Inadequate venting leads to leaking exhaust fans

Hey there, time traveller!
This article was published 19/09/2004 (7865 days ago), so information in it may no longer be current.

HERE we have two questions of a similar nature that appear to have the same cause and effect. I will answer them together.

QUESTION — I have a vent in a bathroom and during cold months there appears to be an ice build-up in the pipes that exhaust the warm and moist air when the fan is operated. When the weather starts to warm up, around the freezing point, water is dripping from the exhaust fan. Also the Gyprock ceiling is cracked.

How can this be fixed?

–Peter Pomialowski, e-mail

QUESTION — I am having trouble with water leaking from my range hood. Every time I turn it on, water starts leaking out all sides and does this for 10 to 15 minutes.

I have inspected the flap on the range hood. It usually opens within a minute of the fan being turned on. I have also gone up into the attic to follow the path of the coiled vent, which is about six feet long and extends to the outside, and it seems to be in good shape. Everything looks OK, but I can’t figure out why it leaks.

We try to run the fan every time we’re cooking on the stove, but sometimes there isn’t time to wait for the water to stop leaking so it doesn’t get used. We occasionally leave the fan on while we’re at work, but the water will start leaking again the next day. I have checked the humidity level in our 850-square-foot side-by-side bungalow, built in 1970, and it is normal, throughout the house.

Do you think this problem has to do with the range hood, the venting, or something entirely different? Any ideas you would have on this would be greatly appreciated.

— Charles and Marcie Foidart, e-mail

ANSWER — Exhaust fans are an integral component of the ventilation system of a home and serve to vent household pollutants and excess moisture to the outside. The bathroom fans are primarily for moisture control and the kitchen exhaust fans for removing cooking odours as well as moisture and other airborne products of cooking.

When these are properly installed and vented they will draw well and cause little concern. When the venting is inadequate or poorly installed, problems like the ones all of you are experiencing are common. Both of your situations are likely due to the same problem, poor venting from the exhaust fans.

The problem with both fans is water intrusion through the fans or their ducting and eliminating the cause of the moisture, rather than the source, is the solution to fixing the problem. It appears that both the bathroom fan and the kitchen exhaust fan in question are vented upwards through the attics of your two houses. The source of the moisture is the warm house air exhausted by the fans, but the water is caused by condensation on the ducting when this warm air hits the cold air of the attic. This will quickly cause condensation on the ducting, which may run back down, as in the kitchen situation or freeze and cause ice and frost build-up, as in the bathroom in question. In the bathroom, the frost may be forming on the inside and outside of the duct and is running down onto the ceiling as well as the fan, when the frost melts.

The bathroom fan may be either a solid or flexible duct, but the kitchen exhaust fan is definitely flexible ducting. Both of these should be replaced with solid metal ducting. The ducting in both situations is either uninsulated or inadequately insulated flexible plastic ducting. This type of corrugated ducting not only is prone to condensation, but also restricts the airflow from the fans and will reduce their effectiveness. Replacing them with solid metal ducting, and wrapping or covering the ducts with several inches of flexible insulation should stop the condensation. The locations where the fan housing or the ducts enter the attic, through the ceilings, should also be sealed and insulated with blown-in foam insulation. This will prevent the frost build-up and eliminate the moisture damage and leaks. Care should be taken to vent these fans as directly as possible to the vent hoods outside. This will maximize the airflow and improve the fan efficiency, as well as end the water problems.

Checking the location that the vent exits the home is also recommended. If either fan is vented through the soffit, under the roof overhang, it may cause further moisture damage to the soffit itself. This arrangement also has a tendency for the flat vents to become blocked with dirt or nests from birds and bees or wasps. If these are present, they should be replaced with properly designed vent hoods. The best exit location is a roof-mounted vent hood, designed specifically for this purpose, with a damper that automatically closes when not in use. Wall-mounted vent hoods are also acceptable, but can often cause paint to peel off siding or stucco to deteriorate from the heat and moisture from the exhaust fan.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the vice-president of the Canadian Association of Home & Property Inspectors (www.cahi.mb.ca). Questions can be e-mailed or sent to: Ask the Inspector, P. O. Box 69021, #110-2025 Corydon Ave., Winnipeg, MB, R3P 2G9. Ari can be reached at 204-291-5358.

trainedeye@iname.com