An air exchanger not only improves the quality of your indoor air but also enhances the overall energy efficiency of your home. This guide aims to help you navigate the selection process and determine the best air exchanger that suits your specific needs.
The Importance of Ventilation in Modern Homes
All homes need ventilation, whether natural or mechanical with an ERV or HRV system. Buildings and houses are no longer constructed leaking heat and moist air the way they used to be; we now build them as airtight as we can, especially Passive House or LEED-certified homes. This makes mechanical ventilation essential in these high-performance homes by installing either HRV or ERV Ventilation Exchanger Systems.
Key Questions and Considerations
How Much Fresh Air Comes in Through the Building Envelope?
A reasonably tight house would have a measured air tightness of about 3 Air Changes per Hour at a pressure of 50 pascals (ACH @ 50 Pa).
For the average home (2100 square feet), this would mean about 75 m³/h or 21 L/s (44 cfm) of outside air under normal circumstances. In real terms, this means that in the average-sized home with average leakage of windows, doors, walls, and roof, the entire volume of air will leak out and be replaced perhaps 3 or 4 times per day.
Assuming a three-bedroom home, Indoor Air Quality (IAQ) would require:
Q = 7.5 cfm/occ · 4 occ + 3 cfm/100 ft² · 2100 ft² = 93 cfm
This is a little more than twice the amount coming in from natural ventilation (44 cfm), which should therefore be considered insufficient for maximal demand.
Choosing Between an HRV and an ERV
Heat Recovery Ventilation (HRV)
HRV is a system that uses the heat in stale exhaust air to preheat incoming fresh air. This reduces the energy required to bring outside air up to ambient room temperature, saving money on heating bills. The ‘efficiency rate’ of an HRV unit determines how much energy will be saved by using that particular device.
Typical efficiencies range from 55% to 75%, but some extremely efficient models are rated as high as 93% efficiency. Although these latter units are significantly more expensive and generally only available from Europe, higher efficiency units are becoming more available in North America.
Energy Recovery Ventilation (ERV)
ERV goes a little further than HRV units, as this type of system also captures some of the humidity in the air to keep it on the same side of the thermal envelope that it came from.
In cold winter climates, an ERV system transfers the humidity from the air being extracted to the incoming fresh (and dry) air to help keep the ambient internal humidity level at a reasonable value (between 40% and 60%) at all times.
In summer, the humidity transfer in an ERV reverses and the humidity in outside air is removed before it is injected into the home. This saves energy by reducing the load on air conditioning systems and/or dehumidifiers.
ERV or HRV – Which to Choose?
The best option between an HRV and an ERV depends on your climate and specific needs:
- Cold climates: If your house is too humid in winter (above 60% RH), then an HRV is the better choice, as it would help get rid of excess humidity while an ERV would tend to keep it at a high level.
- Dry climates: If your house is too dry in winter, then an ERV would be a better choice as it helps retain humidity, eliminating the need (and cost) for you to generate it through other means.
- Hot and humid climates: An ERV is generally better, as it helps manage humidity levels, reducing the load on air conditioning systems.
One thing is for certain though: whichever you choose, an airtight home with an ERV or HRV is an evolutionary leap beyond the leaky houses of the 20th century. If you are building or renovating a reasonably airtight house, especially if going for Passive House or LEED certification, don’t lose sleep over which one to get. Just get one.