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My attic fan has been running constantly
11 June 2005


Q. I've noticed in recent weeks and months that my attic fan has been running constantly, non stop, literally. Even on very cool days and nights. My question is how do I stop this. Is there a switch that I should find. Would it be located in the attic area or the bedroom. Should I call a professional to disconnect it?

A. My recommendation to you is to remove the power from it and leave it that way. I'll tell you why. But first station someone with their head up in the attic where they can see the fan and can communicate with you. If you canít hear shouting from two and half floors away use a cellphone. Switch the breakers at the main panel box until youíve found the right one then unwire the fan motor. If you feel uncomfortable doing that then call an electrician but in any event you need to get control of that motor. It sounds like the fanís thermostat switch has gone haywire or, if there is a single-pole switch to it somewhere, you havenít found it yet.

The name of that type of fan in modern building vocabulary is a PAV, or powered attic ventilator. Contrary to decades of building experience and intuition, researchers at the analytical end of the homebuilding field now tend not to like them much in mode rn houses anymore.

The idea of ventilating an attic is certainly not a new one and for anyone who has had the great misfortune of having to get up into an unvented attic to work during the midst of a hot, mid-summer afternoon can tell you any relief is welcome. Attic space s in this area can reach temperatures of 140ļ F. and above on a hot, sunny day. That's the temperature of medium rare roast beef! Some roof designs, such as hip roofs, didn't allow for easy venting and were often left unvented.

A modern house is a big insulated box. The walls, windows, doors and ceilings are designed and built to impede the flow of air and the transfer of temperature at levels unheard of in times even not so long ago. I know this sounds counter-intuitive, but a properly insulated ceiling assembly under an attic does not need a vented space above it to control interior temperatures. The passive vents such as ridge and soffit venting, gable-end louver or roof vents cut through the shingles real job then beco mes attic space moisture control and is more important in winter than in summer.

Attics in modern homes do fine relative to living space heat control with non-powered convective ventilation. Through field testing and research it has been learned that if the use of a powered vent fan is perceptible in cooling the interior of the hom e then the attic floor is under-insulated. I've found myself in a lot of debates on this point with some old-timers so I repeat the above point twice and give up. It seems to go against common sense but it's been proven.

Powered attic ventilators can and will pull conditioned air from the house. These fans will pull indoor air not from just around the drop stairs or access panels but from any penetration into the attic, such as wires going through top plates of walls, p lumbing pipes, drywall that's not tight, etc. These fans can be counter-productive to your interior cooling efforts. You've already paid good money to cool the inside air and now you're additionally paying for the electricity used by the fan to pull it out.

When the attic fan motors get old and wear out, the bearings can seize up and become a fire hazard. They are not something people keep an eye on. They get installed and then forgotten. I've met people who have lost their homes and all their possessions to fires begun by a malfunctioning attic vent fan. I found a hot, seized fan in an attic last week in an unoccupied house. It was hours or even minutes from causing a fire. I'm not advocating a general panic about these things but when I see a powered attic fan in an adequately insulated and passively vented attic, I recommend that the power to it be turned off and it be abandoned in place. The older I get, the more fire conscious I become. Why take chances?

As for roof shingles lasting longer by the use of a fan, that's never been proven. Again, passive ventilation is proper and sufficient. Roof slope, basic shingle quality, shingle color-- the lighter the better-- and compass orientation-- south facing r oofs age faster-- have more to do with shingle life than any sort of powered venting. When the fan is depowered, just leave it there. The fan assembly will become convective venting.

Another type of powered venting you may have heard about is called a "whole house fan" and that's completely different from an attic only fan. Whole house fans have been around for a long time and were used more commonly before the days of central air-c onditioning. They are big, powerful fans that mount flat through the ceiling of usually the upstairs or central hallway of a house.

When they are turned on with windows open in the house, they pull unconditioned hot air from the house and force it out through the passive attic vents causing gentle cool breezes down in the living level. Using them today in a modern house buys you a couple of weeks time at both ends of the cooling season when you almost but don't quite need the air on. People who don't like air conditioning-- and there are some-- really benefit from using whole house fans.

While youíre up in the attic working to de-power the fan check the insulation level on the floor of the attic. If itís not deeper than six inches, invest in beefing up the insulation to a level of R-30 or above. Call an insulation contractor for a quote for blown insulation and the job can be quickly done-- you'll cut down on your heating bills with added insulation too.

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