Accommodating Ductwork During Framing
When planning the installation of the ventilation system, it is important to provide passageways, or chases, for ducts. Otherwise, these may have to be concealed with dropped ceilings or bulkheads in finished areas of the house. Refer to the Planning Ahead note in the Floor Framing chapter for helpful planning tips.
fan mixes and circulates the outdoor air which is delivered to the distribution ductwork. Coupled ventilation systems are generally less expensive to install since much of the ductwork is already provided by the forced-air heating system. Operating costs may be higher depending on the efficiency of the furnace circulating fan.
Heat recovery ventilators (HRVs) are a packaged type of ventilation system which is engineered to recover heat from the air being exhausted from the house, and transfer this heat to the incoming outdoor air. Numerous studies have concluded that in the long term, HRVs are a cost-effective alternative, whose energy savings more than offset their higher initial capital costs. They also have the advantage of tempering the incoming air such that the need for auxiliary heating is avoided.
Canadian heat recovery ventilator technology is growing in affordability, effectiveness and sophistication. At present, there are two types of heat recovery ventilators manufactured in Canada: plate heat exchanger types, and heat wheel exchanger types.
Plate heat exchangers function using either a parallel flow or counter flow (Fig. 132) of air streams through a plastic or metal core of plates. Outside air and indoor streams flow through alternate, adjacent plates. During the exchange of heat between adjacent plates, moisture from inside air condenses out and is drained to the plumbing system. As a result of this phenomenon, when plate heat exchangers are used, humidification of the house is often required to maintain suitable indoor relative humidity levels.
Heat wheel exchanger types use a heat wheel made from a desiccant material (Fig. 132). Inside air passing through the heat wheel deposits the moisture it contains within the desiccant material, and as the wheel rotates into the outside air stream, the moisture and heat are released into the air supply. Humidification of the house air is usually not required with these HRVs, nor do they require a condensate drain.
Both types of HRV may be installed as a stand-alone ventilation system or as a ventilation system coupled to a forced-air heating system. Properly installed, HRVs deliver a balanced flow of supply and exhaust air, neither pressurizing nor depressurizing the house. This makes them ideally suited for installation in homes with spillage-susceptible combustion appliances such as fireplaces and woodstoves. As well, some types of HRVs can operate in a recirculation mode to distribute heat from woodburning appliances throughout the house.
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