By Jon Eakes

Both the HRV (Heat Recovery Ventilator) and the ERV (Energy Recovery Ventilator) were developed together in the early 1980s with the promise that the HRV would dehumidify the tightly sealed house. The ERV promised to do the same job while returning 50 per cent of the moisture back to where it came from, thus not drying out a house too much.
Unfortunately, because ERVs actually retained moisture in their exchange cores, they quickly self-destructed with Canada’s freezing temperatures. ERVs literally disappeared from Canada, but became the ventilation mainstay in the hot humid air conditioning climate of Florida. They worked just the opposite down there, keeping outdoor moisture outdoors, thus keeping humidity levels in the air-conditioned house lower, easier to cool, and more comfortable. And they never froze.
In the meantime up in Canada we sealed houses so the indoor relative humidity went up, and we ventilated the houses with HRVs so the relative humidity went down, sometimes so far down that we needed to put humidifiers back into operation. We built bigger houses with fewer people and they got too dry, while smaller houses with large families got too wet.
Obviously we needed more air control options. In 1995, 15 years after its initial development, this technology came back to Canada when Venmar concluded some serious research into creating an ERV that could stand up to freeze/thaw conditions and they brought out their Duo Thermal Wheel ERV, followed a few years later by their EKO Cross Flow ERVs.
Although it has been 18 years since the ERV has come back to freezing Canada, few contractors understand them enough to know when they might actually be the preferred ventilation strategy.

Many ERVs Won’t Work in Canada
Although today there are more than 40 companies making ERVs, you must be very careful to check the Home Ventilating Institute (HVI) listings to identify the few models that are rated to last for 10 years at temperatures down to -25C.  For several years Venmar had the only unit capable of doing that, although now there are a few more on the market.

Understanding the Technology
During the Canadian winter an HRV recuperates heat from the outgoing stale air and transfers it to the incoming fresh air, bringing their two temperatures close to the same levels, even though the outgoing air is moist and the incoming air is dry. An ERV does some of the same temperature transfer, but also brings over warm moisture into the fresh air stream.
Leaving more humidity in the house is only part of the story. There is energy stored in that moisture. Very little energy is required to bring this moisture up to room temperature compared to cold vapour coming off a humidifier that just soaks up energy from the space heating system. An ERV recuperates a great deal of “embodied energy” or what is called enthalpy or latent heat, and that is where a good part of its energy recovery comes from. Wet or dry air at the same temperature is much like a cast iron fry pan and a sheet of aluminum foil, both at the same temperature. The massive fry pan has enough energy stored in it to cook your skin; the aluminum foil will be cooled instantly when you touch it because there is little mass to store energy.

Cross flow HRV core
Cross flow ERV core

The ERV comes in two basic types: Thermal Wheels that rotate slowly through the two air flow streams, like the DUO; and Cross Flow cores like the EKO, that look like those used in a traditional HRV. They both transfer about 80 per cent of the air temperature, while the more massive thermal wheel will recuperate 75-80 per cent of the latent energy in the moisture and the cross flow design 65-70 per cent.
The shift in core technology is most noticeable when comparing the smooth channels of a cross flow HRV to the rough fibrous channels of the cross flow ERV. You can see my red tool box on the other side of the HRV core, and barely make out a flashlight beam peeking through the ERV core.
The Venmar EKO 1.5 ERV is an interesting unit to illustrate the technology. One unique feature of this unit is that Venmar has built it so that the ERV core could be exchanged for an HRV core in the same installed machine—both being cross flow exchangers. That could be particularly useful if the occupational loads on the house were to change, such as from a quiet retired couple to a growing family and much more moisture. Or if an indoor hot tub were added to the house and now dehumidification were necessary. Or you just didn’t know which unit to choose in the first place. Or to the extreme of using the HRV core in the winter and then the ERV core for the air conditioning season, effectively choosing both machines.

How Much Ventilation Should We Have?
For a long time it has been assumed that CO2, generated by human breathing, was a good indicator of indoor pollutants and was used to establish our norms for ventilation rates. But sometimes those official ventilation rates caused problems of their own—from drying out the house to introducing pollution from outside the house that was worse than what was initially inside—and the occupants simply turned the units off. With time, experience and better sealed houses, we have discovered that the ideal ventilation for any given house was far more variable than previously thought.
Some even question our established ventilation calculation formulas. A recent study published by BuildGreenAdvisor.com, Ventilation Rates and Human Health, asked the question: “Have researchers found any connection between residential ventilation rates and occupant health?” The answer was there were not very many studies on the subject and some of them said there was no relationship between the two. Joseph Lstiburek, with his typical frankness, put forward that with a lack of scientific basis for ventilation rates we would be far better off starting by controlling pollution sources. Many cases were also cited where outdoor air was worse than indoor air and any ventilation only polluted the house. I only mention those thoughtful reflections to point out how much each house needs to be studied on its own.
No builder can provide a specific ventilation rate to a house; he can only provide a planned ventilation potential and then, if it works well for that house, the occupants may actually use it.
A healthy air quality strategy is far more than just choosing the model of air exchanger. It is minimizing the need for ventilation by reducing pollution sources, providing point of pollution extraction, maximizing air circulation throughout the house and then taking into account the building’s outdoor environment and the lifestyles of the occupants. Using automated control systems, some measuring outdoor temperatures, others measuring pollution levels, but all offering easily variable ventilation speeds, help these machines to adapt to daily reality and user demands.
Now that cold weather technology has been proven for the ERV, you really do have one more tool to design ventilation strategies that specifically suit your individual customer. Just make sure that you install an ERV that is HVI certified for 10 years down to -25C and not a Florida import.