Indoor air quality problems have soared since the late 1970’s when construction technology succeeded in developing energy efficient "tight" houses. Heat recovery ventilators have long been popular in Canada and are becoming popular in the USA. In fact, Canada has a national ventilation law. In 1992 there were more than 125,000 units sold there. In the U.S., there are three states which have ventilation laws and there are more than 15,000 units currently in use in the USA.
Pollutants inside houses, which once escaped through cracks around windows and doors, are now trapped inside creating an indoor environment that is often 2 to 5 times more polluted than outside. Pet dander, mold spores, dust mites, allergens, tobacco smoke and other pollutants add up to poor indoor air quality. An ERV (Energy Recovery Ventilator) or an HRV (Heat Recovery Ventilator) is a great way to fight poor indoor air quality by bringing fresh air into the home while also expelling stale air.
HRVs and ERVs use internal fans to pull fresh air in and move stale air out of a home. The key to efficient ventilation is the HRV or ERV core - which heats or cools incoming fresh air, recapturing 60 to 80 percent of the conditioned temperatures that would otherwise be lost.
HRV vs. ERV – what’s the difference?
There are two types of energy-recovery systems: heat-recovery ventilators (HRV) and energy-recovery (or enthalpy-recovery) ventilators (ERV). Both types include a heat exchanger core, one or more fans to push air through the machine, and some controls. The main difference between a heat-recovery and an energy-recovery ventilator is the way the heat exchanger core works. With an energy-recovery ventilator, the heat exchanger transfers a certain amount of water vapor along with heat energy, while a heat-recovery ventilator only transfers heat. The part of the country in which you live will dictate that type of unit that is right for your needs. Generally speaking – HRVs are usually recommended for colder climates with longer heating seasons. ERVs are used for warmer, more humid climates with long cooling seasons.
Sizing and Installation
HRVs/ERVs are typically sized to ventilate the whole house at a minimum of .35 air changes per hour. To calculate minimum CFM requirements, simply take the square footage of the house (including basement) and multiply by the height of the ceiling to get cubic volume. Then, divide by 60 and multiply by .35.
The best way to configure the installation for an ERV or HRV is to create a dedicated system of ducts to exhaust stale air from problem areas (bathrooms, kitchens) and bring in fresh are to common areas (bedrooms, living rooms). While this is preferred, it is often impossible – especially in a retro-fit situation. The most common, and simpler installation is accomplished by connecting the HRV/ERV supply and exhaust ductwork directly to the return air duct of the home’s existing forced air heating and cooling system.