ERV – keeping the heat!

While we are on the ventilation theme, let’s stick with that subject for a little longer. But rather than fanfare to ceiling fans, how about shedding more light on heat recovery and indoor air quality (IAQ)?

Mechanical ventilation in a relatively airtight building like ours is important to maintain a good IAQ. The energy recovery ventilator (ERV) provides the necessary ventilation. It flushes stale air and pollutants out of the building, but without the usual energy penalty. That is thanks to a heat recovery process, which I schematically described in a previous post.

But how exactly is the heat recovered? Let’s take a closer look at the mechanics of the ERV.

There are two sides to the unit. One side connects to the building exterior with two ducts. One of the ducts delivers fresh air from the exterior to the ERV. The other duct exhausts stale air from the ERV to the building exterior.


I covered the fresh air supply and exhaust in previous posts.

If we turn the ERV around, we look at the side that connects to the building interior. One of the ducts supplies fresh air from the ERV into the building, while the other duct returns stale air from the building to the ERV.


Each side (exterior and building side) has an efficient ECM blower motor that is connected to an impeller. The impeller, like a fan, provides the velocity in the two air streams. The airflow can be modulated from 30 to 200 cubic feet per minute (cfm).

The key in this set up is that the two air streams, the fresh air stream and the stale air stream, are physically separated from each other. The fresh air cannot mix with the stale air.


The only connecting element between those two air streams is the enthalpy wheel, which is powered by its own small ECM motor.

ERV-05 ERV-06

Let’s take a winter scenario: The enthalpy wheel rotates across the two air streams when the ERV is running. In that process, it picks up the heat and moisture from the stale air stream coming from the building and transfers it to the fresh air stream entering the ERV. And vice-versa, it picks up the cool from the fresh air stream entering the ERV and transfers it to the stale air stream that is leaving the building.


What makes that heat and moisture transfer possible are the six filter pies in the enthalpy wheel. To learn more about the filter pies, I talked to Matt Baker at UltimateAir:

“ [The] material [of the filter pie] is a woven blend of nylon and a type of regenerated cellulose similar to rayon. The nylon is primarily responsible for heat [recovery] and the [regenerated cellulose] handles the moisture transfer”.


It’s amazing that these synthetic mesh pillows recuperate and transfer heat and moisture that effectively.

Did you know that the “E” in ERV is actually standing for enthalpy (recovery ventilator) and not energy (recovery ventilator). Because most people don’t know what enthalpy is, but have a concept of energy, that term stuck.

Related posts:

1st floor ventilation details

1st floor ventilation planning

ERV recap

Installing the ERV

Picking an ERV

Vetting ventilation

Blower door test – after insulation

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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