Ventilation system

The importance of ventilation in today's more energy efficient homes is universally recognized. Because of the energy savings generated, the system of choice is often a heat recovery ventilator (HRV).

An HRV is a mechanical ventilation device that helps make your home healthier, cleaner and more comfortable by continuously replacing stale indoor air with fresh outdoor air. HRVs are set apart from other mechanical ventilation devices by their ability to exchange heat between the supply and exhaust air streams, which in turn reduces the cost of heating or cooling the healthy fresh air that circulates through the home. HRVs are sometimes called air-to-air heat exchangers because they preheat or cool incoming air using exhaust air.

The need for mechanical ventilation

Wood-frame houses use a mechanical ventilation system to remove pollutants from the home by replacing stale indoor air with fresh outdoor air. These pollutants come from a wide range of sources - household contents and materials, people and their activities as well as family pets
A properly installed, operated and maintained HRV exhausts indoor air pollutants and excess humidity to the outdoors while distributing fresh air throughout the house. During the heating season, the HRV captures heat from the outgoing air and uses it to preheat the incoming fresh air. During the air-conditioning season, an HRV can reverse this heat-exchange process, removing some of the heat from the incoming air and transferring it to the outgoing air.
In many homes, air infiltration through doors, windows and other openings in the building shell is too random and does not always provide adequate ventilation, which is just as important in a conventional home. Mechanical ventilation is needed in many conventional homes in order to evenly distribute fresh air throughout the home and maintain a healthy living environment. An added benefit of mechanical ventilation systems is their capability to filter the incoming fresh outdoor air.
Some Typical Household Pollutants
You can eliminate or reduce certain sources of indoor air pollution by understanding where household pollutants come from. However, no matter how careful you are, there will always be some pollutants in your home and, therefore, a need for ventilation.

Pollutant	Source
Excess moisture (humidity) and moulds	A crawl space with an exposed earth floor, people, clothes drying indoors, cooking and washing, plants, firewood stored indoors, etc.
Urea-formaldehyde	Some types of particle board, panelling, carpeting, furniture, textiles, etc.
Radon	Soil and ground water
Tobacco smoke	Smoking
Household chemicals	Cleaning products, certain hobby supplies, paints and solvents, aerosols, etc.
Odours, viruses, bacteria and dandruff	People and pets
Combustion by-products (including carbon monoxide, nitrogen oxides, carbon dioxide and particulates)	fuel burning appliances, including furnaces, heaters, range/ovens, gas clothes-dryers, fireplaces, wood stoves, etc.*

 

The HRV

An HRV system generally consists of the following components:

* insulated ducts for incoming (fresh) and outgoing (stale) air, along with exterior hoods;

* ductwork to distribute fresh air throughout the home and to return stale air to the HRV;

* fans to circulate air throughout the home and to exhaust stale air to the outdoors;

* a heat-exchange core, where heat is transferred from one air stream to the other;

* filters to keep dirt out of the heat-exchange core;

* a defrost mechanism (some units use a preheater) to prevent freezing and blocking of the heat-exchange core when the temperature of the incoming air is cold (not shown);

* a drain to remove any condensation from inside the HRV (may not be required with all models);

* Operating controls to regulate the HRV according to ventilation needs.

Air Distribution

There are two standard ways to distribute fresh air throughout the home - through ductwork installed specifically for the HRV (direct ductwork) or through the ductwork of a forced-air furnace system. A direct-ducted system is commonly found in homes that do not have forced-air heating, such as those with electric baseboard, hot water or radiant heating. In this case, the fresh air is distributed through ducts to the bedrooms, living room, dining room, basement and other rooms, from where it disperses throughout the house. Exhaust air ducts take the stale air from rooms that have high moisture and pollutant sources back to the HRV and from there to the outdoors.
An HRV can also be installed to work in conjunction with a forced-air furnace system. In this case, the HRV's fresh-air duct is connected to the furnace's main return air duct. The fresh air enters the furnace and is distributed throughout the house using the regular system of ductwork. In such a configuration, the furnace blower should run continuously at low speed when the HRV is operating to ensure a regular flow of fresh air throughout the house. (The furnace blower can also be wired to operate at the normal higher speed for heating and cooling. However, this higher speed can be noisy and may make the rooms feel cool if used to distribute ventilation air continuously.) Separate, additional ductwork may be needed to transfer stale air from the wet rooms to the HRV.