these lower cost systems:

  1. make sure the system is actually balanced for supply versus exhaust flow
  2. if the ducts run through unconditioned spaces make sure they are as short as possible,
  3. if the ducts run through unconditioned spaces add more insulation than the supplier provides (about twice or R2.4+) and
  4. if you are not conditioning the entire home, look into the ability to turn off ventilation to the unheated spaces.

I am flagging #4 with a big caution: we do not recommend this as it can lead to significant moisture issues.

We’d add the following suggestions.

A. Allow for additional time for balancing these systems during installation commissioning. The experience in Australia suggests it can take an entire day of engineering time: not cheap.
B. Leakage test the ducts, as this is a common installation defect.
C. Specify electrically commutated (EC) fans. That way the electrical usage is low enough for the recovery to be effective.
D. Only put the distribution ducting inside the thermal envelope.

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