What to do when your bath fan fails

Bath fans are an incredibly important part of a high-performance home.  Unfortunately they are also a frequently failed item when we perform final testing for programs like Energy Star and GreenBuilt NC.  This week’s blog focuses on how builders can be successful with bath fans.

Why are bath fans are important?

The most important job of a bath fan in our climate is to remove moisture from showers.  This has become an even bigger job with the trend toward open shower designs where there is no shower door and moisture can more easily escape the shower into the bathroom and other parts of the home.

Residents of Western North Carolina battle bathroom humidity in the summer and winter.  Summer is obviously not a time when you want to be adding any more humidity to your home.  But a tight home probably doesn’t need it in the winter either.  If your bath fan isn’t doing its job, you might notice window condensation happening on cold days.  That’s a sign that your indoor humidity is too high.

Set yourself up for success

The most important thing to do is buy a good bath fan to begin with.  When we test your bath fans, we want them to be removing 50 cfm of air from the bathroom.  We find that Energy Star rated or “low-sone” (typically 1 sone or less) fans are more likely to actually pull 50 cfm.  “Low-sone” means quiet, and a quiet fan is usually a higher-quality model.  As an added bonus, homeowners are more likely to actually USE a fan that doesn’t sound like a jet engine is taking off.

It’s also important to realize that if you buy a fan that is rated at 50 cfm, it’s unlikely to actually remove 50 cfm.  This is because they’re tested under ideal conditions, and your personal combination of ductwork, exterior penetration, and exterior cover is unlikely to be 100% ideal.  Buying a 70 or 80 cfm rated fan is safer.

Some fans come with the ability to accept a 6” duct.  I really like these models, and encourage you to use the 6” duct with them.

“Building science” types often like to look smart by showing photos of bath fans that are installed backwards so that a 180 degree bend in the duct is required to get to an outside wall.  It happens, and should be avoided if at all possible.  I get it.  Sometimes you can’t get it centered over the shower if you turn it around.  No one wants to look at an exterior penetration on the front of the house.  Do the best you can.  If you can’t find any other option besides duct gymnastics, you might need to get a bigger fan or hard duct it.  If you can’t look at your ducting and believe that it will actually allow air to move through it, don’t hang drywall.

There's never going to be an easier time to do something else with this bath fan ducting.
There’s never going to be an easier time to do something else with this bath fan ducting.

Be aware that bath fans depend on at least three people for proper function: the electrician, the HVAC installer, and the siding guy.  The electrician installs and wires the fan.  The HVAC installer attaches the duct (usually flexible) and will often leave some extra length because the envelope penetration hasn’t been cut yet.  The siding guy has to cut the hole for it.  Someone then has to pull the duct through and attach the exterior cover.  No wonder the finer details sometimes slip.

Still failing the test? 

If the flow is really low (less than 25 cfm), your fan has serious problems.  The good news is that these are often the easiest to fix.  Most of the time if you go outside, you’ll notice that the flaps on the exterior cover aren’t popping open when you turn on the fan.  Sometimes they just get stuck or painted shut, and if you hit them with a pole or rake you can get them moving again without even getting on a ladder.  Sometimes you have to get up there and remove the cover and remove an obstruction – usually insulation or a bird’s nest.  Always be careful reaching in there.  Bees nests are not an uncommon thing to find.

More rarely, there’s something blocking the fan that you have to remove from the interior.  This could be a plastic cover or knockout plate that was shipped with the fan that someone forgot to remove.

If the flow is higher (between 25 and 50 cfm), it’s probably a duct problem.  The following technique will usually fix the problem.  Go outside and remove the exterior vent cover.  Grab the end of the duct and pull it through the exterior penetration.  Pull it tight.  Usually you’ll have at least a foot of extra flex duct.  Cut off this extra duct, and fasten the end of the duct securely, tightly, and permanently to the hole.  The goal here is to keep the duct lined up with the hole and directing all of the exhaust air out of the home.  A secondary goal is to keep pests or air infiltration from entering the home around the duct.  Finally, put the exterior cover back on and retest the fan.  You can usually pick up 10-20 cfm using this method.  In some really egregious cases, we’ve added 45 cfm by doing this.

If you have access to the ducting from the attic, diagnosing it from there is also worth a try.  It is possible to get a fan that’s just a lemon, but it’s rare.  If you try these things and can’t fix the problem, the easiest thing is usually to replace it with a larger fan.

ASHRAE Standard 62.2, Energy Star, and most green building programs also allow a 20 cfm continuous fan to be used in lieu of a 50 cfm intermittent fan.  If you have 20 cfm or more you can turn it into a continuous fan by removing or labeling the switch.  Although it’s easy, I advise against this approach most of the time because I don’t feel that it does as good a job of removing moisture.  However, for a seldom used bathroom in a home with exhaust-only ventilation, this can sometimes be acceptable if the fan is low-sone.

Taking the time to do these things won’t guarantee zero humidity problems, but it really helps.  If you follow these steps, you can eliminate the vast majority of bath fan problems in your new homes.

Copyright 2013 Amy Musser