Buildings

The Scientific Spin on PAVs

Cooling off hot attics with powered attic ventilators (PAVs) seems like a good idea. After all, doesn't cooler attic air mean less work for the HVAC system, longer shingle life, and reduced energy costs? Unfortunately that's more myth than fact. Studies have shown that under the best of circumstances, PAVs consume more energy than they save. Under the worst of circumstances, they can start a house fire by causing flame rollout from combustion appliances. Let's examine four important facts that help dispel the myth of PAV benefits:

1. Radiant heat is the root of most evil.
   In attic spaces, hot air (convective heat) is a minor factor compared with the radiant heat from the sun on the roof. Radiant heat, if not controlled, will be absorbed and transmitted to interior spaces via the underlying building components (conduction heating). PAVs only target the hot air.
2. Some codes should never be broken .
   The problem of radiant heat being transmitted into interior spaces can and should be addressed by bringing the attic space insulation up to current building codes. That means using the right grade of insulation and installing it properly, leaving no misalignment, gaps, or compression to negate its benefit.
3. The path of least resistance can be costly and dangerous.
   When the PAV starts running, it has to suck air from somewhere. Studies have found that conditioned air in the house is typically being sucked into the attic through numerous small and sometimes not so small openings between the attic and interior space. This happens because it's easier to draw from than the intended outdoor air supply. In addition to sucking dollars out of the homeowner's pocket, this situation becomes life threatening if the sucking is strong enough to draw toxic carbon monoxide fumes into the living space or cause flame rollout from combustion appliances.
4. And the walls can come tumbling down.
   As cool interior air gets sucked into the attic, the interior space tries to equalize pressure by sucking warm moist air down chimneys or through cracks in ducts, walls, and floor systems. The end result is that the interior space is actually being warmed — in stark opposition to the PAV's objective. The damage doesn't always stop with wasted energy dollars. That warm moist air can flow in and condense on cold metal air-conditioning equipment and ductwork. The condensation will generally find its way to wood or sheetrock surfaces and can result in damage such as falling ceilings.

For years, Advanced Energy's Bruce Davis has been involved with studies that demonstrate the disadvantages of PAV usage. A study of eight houses with PAVs in the Research Triangle Park area of North Carolina revealed that the house was being depressurized to varying extents in every case.

Rumor has it that Davis has mounted a couple of PAVs on his research lab wall to serve as hunting trophies of a beast that ought to be driven to extinction. If builders and homeowners insist on perpetuating the species, Davis advises that harmful effects from PAVs can be reduced by:

• Ensuring that the ceiling is sealed airtight to prevent air transfer and moisture problems
• Using large attic vents to provide adequate air intake
• Confirming through performance testing or equipment specifications for combustion equipment that combustion products are not being backdrafted into the space

For More Information

Contact Advanced Energy at (919) 857-9000 or moreinfo@advancedenergy.org.