Non-Working Fans? It Could be the Ducts

Before we started work on one of our earlier projects, the client told me that the vent installed in the bath was still fairly new and only a few years old. He had it and two others in the upstairs baths installed as replacement units by another contractor, who had been highly recommended by numerous neighbors. He wanted to reuse this ventilator, but told me both it and the ones upstairs did not seem to be working properly. He bought them because they were highly-rated, powerful and ultra-quiet models, but since their date of installation all three were very noisy and seemed to draw even less air than the old builder fart fans they had replaced.

Right after listening to his problem, I turned the fan on and off a few times. He was right, there was something wrong. When you turned the fan on, for the first 4 to 7 seconds, it sounded as if the fan would not even turn on, then the sound of the motor working very hard would start and continue. I, at 6’7″ tall, could easily feel very little air movement as I placed my hand on the vent.

I responded that the fans appeared to be a NuTone ultra quiet fan, a good quality products that should be both quiet and powerful, and that we would investigate the fan in this bath after we began construction. In my mind and while measuring the room I had already checked the first possible cause of this problem in this bath.

Make-Up Air: If you have a fan drawing air out of a room, you must make sure there is a path for the same amount of new air to enter the room from the rest of the house or outside the home. If the air you remove is not simultaneously replaced by the same amount of air you are removing, a vacuum is created which will bring the air movement and motor to a halt. Little or no air movement and a very noisy fan that is overworking is a symptom of too little or no make-up air.

The most common and easiest location for this make up air space is the space between the bottom of the door of this bath and the floor. (A secondary, rarely used, way is to place a through vent from the bath into an adjoining room or hall). That air space must be of near equal size in square inches of total opening as the size of the vent leaving the fan. All NuTone fans require 6″ diameter vents to operate within specs, so using high school math, a 6″ diameter duct has a surface area of πr² 3.14 x 9″ = 28.26 sq. in. The door in this bath is 30″ wide, so the space between the top of the floor and bottom of the door should have been almost 1″ — specifically, 0.9 inches tall. At that time, it was only about a 5/8″ space. This slightly undersized opening for the fan would make the fan work a little harder and noisier, but not enough to cause the seriousness of the symptoms seen. So there was something else, much more serious, contributing to the problem that I could only find after demo. 

After starting our work and removing the drywall from the walls and ceiling of this bath, the cause of the problem was obvious. As stated, this NuTone 110 cfm vent requires a 6″ duct. The original installer simply connected this NuTone fan to the original 3″ fan’s ducting with a 3″ to 6″ transition. If the 3″ duct only ran a few feet to an exterior wall, that would have caused the fan to work a little harder, but in this case, that 3″ duct was almost 40 feet long, passing over a laundry room and through a long section of garage. Making it even more difficult, the spaces this existing 3″ duct was passing through had other furnace ducting, wires, and structural obstacles that made it impossible to run a new 6″ duct through the same path. Our only solution for this bath was to replace the fan with a Panasonic model with all the same specs except one: it only needed a 4″ duct. That is the only reason why on jobs that require only an 80 or 110 cfm duct I prefer Panasonic fans. We were able to run the new 4″ duct through the spaces doing minor damage to drywall on soffits in the garage to make the venting in this bath work the way it was intended and the client had originally hoped for.

The symptoms this fan had were caused partially by the make-up air space being too small, but mostly by the incredible resistance of this 6″ exhaust vent being connected to a 40 foot long 3″ duct. The fan behaved like it could not move any air, the pressure on both the intake and exhaust sides were so great it took the motor 7 seconds to start moving fast enough that it could even be heard, and it never was able to overcome the resistance enough to move air at the units specified cfm rating.

Another benefit to changing this fan to a Panasonic model with its 4″ duct is that the make-up air under the door did not need to be increased. Since the Panasonic fan was engineered to be quiet and operate at full efficiency with a 4″ duct rather than a 6″ duct, the πr2 equation above with the new Panasonic variable became is 3.14 x 4″ = 12.56 sq. in. Divide that by the 30″ door and the clearance needed between the door bottom and floor is only 0.4 inches, less than ½ inch, so our existing 5/8″ clearance was fine. This is another reason I prefer to use the Panasonic brand for these 80 and 110 cfm fans. Over that cfm rating (130 or 150), both Panasonic and NuTone fans use the same 6″ ducting.

The client asked us to investigate the other two baths with the same problem on the second floor. In these baths we found the same problem, but it was far easier to fix. Both were connected to the same 3″ diameter ducting leading out the roof. In this case the solution was simple and did not require replacing the NuTone 6″ vent fan since these are great fans that work as advertised when installed properly. This ducting was only 15 to 20 feet long for each bath and was completely accessible in a spacious attic crawl space. We simply replaced the 3″ duct with a 6″ duct. The roof jack air escape was a standard 6″ cap that did not need replacement. The original builder used a very common 6″ roof cap, then saved pennies by installing the 3″ ducting, typical builder thinking.