A modern airtight home needs mechanical ventilation. Homes are no longer built to leak heat and moisture; we now build them as tight as we can. This makes mechanical ventilation essential in a high performance home.
In residential and multi-family residential construction the question often comes up, “Do I need an HRV or an ERV?”
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 per cent, but there are now some models as efficient as 93 per cent coming on the market; at present they are also significantly more expensive.
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 from which it came. 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 per cent) 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. A high efficiency of humidity transfer would be around 65-70 per cent but this value depends on actual humidity levels on either side of the envelope.
In the colder climates such as the interior and northern parts of British Columbia, an HRV is the best solution, but for the Lower Mainland area of BC an ERV is the better choice.
The ERV transfers moisture across the core as a gas and no liquid water is in the core thus no condensate and no liquid for bacteria to form, either. As there is no additional condensate forming in the unit, a drainage system is not required for ERVs thus simplifying installation and reducing overall costs.
At present there are no ERV/HRV installation certification programs in North America but the incorporation of an ERV will assist designers in achieving a higher energy-efficiency standards. Air to air recovery units reduce overall energy consumption for the home owner. ASHRAE Standards 90.1 and 62.1 and 2 demand a certain level of fresh clean air and ERV/HRV units assist in this design criteria.
When designing and installing ERV units, be sure to allow for a proper control system. Quite often there is a main controller in the hallway near the bedrooms, which will control the ERV based on the overall home requirements, with additional timer controls (20/40/60) in the bathrooms controlling the ERV based on occasional increased venting requirements.
The responsibility for wiring these controllers must also be identified in the specification documents.
Energy modellers are now involved in design and are trying to achieve two watts/cfm to meet new energy standards. ECM motors are now becoming more cost effective and incorporation of these motors will assist in matching the correct field performance. Balancing the ERV after installation should also be included in the specification.
by / Norm Grusnick, P. Eng.
Commercial products manager, ECCO Supply