Heat recovery ventilation - not all systems are created equal

Would you trust the fuel efficiency test of a car starting at Whakapapa Village, and driving down to Lake Taupo? For those not familiar with New Zealand geography: Whakapapa is an alpine village at an elevation of about 1200 m; Lake Taupo sits below it at about 330 m. Or the efficiency rating of a freezer that was tested in a garage on Svalbard – a town in the Arctic Circle?

Don't get me wrong: properly designed and installed heat recovery ventilation is great, for your health and for your wallet. But many heat recovery ventilation systems make performance claims that lack substance. Often, efficiency statements are purposefully vague (“up to 95%”). But even when a concrete number is provided, it pays to check how this was established. Has the heat exchanger been tested for airtightness? If not, then the supply air temperature used to determine performance may simply be the result of recycling stale, outgoing air. This may be great for comfort, but it comes at the detriment of indoor air quality – the main purpose of having a ventilation system. Has the waste heat from fans been deducted from the efficiency considerations? If not, a high supply air temperature may well be the result of wasteful, poorly performing fans, and nothing to do with the efficiency of the heat recovery. And if the test is performed in a warm environment, poorly insulated systems will seemingly be better suited to providing warm air than properly insulated ones.

The Passive House certificate of ventilation components excludes all these influences. For example, it is not interested in supply-air temperature at all; it requires testing for airtightness, thermodynamic and electrical efficiency. For units that are not certified to this method, heightened scrutiny of any test results is required.

What to look out for when you are in the market for heat recovery ventilation?

Is the unit a Passive House certified component? 328 units are, many of which are available in Aotearoa, check here: https://database.passivehouse.com/en/components/list/ventilation_small

If the unit you are interested in is not on the list, ask the supplier which tests have been performed to ascertain performance, and get a test report. A tell-tale sign of mediocre performance will be the type of heat exchanger used, so ask about this, too. A cross-flow heat exchanger will not be as efficient as a counter-flow heat exchanger.

Good systems:

• use a counter-flow heat exchanger;
• employ centrifugal fans with forward curved blades and electronically controlled DC motors;
• use semi-rigid or rigid ducts that are anti-electrostatically coated when they are made from plastic;
• have energy-efficient provisions for frost protection;
• include an acoustic concept;
• have an automatic thermal bypass for summer;
• allow for easy filter changes;
• have outlets that are optimised for comfort, including acoustic comfort;
• come with commissioning as part of the deal.

Please note: the costs of a ventilation system are eminently determined by a cost-optimised design rather than the cost of components. Reducing duct length, complications with installation, and the number of outlets is sensible value engineering, skimping on good componentry is not!