The installation of double-duty exhaust fans in existing dwellings is common. This practice can save money and time on the job and provides a simpler ventilation solution for the occupants. But is it a good idea?
1) What is a double-duty fan?
The ASHRAE 62.2-2016 standard requires local (spot) ventilation in kitchens and bathrooms. These requirements are 100 CFM and 50 CFM, respectively, if the fans are occupant controlled. Local ventilation must exhaust air from these rooms.
The standard also requires dwelling-unit ventilation to be sized with the RED 62.2-2016 tool, or other appropriate calculation aid. Dwelling-unit ventilation may be balanced or unbalanced (exhaust- or supply-only). Many installer/designers provide these two functions — local and dwelling-unit ventilation — with one high-quality exhaust fan rated for continuous operation.
2) Does the ASHRAE 62.2-2016 standard allow double-duty fans?
Yes, in both existing and new dwelling units. Incidentally, ASHRAE 62.2-2013, ASHRAE 62.2-2010, and ASHRAE 62.2-California also allow double-duty fans. RED provides free web apps for each of these versions of the standard.
3) Do double-duty exhaust fans save money?
Yes and no. If you are installing fewer fans, maybe just one double-duty fan, you will save on installed costs, both labor and equipment. We all know we can save lots if the electrician spends less time on the job. Fewer fans also mean less ductwork and time-consuming terminations.
However, there might not be a savings on ventilation operating costs. Chances are if you use a double-duty fan, it will run longer than if you had installed separate local and dwelling-unit fans.
4) Are double-duty exhaust fans appropriate for all climate zones?
ASHRAE 62.2-2016 requires all local (spot) ventilation to be exhausting fans. This applies to all climate zones. On the other hand, dwelling-unit ventilation may be balanced, exhaust-only, or supply-only.
Most residential ventilation experts recommend exhaust-only dwelling-unit ventilation for cold climates and supply-only for warm climates. The reasons have to do with the pressures created in dwellings from the unbalanced ventilation and potential condensation in building assemblies. So installing double-duty fans in cold climates makes more sense than in warm climates.
Although more expensive to install, balanced dwelling-unit ventilation works in any climate because no pressures are created if the ventilation is, indeed, balanced. Ducted HRVs and ERVs can also be installed to provide double-duty ventilation. Air is exhausted from a bathroom at a continuous rate of at least 20 CFM by the HRV/ERV; this provides the local bathroom ventilation (this is not recommended for a kitchen). Additional ducts, registers, and grilles from/to the HRV/ERV provide the dwelling-unit ventilation and the required design rate.
5) Do double-duty fans generally provide better IAQ than separate local and dwelling-unit fans?
We think in almost all cases the answer to this question is “no”. The best place to exhaust contaminants like moisture and bad cooking stuff is at the source with a local bathroom fan or kitchen range hood. But the best location for the dwelling-unit ventilation fan is seldom in a bathroom or a kitchen; a central location where the occupants spend lots of time is preferred.
6) In conclusion?
Yes, it is a good idea to install double-duty fans, especially in colder climates. Use caution in warm climates. Remember, double-duty fans save money, but are usually not the best way of providing the ideal IAQ for your customers. Whenever possible when installing a double-duty fan, locate it in a centrally located kitchen or bathroom.
