SHOW WHERE YOUR
INDOOR WINDS BLOW
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The big whooshing sound that you never hear may be robbing your home of a third or more of your heating and cooling efficiency. Research conducted by scientists working for government, utility companies and heating-ventilating-air conditioning (HVAC) manufacturers show that most consumers get only two-thirds of the heating and cooling they're paying for mainly because of leaks in home air distribution systems, especially duct work.
Perhaps you have heard about modern ultra-high efficiency furnaces and air conditioners with so-called SEER ratings as high as 12, compared with an average 8 or 9 for equipment marketed even 10 years ago. However, these 12 SEER units may only be delivering 6 or 7 SEER performance if you have leaking or broken ducts. Connecting high-efficiency equipment to a poor air distribution system is like transplanting a heart into a person with clogged arteries.
Inefficient operation makes it hard to control summer humidity without dropping indoor temperatures excessively. More serious, HVAC system leakage increases dirt and dust inside your home and can even lead to health hazards.
For instance, return leakage from filthy attics and crawl spaces often bypass filters. This fouls evaporator coils and ducts and encourages microbial growth that may contribute to allergies. Leaks can cause negative pressures in the combustion area of an HVAC system, which can backdraft furnaces, water heaters and fireplaces and lead to carbon monoxide poisoning. Leaks in a garage return duct can suck in gasoline fumes, car exhaust, pesticides and any other harmful chemicals that might be stored there.
One hint that you might have leaky air ducts is if your home is hard to keep clean no matter how much you dust or vacuum. In particular look for dirt around the air registers. However, there is no easy way to detect most leaks because much ductwork is inaccessible and air is invisible. Most leaks do not come from gaping holes but from tiny gaps and cracks too small to notice.
This is why advanced HVAC contractors are turning to sophisticated "blower door" technology to perform whole house energy audits. The blower door was developed at the height of the 1970's energy crisis by scientists at Princeton University to measure leaks in a building's thermal envelope - i.e., how much air escapes from windows, doors, walls and ceiling. It is now widely used by energy efficiency researchers and building engineers worldwide, and in recent years has become affordable to many HVAC contractors. Blower doors have been highlighted in a CBS News Special Report hosted by Dan Rather, on PBS's "This Old House" series and in National Geographic magazine.
A blower door consists of an expanding panel system that fills a doorway (the house door does not need to be removed), coupled with a powerful variable speed blower and a set of pressure gauges. Advances units also have a built-in computer and printer. After preparing the house for a test, the blower door operator adjusts the blower speed to create a small pressure difference between indoors and outdoors. The pressure differential is the equivalent of having a 10-15 mph wind blowing on all sides of the house simultaneously.
This induced wind causes air to move through any leaks at a slightly exaggerated rate. The work crew injects a cool chemical smoke into the draft that pinpoints where leaks occur. If the fan is operating in its depressurization mode by sucking air out of the house, an equivalent amount of outside air rushes in to replace it through all the cracks, gaps and holes. If the fan is adding pressure by blowing air into the home, just as much indoor air will be pushed to the outdoors. The smoke will make all this visible.
It is fascinating to watch a blower door demonstration. You can see smoke leak past the indoor window, for example, and then out through gaps in the storm window. Until blower doors enabled researchers to take a close look at air leakage in homes, everyone assumed that windows and doors were responsible for the vast majority of building air leaks. Now they know that less than 20% of most homes' leakage come from these prime suspects. Recent research has shown that the worst leaks were completely overlooked. They are in places homeowners never expect - into the interior walls and baseboards, through the attic floor, around plumbing pipe penetrations. The smoke doesn't lie.
The most common problem is the use of wall cavities as return air ducts. These are never airtight and often draw air down from attics or up from crawl spaces. Floor joist spaces often suck in air at the sill plate and band joist area. Supply air ducts that run outside the conditioned space usually leak, sometimes catastrophically. When the blower door pressurizes the house with the air handler off, the smoke will usually get pushed into both return grilles and supply registers as it finds its way to openings to the outdoors. This simple demonstration shows that there are hidden problems without the need for the homeowner to crawl around a dirty attic or crawl space.
Blower doors not only detect leaks but also measure the extent of the leakage. By calculating air flow rates and the pressure created, blower door operators can determine a value called the Equivalent Leakage Area, or ELA. The ELA is the size of a hole the home would have if all the cracks, gaps and holes were somehow brought together in one place.
Some leakage is normal, even desirable, to ensure adequate fresh air ventilation. Most homes need an ELA of between 1.25 and 1.75 square feet to maintain suitable indoor air quality. The vast majority have far more leakage than they need. Contractors performing blower door energy audits routinely find homes with an ELA measuring up to eight square feet - the equivalent of an open window!
Think of what that does to your energy bills when you try to heat your home in winter or cool it in summer. A blower door home inspection is the best investment you can make when you consider all the BTU's that may be pouring into the great outdoors.