ERV performance test

I have shared anecdotal evidence about the ERV performance.

Last fall, our heating season started a full month earlier, because our ERV was out of commission and in need of repair. At that point I got interested in actually measuring the performance – or to be more precise – measuring the heat recovery rate of the ERV.

Performance data is usually available through the manufacturer (UltimateAir). But that data can be a double sided sword.

The beauty of the standardized testing conducted on behalf of the manufacturers is that you can compare apples to apples–compare one product against a competing model. The beast is that the standardized testing doesn’t necessarily tell you what performance you can expect in your house. Laboratory conditions often have little in common with our homes.

When UltimateAir says the heat recovery rate is up to 96%, what would that mean in terms heat recovery on a particular winter day in our home? To find out, I purchased a temperature sensor with two probes and began measuring.


Methodology – so you know what I did…

I measured the air temperature on the side of the ERV that is connected to the building exterior. One probe was placed in the fresh air supply stream inside the ERV (1). The other was placed on the exhaust duct (2), as the exhaust stream inside the ERV is sealed off.


Once I got stable readings I switched the probes to the ERV side that connects to the building interior. One probe was placed inside the ERV, in the exhaust air stream from the building (3). The other was placed on the duct (4) that delivers fresh air from the ERV into the building.


I began the measurements with the ERV off, and at low speed (60 cfm), medium speed (97 cfm) and high speed (200 cfm). I had no means of measuring the actual air flow, but relied on the air flow rates published by UltimateAir.

For the duration of the testing I also recorded the temperature inside and outside of the building, relying on our thermostat and an outdoor thermometer. Items I did not measure, but which may impact the heat recovery rate, are humidity and dew point.

I expected the heat recovery to vary depending on the outdoor temperature, and thus picked different days for the testing to get a reasonable temperature spread.

Apparent sensible effectiveness (ASE)

I had the measurements. But how do you calculate the energy recovery rate?

First off, we need a unit, a metric – in this case the apparent sensible effectiveness (ASE). To get to an apples to apples comparison, the CAN/CSA-C439-00 standard sets forth a testing protocol, which includes information on how to calculate the apparent sensible effectiveness:

The temperature rise of the outdoor air passing through the ERV is divided by the temperature difference between the outdoor and indoor air. The results are expressed in percent.

With outdoor temperature at 48.2 F falling to 46.2 F, and indoor temperature at 65 F to 66 F, the calculated ASE was:

  • 96.7% at low speed (60 cfm)
  • 91.8% at medium speed (97 cfm)
  • 93.5% at high speed (200 cfm)

With outdoor temperature at 34.5 F falling to 32.4 F, and indoor temperature at 65 F to 66 F, the calculated ASE was:

  • 94.3% at low speed (60 cfm)
  • 93.5% at medium speed (97 cfm)
  • 91.6% at high speed (200 cfm)

With outdoor temperature at 20.7 F rising to 25.7 F, and indoor temperatures at 65 F to 66 F, the calculated ASE was:

  • 95.2% at low speed (60 cfm)
  • 90.0% at medium speed (97 cfm)
  • 88.4% at high speed (200 cfm)

With outdoor temperature at 8.5 F rising to 10.0 F, and indoor temperatures at 65 F to 66 F, the calculated ASE was:

  • 95.0% at low speed (60 cfm)
  • 87.7% at medium speed (97 cfm)
  • 82.7% at medium speed (200 cfm)

Cool looking graphs, but what does this mean?

The ERV performance falls in line with our anecdotal observations. We can expect plenty of fresh air with a heat recovery effectiveness around 95% at the low ventilation speed of 60 cfm. The effectiveness begins to drop off a little with colder outside temperatures, and even more so with higher ventilation rates (i.e. 97 and 200 cfm).

Because of the ERV’s effectiveness, we don’t open our windows during the winter. We never felt the urge! The ERV provides a comfortable environment with good indoor air quality (IAQ). It lets the stale air and the pollutants go, but keeps the thermal energy.

Is the published performance on track?

I am somewhat suspicious of performance data published by the manufacturer, as the numbers may be inflated to make the product look good. Our testing appears to indicate the the published performance data by UltimateAir is on track.

The RecoupAerator product literature claims an energy transfer rating of up to 96%. Our testing revealed that at the low ventilation rate of 60 cfm, it is reasonable to expect an effectiveness around 95%.

More detailed performance is published in the third party test report for the RecoupAerator. Our test results for apparent sensible effectiveness at 34.5 to 32.4 F came in just below the third party test report results for the RecoupAerator at 32F:


I believe that the slight discrepancy in results can be explained by my test procedure differing from the laboratory procedure, which makes an apples to apples comparison difficult. But I now feel confident that the third party performance data published by UltimateAir is on the mark.

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About Marcus de la fleur

Marcus is a Registered Landscape Architect with a horticultural degree from the School of Horticulture at the Royal Botanic Gardens, Kew, and a Masters in Landscape Architecture from the University of Sheffield, UK. He developed a landscape based sustainable pilot project at 168 Elm Ave. in 2002, and has expanded his skill set to building science. Starting in 2009, Marcus applied the newly acquired expertise to the deep energy retrofit of his 100+ year old home in Chicago.

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