Ceiling whole house fan system (C) Daniel Friedman Exhaust Fan Ventilation System Guide
     


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How to diagnose & fix exhaust fan problems: this article explains using exhaust fan ventilation systems to improve indoor air quality in homes.

We include exhaust vent system troubleshooting checks: Here we provide air handler unit or blower assembly troubleshooting by expanded annotated information from the US EPA [5] who provided suggestions for investigating the air handling unit during an indoor air quality investigation.

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Troubleshooting Local Exhaust Vent Systems - Quick Checks

Clogged dryer vent fan cover (C) D FriedmanThis article includes excerpts or adaptations from Best Practices Guide to Residential Construction, by Steven Bliss, courtesy of Wiley & Sons.

  • Does the exhaust fan run? If not check for loose electrical connections or a blown fuse or tripped circuit breaker; the fan motor may also be damaged, or the fan itself jammed.
  • Is the exhaust fan used when it should be? People often leave exhaust fans off when they should be run to remove unwanted odors or moisture, perhaps because the fan is too noisy.
    See FAN NOISES

    and Fan Noise Control
  • Does air flow out of the exhaust fan? If not check for a blocked fan duct or a dirty or clogged screen at the fan inlet or outlet. See examples of clogging
    at Dryer Vent Covers
  • Is the quantity of air being exhausted sufficient to remove odors and unwanted moisture? If not, and presuming the fan ducting is correctly installed, routed, and clean, you may need a higher capacity fan. But first check that sufficient make-up air is able to enter the room where the fan is in use.

Exhaust-Only Ventilation for Improving Indoor Air Quality

As detailed in Best Practices Guide to Residential Construction:

Exhaust-only ventilation is the most common approach, due to its simplicity and use of familiar components such as bathroom fans. However, unless houses are built very tight, there is little control over where fresh air enters the building.

Also, building depressurization can be a problem, particularly with high-capacity fans. In addition to the increased potential for backdrafting, a depressurized house tends to draw more soil gases, including radon if it is present. And in hot, humid climates, moist air infiltrating through exterior walls can condense on interior finishes such as the back face of vinyl wallpaper that is chilled by air conditioning.

Below we discuss three exaust-only ventilation approaches, followed by a discussion of Supply Only Ventilation
and
then VENTILATION, BALANCED.

  1. Single Port Exhaust Ventilation
  2. Multiport Exhaust Ventilation
  3. Ventilating Heat-Pump Water Heater

Single-Port Exhaust House Venting Systems

Single port exhaust ventilation system (C) J Wiley, Steven Bliss

The simplest and least expensive central ventilation system consists of an automatic timer wired to one centrally located bathroom or laundry fan so it cycles on and off for a portion of every hour or for the 8 to 12 hours per day when most people are home, typically mornings and evenings.

See the figure at left (click the image for details) about a single port exhaust house ventilation system.

The simplest ventilation system uses a single, centrally located exhaust fan that runs on a timer or continuously at a low speed. The fan may also serve as a bathroom or laundry fan, but a dedicated fan is optimal.

Passive air inlets are sometimes installed but will only work properly in very tight homes.

Illustration Source: Recommended Ventilation Strategies for Energy-Efficient Production Homes, 1998, by Judy A. Roberson, et al., Lawrence Berkeley National Laboratory, appearing in the text cited above.

Exhaust fan control switch: Since the house ventilation exhaust fan is doing double duty as a bath or laundry fan, it must have a manual override switch for intermittent use. In larger homes, two fans at separate locations can be used. Another upgrade is to use a dedicated fan in a central location, such as a hallway ceiling, which will provide better distribution of both exhaust and supply air.

Exhaust fan noise: For the house ventilation system to work well, it is important to use a quiet exhaust fan of one sone or less and choose a central location. Also, the door to the bathroom with the exhaust fan must be undercut by 3/4 to 1inch, along with doors to all 4 bedrooms and other rooms that require ventilation. An alternative is to connect the rooms with through-the-wall transfer grilles.

The biggest drawback to exhaust-only ventilation is that there is little control over distribution of the incoming air. Makeup air will come via the path of least resistance. In a leaky house, this might be a window or drop ceiling in the bathroom with the exhaust fan, leaving the rest of the house un served by the ventilation system. For this reason, single-port exhaust-only ventilation works well only in relatively small, tight houses.

  • Passive air inlets. Some contractors install passive air inlets in an effort to direct makeup air into bedrooms and main living areas.

    For these to work properly, however, the house must be extremely tight and doors must be left open or be cut at least an inch above the carpet. If a house is too leaky or rooms are cut off from household airflows, the inlets will function like other random holes in the building shell, leaking air inward or outward, depending on the wind, stack effect, and imbalances in the HVAC system.

    The inlets typically require at least 10 Pascals of negative pressure to operate. They do not eliminate depressurization as sometimes thought. In fact, they require it to work properly.

Multiport Exhaust House Venting Systems

Single port exhaust system for indoor air quality (C) J Wiley, Steven Bliss

This type of system uses a more powerful exhaust fan that is remotely mounted, typically in the attic or basement. See the figure at left for details of a multi-port whole house exhaust fan vent system).

A multiport exhaust system improves air distribution by picking up air from bathrooms and main living areas. These are often used in conjunction with passive air inlets.

Exhaust-only systems are best used in homes with electric heating or sealed-combustion appliances where backdrafting is not a concern.

Illustration Source: Recommended Ventilation Strategies for Energy-Efficient Production Homes, 1998, by Judy A. Roberson, et al., Lawrence Berkeley National Laboratory, appearing in the text cited above.

The multiport house exhaust fan system is ducted to exhaust grilles in bathrooms, laundries, and other wet areas, and sometimes to a centrally located pickup point in the main living space. A room with no outside walls would also benefit from a pickup point.

Systems typically run on a low background speed with timer switches in bathrooms for higher-powered spot ventilation. If installed correctly, these systems are very quiet and provide good distribution of ventilation.

Multiport exhaust systems may incorporate passive air inlets (see description above) that install either in windows or through the wall, providing some control over supply air. The inlets, typically three or four for a small house, go in bedrooms, main living areas, and other occupied rooms, such as dens or home offices. Inlets should be placed high on the wall away from beds, chairs, or other places where drafts might cause discomfort. Placement near a window is preferred.

Because these systems use more powerful fans that depressurize the house, they should not be used in houses with fireplaces or atmospherically vented combustion appliances. They are also not recommended in hot climates, since hot, moist exterior air may be drawn into walls and condense behind interior surfaces chilled from air conditioning.

Packaged multiport house exhaust venting systems are available from American Aldes, Fantech, and a few other few manufacturers (see Resources, page 297 in Best Practices Guide to Residential Construction)

Ventilating Heat-Pump Water Heater

This variation on exhaust-only ventilation passes the exhaust air through a heat-pump water heater, reclaiming heat from the outgoing air stream. Some systems can be reversed in summer, functioning as a supply ventilation system while cooling and dehumidifying the incoming air. A packaged heat-pump ventilating system is available from Therma-Stor.

-- Adapted with permission from Best Practices Guide to Residential Construction.

 

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