Coupling a ventilation system with a forced air heating system to provide the necessary distribution of outdoor air is relatively simple. A duct brings air from outdoors to the heating system’s return air plenum. Whenever the principal ventilation fan is activated, the furnace fan is automatically activated to distribute the outdoor air. (See Sentence 9.32.3.4.(9)) Where no auxiliary supply fan is installed as per Sentence 9.32.3.4.(8), the furnace fan also drives the flow of outdoor air in through the outdoor air duct. Use of an auxiliary supply fan allows the size of the outdoor air supply duct to be reduced.
This system tempers the outdoor air before it reaches occupied areas of the house by mixing it with return air in the furnace’s return air plenum. It is important that thorough mixing occur before the cold air reaches the furnace’s heat exchanger, otherwise condensation could reduce the service life of the heat exchanger. The 3 m minimum distance between the furnace and the outdoor air duct connection is one means of addressing this concern. However, a well-designed mixing device is likely to be more effective, as are certain arrangements of the outdoor air duct’s connection to the return air plenum. Figures A-9.32.3.4.-A and A-9.32.3.4.-B illustrate one such device and arrangements that have been shown to be effective in research carried out by Canada Mortgage and Housing Corporation (“Testing of Fresh Air Mixing Devices,” IRTA Research for Research Division of CMHC, March 1993).
Simple Air Mixing Device
Connection of Outdoor Air Duct to Return Air Plenum
Even if the outdoor air is well mixed with the return air, in very cold weather the resulting mixed air temperature could still be lower than what the furnace heat exchanger can tolerate if there is too much outdoor air. That is why Article 9.32.3.4. includes several provisions, including Table 9.32.3.4. and the requirement to actually measure the outdoor airflow (see Sentence [b-9.32.3.4.#^table-93234hplca|9.32.3.4.]), to guard against this possibility. In some cases, it will not be possible to use the forced air heating system to circulate the outdoor air unless additional heating devices are used to temper the outdoor air before it reaches the furnace heat exchanger. This would be the case, for example, in a highly insulated house with a small furnace that is located in a very cold region.
The maximum outdoor airflow permitted by Table 9.32.3.4. must equal or exceed the “normal operating exhaust capacity” of the principal ventilation fan, as determined in accordance with Sentence [b-9.32.3.3.#^table-93233hplca|9.32.3.3.]; otherwise there is an increased possibility that the mixed airflow over the furnace heat exchanger in cold weather will be colder than what the heat exchanger can tolerate. No values are listed in Table 9.32.3.4. when the maximum flow permitted exceeds the maximum capacity found in Table 9.32.3.3. since no higher outdoor airflow is required to match the flow of the principal ventilation fan.
Sentence 9.32.3.3.(9) is intended to avoid having the principal ventilation fan exhaust the outdoor air brought in through the outdoor air supply duct before it is circulated to the dwelling. The design of some advanced integrated mechanical systems is such that some portion of the outdoor air is exhausted before being circulated but this is taken into account in the design of the system and the total amount of outdoor air brought in is adjusted accordingly. This provision is not intended to preclude the use of such systems.
The duct bringing outdoor air to the furnace return air plenum must be equipped with a manual damper [see Sentence 9.32.3.4.(6)] that is adjusted (see Sentence 9.32.3.4.(10)) to balance the outdoor airflow with the flow through the principal ventilation fan. It is recommended, but not mandatory, that a motorized damper also be installed in this duct and that it be wired to be fully open when the principal ventilation fan is operating and fully closed when the principal ventilation fan is not operating. This damper will allow ventilation to occur only when the occupants have called for it by turning the “Ventilation Fan” switch to “on.” The absence of such a damper can lead to unwanted ventilation, which can result, in turn, in excessive dryness and increased heating costs in winter, and increased loading on air-conditioning equipment in the summer.