Before the energy crisis of the 1970s, homes had what you can call natural ventilation thanks to lack of insulation and gaps around doors and windows. This meant poor comfort, heat loss and little control over air quality. These days we have much better insulation so we don’t have to lose heat and houses are built at much higher standards of air tightness– but that creates a new problem of getting fresh air in homes now.
Air recirculation became an important issue. Early air exchangers were an answer to the problem; HRV and ERV units are newer and better versions of these devices. Both are air exchange systems that help to enhance indoor air quality and minimize heating.
Both will help…
- Exhaust, stale, polluted, humid air.
- Reduce condensation, mould, mildew, etc
- Transfer heat from stale, damp, exhaust air to preheat (or cool, in summer) fresh incoming air.
- Save energy costs by helping the furnace and/or a/c to work less.
An HRV captures and recycles heat. Polluted exhaust air is used the heat to warm incoming fresh air. So in the winter, by the laws of physics, whenever air is warmed, it’s humidity is reduced as warm air can absorb moisture. Thus cold incoming outdoor air is automatically dehumidified when it is warmed by the re-cycled heat.
At the same time, humid warm indoor air, which has absorbed moisture and contains other indoor air pollutants, is exhausted. As the exhaust air passes through the HRV, the exhaust air heat is captured by the Heat Exchanger which heats the incoming cold outdoor air.
In the summer, homes WITH A/C, the HRV heat exchanger captures cooled energy, and used it to cool the warm outdoor air as it is introduced into the home, for ventilation. This reduces the energy on the a/c system while providing 100% fresh air ventilation.
The HRV can only dehumidify outdoor air when the outdoor air is colder than the inside air (law of physics as stated earlier). In homes without a/c it is possible for the indoor air temperature to be several degrees cooler than the outside air temperature. If this is the case, the HRV will cool the incoming fresh air, by recycling the cooler energy that is retained indoors, and using this energy to cool the outdoor air as it is introduced into the home.
How HRVs work
Since they transfer moisture (core absorbs moisture from air), they help prevent excess dryness in the cold season and reduce the demands on the air-conditioning system in the warm season. ERVs are best suited for colder, drier climates in winter as they help limit the amount of humidity expelled from your home.
Venmar diagrams this best…
Important points about ERV…
- No Drain Install Required – An ERV will not produce any condensation, therefore saving the cost of the drain pan and installing a drain.
- 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.
- Energy Savings – An ERV can also create energy savings by removing or reducing the need to operate a humidifier in the winter and less energy to be used by a/c in summer.
So… HRV or ERV?
Several factors play a role.
1. Climate – Generally HRVs work better in the milder, more humid winters and Moderate humid to dry summers. ERVs are better for hot humid summer climates.
As you can see from Lifebreath link below, the lower mainland would fall between zone 2 or 4 which could warrant either.
ZONE 1: True North – Cold Winter and Dry Summer – HRV HRV is best for a very cold and prolonged winter season as the robust aluminum HRV core will have a long life.
ZONE 2: US Canada Border – Cold Winter and Hot Humid Summer – HRV or *ERV with Defrost HRV is best for a cold or prolonged winter. ERV will help reduce the AC load in the summer by transferring moisture. ERV needs a defrost system for areas experiencing cold (freezing) winter temperatures.
ZONE 3: Central US – Moderate Climate – HRV or ERV HRV and ERV will work equally well. ERV will help reduce the AC load in the summer by transferring moisture. ERV does not require a cold weather defrost system.
ZONE 4: Marine Climate – HRV or ERV Humidity and temperature are moderated by HRV or ERV. ERV typically does not need a cold winter defrost system with some exceptions in mountainous areas.
ZONE 5: Arid – Hot and Dry – HRV HRV will transfer energy summer and winter. With little moisture to transfer an ERV is not needed.
ZONE 6: Gulf Region – Hot and Humid – ERV ERV is recommended. Primary benefit is transferring energy and moisture thoughout summer and winter without negatively impacting AC load.
2. How AIRTIGHT is your home? The less air leaks, the more humidity stays in, making the HRV the way to go.
3. Type of heating system – In a wood-heated environment, which is more likely to be drier, an ERV will promote a healthier humidity level.
4. Home SIZE – In general, HRV units are best suited to small or medium-sized homes as humidity can quickly accumulate. ERVs, therefore,will better serve larger homes where the air tends to be drier.
5. Number of Occupants – An HRV fits the needs of a larger, active family that generates a lot of humidity. In a home with a few people less humidity is generated and the drier the air will be. In this situation the ERV is the better choice.
***Studies have been done to support the use of ERVs***
The National Research Council of Canada- Residential energy-efficient moisture control through ERV http://www.nrc-cnrc.gc.ca/ci-ic/article/v17n4-13
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.
”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”
And in extreme dry winters studies proved ERV more effective at retaining moisture within the home.