Energy Recovery Ventilator
ERVs transfer heat but also transfer some of the humidity (or moisture) of the more moist airstream to the less moist airstream. This can help to keep humidity outside in hot humid times and keep moisture in when cold dry winters could make it too dry inside.
In cold climates the outside air that is brought into the building is dry because cold air holds less water vapor than warm air. When using an HRV, the humidity generated inside the building from plants, people showering and cooking, and moisture from leaks or basements is either exhausted or condensed out, leaving the building too dry -again, in cold climates. When using an ERV, 40-60% of the humidity that would normally be exhausted is transferred to the fresh incoming air helping to maintain the relative humidity at a comfortable level. An ERV can also create energy savings by removing or reducing the need to operate a humidifier in the winter. Here are a couple of the studies supporting the use of ERVs in cold climates:
No Drain Required
In most conditions, an ERV will not produce any condensation, therefore saving the cost of the drain pan and installing a drain. By using an ERV and avoiding drains, European manufacturers are able to install ERV units in the walls of a home, where there is no place to put a drain.
The National Research Council of Canada- Residential energy-efficient moisture control through ERV http://www.nrc-cnrc.gc.ca/ci-ic/article/v17n4-13
NRC studies showed that ERVs may have benefits over HRVs both in cold dry winters and humid summers due to ERVs having the ability to transfer moisture through membranes
Natural Resources Canada
ERV Cold Weather Assessment a winter study was carried out using the twin houses at the NRC Canadian Centre for Housing Technology in Ottawa to determine the effect of ERV vs HRV and there were several key conclusions.
The ERVs total effectiveness was 61% vs the HRV of 51%
The HRV core produced significant amounts of condensation under all conditions. The ERV core never produced any condensation; this means that all of the moisture it removed from the exhaust air stream was transferred to the fresh air stream rather than condensed
The ERV core recovered up to 1.8L/day more moisture than the HRV core during the 20% indoor humidity portion of the experiment, and 2.5L/day during the 30% indoor humidity portion.
Heat Recovery Ventilator
An HRV consists of two separate air streams - one collects and exhausts stale indoor air; the other draws in outdoor air and distributes it throughout the home. HRVs provide fresh air while also saving energy by reducing the heating (or cooling) requirements.
Typically an HRV will freeze if the temperature drops below zero degrees Celsius requiring some form of defrost to be used, such as a pre-heater which uses energy; or recirculation which does not provide fresh air. A European manufacturer completed cold weather testing at a 3rd party lab and found their counter-flow ERV core began to freeze below -8C° compared with the equivalent size and shape HRV at just below zero C. The lower frost point results in the ERV requiring less defrost, and thus saving energy and money.
HRVs have great drying ability in the cold months. As such, on the bottom of HRVs there are drain holes for a condensate drain. This drain connects into one tube after a few inches via a plastic T. After the T there's a loop or P-trap and then it runs into either a drain or a bucket.
If the drain holes in the unit get clogged, the water at the bottom can build up. Due to the efficiency of modern units this water can end up freezing.
At Fresh Air Ventilation in Maine we primarily install ERVs. This is due to the fact that this great drying ability HRVs have can often make it too dry. When it's too dry, the dehumidistats which turns the unit off and on based on humidity- losing the benefit of fresh air. Even without the dehumidistat, occupants often turn the unit office, and end up forgetting to turn it back on. The "Comfort Zone" is between 30 and 60% Relative Humidity, which is the amount of moisture in the air- relative to the temperature.
Ventilation is no longer a question of If one should be installed, but a question of How. The public, in trying to educate itself often-times finds misconstrued, outdated and convoluted information. The climate zone problem stems from concern over the recovery core freezing, this is not as much of a concern today.
The question is then simply: "Which do I choose? HRV or ERV?" Well, recent studies have suggested that ERVs should be used in cold dry climate zones because of improved comfort, there's no drain required for condensation (more on this below) and they use less energy for defrost.
Maps like the one below on the left have shown that HRVs should be used in cold dry climates. By using energy modeling software and looking at the energy savings and comfort benefits, a new map of where ERV or HRVs should be used has been developed. Shown below on the right.
A summer field study on HRV and ERV performance for a range of summer conditions was carried out using the twin houses at the NRC Canadian Centre for Housing Technology in Ottawa. The ERV was found to provide better humidity control (i.e., providing a lower level of indoor relative humidity), as well as lower air conditioning electricity consumption, measured as the cumulative saving of 12% over a week.
A winter field study investigating the impact of ventilation rates on indoor air quality andthe respiratory health of asthmatic children in Québec City was carried out in over 100 homes with the Institut national de santé publique du Québec. In a large number of homes, the relative humidity (RH) was found to be too low in winter. Because the introduction of more cold, dry outside air further reduces RH, low-RH homes were equipped with ERVs (instead of HRVs) to increase the ventilation rate. Both HRVs and ERVs performed equally in terms of providing better indoor air quality, characterized by a significant reduction in the concentration of a number of gaseous pollutants of indoor origin. However, ERVs were additionally effective in maintaining an acceptable indoor RH.