Why Does My Bathroom Fan Continually Run?

Years ago, air leaks from a home’s exterior to the interior were very common and occurred where wiring and plumbing runs inside framing members between living and attic space existed as well as small gaps at windows and doors , for example.

When energy prices started going up in the 1970s, newer homes were built better insulated. This trend has continued into the 21^st century. Required higher R-values for home insulation and better air sealing in attics, siding penetrations, etc. meant less air flow between the home’s interior and exterior. If you go into a home being constructed today, you will notice air sealing at top plates where wiring and plumbing enters the attic through framing. This air sealing limits the amount of conditioned air loss from the home into the attic. Less energy usage to heat and cool the home was the design objective but caused, as a consequence, less fresh air and more stagnant air in the home. The longer the home’s interior stays warm in the winter or cool in the summer means the HVAC system needs to run less often. Heating and cooling constitute a large portion of the home’s monthly energy costs.

Stagnant stale air is not good for various reasons and special considerations are needed to help make sure fresh air is introduced into the home on a regular basis. Stagnant air can encourage mold growth and also allows objectionable odors and high humidity levels (such as from cooking or bathing) to linger for a long time. In many cases, newly-built homes include one or more mechanical fans that run either intermittently or run continually. In our area, many of the builders are installing continuously-operating fans in bathrooms as part of the normal bathroom fan system.

Air changes per hour (abbreviated ACH or ACPH) is a measurement of how many times per hour the air volume in an area is refreshed hourly. Most rooms in a home ideally should have between 3 and 9 ACH. As an example, think of a 10’ x 10’ x 8’ room which is 800 cubic feet. If that room has an air exchange rate of 5 ACH, that would mean that approx. 4,000 cubic feet of air is being removed from that room hourly. When air leaves a space, replacement air must then enter the space from some other location, otherwise you’d have a vacuum.

These extra mechanical ventilation systems help encourage good air quality and are considered part of the home’s mechanical ventilation system. Since air can less easily leak into or out of modern homes due to better construction codes, the air changes need to be done differently. That’s where mechanical ventilation systems come in.

This can be achieved in different ways. Some homes have a central system for accomplishing this, such as an HRV (Heat Recovery Ventilation) which helps exchange interior and exterior air while minimizing heat loss in this air exchange. Some homes use continually running kitchen exhaust fans. In our area, however, I am seeing these ventilation systems incorporated into dual-speed bathroom fans which is a simpler and cheaper method. The higher speed is what you control with the fan’s wall switch, but the lower speed is how the fan is operating in its ‘home ventilation mode’, if you will. At the lower speed (approximately 20~30 cfm), the fan is very quiet and you may not even notice it running. At regular fan operation speed (when you turn on the wall switch), it runs in the 70~140 cfm range. CFM is ‘cubic feet per minute’ and is a fan rating of how much air a fan is moving per minute.

A properly designed and installed Energy-Star rated ventilation fan should help ensure fresh air within the home and at a low electricity cost (often in the $0.75 ~ $2 per month range depending upon your electrical rate). The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) sets standards on ventilation requirements in homes. These ventilation systems should have an over-ride on/off switch, such as for servicing the unit. Depending upon the installer, this switch could be one of numerous places, such as in the bathroom itself, near the home’s HVAC thermostat, or simply may be a properly labeled circuit breaker in your breaker panel. If buying a brand new home, it is recommended to ask the builder where this switch is located. Most of the fan systems, such as those found in bathrooms, have settings for fan speed and the optimum speeds (high and low speed) are set by the installer.

Depending upon the home’s design, a supplemental in-take vent may be incorporated into your HVAC system to bring in some exterior fresh air. Remember, the ventilation fan is sucking small amounts of conditioned air out of the home; a supplemental in-take may be installed to help replace this air. In many cases, however, no extra in-take system is needed since even a brand new home will still have some small air leaks that can introduce sufficient fresh exterior air into the home to help replace the air being removed to the exterior. The fan creates a slight negative air pressure which will allow small amounts of exterior air to still enter at minor voids, etc.

All bathroom ventilation fans are required to discharge to the home’s exterior and should have a proper exterior vent cover. They may not terminate within attics, at soffit or ridge venting, into living space, or into basements or crawl spaces.

See the House Ventilation 101 website for more info.

Matthew Steger is a Certified Level 1 Infrared Thermographer, an ASHI Certified Inspector (ACI), and an electrical engineer. He can be reached at matthew@thehomeinspectorsnotebook.com. No article, or portion thereof, may be reproduced or copied without prior written consent of Matthew Steger.