Whether natural or mechanical, homes need ventilation. They are no longer built to leak heat and moisture the way they used to be; we now build them as airtight as we can. This makes mechanical ventilation essential in a high performance home.
How much fresh air is enough?
How much fresh air comes in through the building envelope?
What is the difference between an HRV and an ERV?
How do you choose between an HRV and an ERV?
How much fresh air is required and the best way to provide it are important issues. Energy recovery from exhaust air is becoming common place in cold regions, and two types of equipment can do this- an HRV (Heat Recovery Ventilation) and an ERV (Energy Recovery Ventilation.
Both HRVs and ERVs are somewhat new to mainstream home construction, and can often be confused. In an effort to clear that up, we will first explore why ventilation is so crucial, then explain the options and their best applications.
Up until the last few decades, houses were so leaky that sufficient cold dry air seeped in to meet the needs of occupants, and ensure homes had no moisture damage. These houses were said to 'breathe', but that would be like breathing through your skin instead of through your nose.
It meant that cold, dry winter air would need to be warmed as well as humidified, while hot and humid air would enter in the summer.
Nowadays, in the name of energy efficiency, houses are built to much higher standards of air tightness, so a mechanical ventilation system is essential for the following reasons:
- To provide oxygen for occupants since people deplete oxygen as they breath. In a reasonably airtight home with no ventilation you would feel the effects of that in quite short order.
- To remove contaminants – because along with the toxins emitted by the human body (ammonia, benzene, carbon monoxide and methane to name but a few), chemicals in building materials and furnishings continue to off-gas for many years after installation.
- To remove the excess humidity generated by normal human activity in order to ensure building durability and efficiency in heating.
Health Canada recommendations for optimum indoor air quality © Health Canada
How much fresh air is enough?
It is very difficult for humans to detect low levels of contaminants in their air, even when they represent a health hazard. An ideal ventilation system would include sensors that could detect the presence of excessive humidity and all harmful agents in order to provide fresh air accordingly, but no such system exists yet.
Therefore, our best option at present is to err on the side of safety, and provide a minimum fresh outdoor air supply at all times. Most building codes rely on the *ASHRAE standard 62.2 (or some variation of it) to establish ventilation norms for homes.
*ASHRAE (American Society of Heating, Refrigeration and Air-Conditioning Engineers) is the most respected and authoritative source for interior air quality standards. ASHRAE 62.1 and 2 are the recognized standards for ventilation and indoor air quality (IAQ).
According to the current version of the ASHRAE standard (2013), the calculation for individual homes is as follows:
This equation assumes the number of occupants equal to the number of bedrooms plus one, a reasonable assumption for an average family where each child has their own room. The calculated value refers to the minimal installed rate (Q). Some codes (like the National Building Code of Canada) insist on insuring a capability of providing half this value on a continual basis.
Note that the 3 cfm/100 ft² of living area used is triple that of the previous version (2003), since the extra 2 cfm were considered until now to be provided by the air leaks in the envelope. Worth noting, key professionals in the industry are still debating the value of this update, read more.
In Canada, the ventilation capacity is prescribed according to the type and number of rooms to take into account the activity of the occupants rather than the dimension of the living space.
The National Building Code refers to standard CAN/CSA-F32 for establishing the ventilation rate of a residence. But, at any rate, the intent is to ensure around 0.3 ACH, which is the international norm to evaluate ventilation systems.
How much fresh air comes in through walls?
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, that means in the average sized home with average leakage, 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. In the case of air leakage only, the quality of the air coming in through the envelope should also be under scrutiny since the envelope itself is serving as the air filter, which means any VOCs or mould spores trapped inside the wall assembly can easily find their way into the living area.
Note, however, that the previous ASHRAE standard would have yielded 51 cfm, barely more than the amount coming through leakage in the building envelope.
Choosing between an HRV and an ERV
Heat Recovery Ventilation (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.
Similar to the human breathing system as mentioned above, this exchange of air is performed in a single area of the home, the lung of your home, your ventilator core.
Note that outgoing air and incoming air never mix in the heat recovery process; they simply pass in separate channels in the ventilator core, allowing an exchange of heat through conduction.
The 'efficiency rate' of an HRV unit determines how much energy will be saved by using that particular device. Although it requires the operation of a fan on a continual basis, the energy recovered from the inside air is many times that of the energy required for the fan.
Typical efficiencies range from 55% to 75%, but some extremely efficient models are rated as high as 93% efficiency. At present, these latter units are significantly more expensive and only available from Europe. However, when you factor the value of energy savings over the unit’s full life cycle, shipping these costly units across the ocean can still make it a financially and ecologically sound investment.
Energy (or Enthalpy) Recovery Ventilation (ERV) goes a little further than the HRV scheme, 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.
So in winter, the system transfers the humidity from the air being extracted to the incoming fresh (and dry) air to help keep the ambient humidity level at a reasonable value (between 40 and 60%) at all times.
In summer, the humidity transfer reverses and the humidity in outside air is removed before it is injected into the home. This saves energy by reducing the load on your air conditioning system and/or dehumidifier. A high efficiency of humidity transfer would be around 70% but this value depends on the actual humidity on either side of the envelope.
One important note is that whatever you choose for your needs, there will always be a power on/off switch. If your system is too noisy, you will likely turn it off for long periods of time even if you really need it. Ensure you have a quiet system and that it is installed properly to avoid the temptation of turning off a piece of equipment that represents both a financial and health investment.
How to choose:
The best option between an HRV and an ERV depends on your climate and specific needs. If your house is too humid in winter (above 60% RH) then an HRV is the better choice, as it would surely get rid of excess humidity while an ERV would tend to keep it at a high level.
If the opposite is true and 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.
In summer time, the use of an HRV will usually increase the humidity level inside your home, so an ERV is better in hot and humid zones. But a dedicated dehumidifier will likely do the trick much better. At the very least, the ERV will lower the load on the air conditioning system, even if it can’t keep up with the high humidity level on the outside.
So in the end, there is not one right choice. It depends on your climate, your lifestyle and your home. In a perfect world we would have one of each, short of that we are left to make a choice.
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, so if you are building or have a reasonably airtight house, don't lose sleep over which one to get – just get one.