improve your furnace's efficiency

A faulty furnace not only burns money, it pollutes the atmosphere. Here are suggestions from the Boulder Green Building Guild to improve your furnace’s efficiency.

By Kim Masters

Upgrading your heating system is one of the best ways to make your house healthier and cheaper to operate. By combining equipment upgrades with appropriate insulation, weatherization and thermostat settings, you can slash your energy bills and pollution output in half.

In Colorado, heating costs are two-thirds of the utility bill. In the U.S., heating systems are also responsible for more than a billion tons of carbon dioxide (CO2) released annually into the atmosphere. An energy-efficient furnace goes a long way toward reducing both of these.

Here are a few suggestions for forced-air heaters, the most common residential heating system in the U.S.

up the efficiency

Gas furnaces generally have the highest efficiency ratings, followed by gas boilers and oil furnaces, and lastly, oil boilers. Upgrading your furnace or boiler from 65 percent to 90 percent efficiency in an average cold-climate house saves 1.5 tons of CO2 emissions if you heat with gas and 2.5 tons if you heat with oil. Energy-bill savings can be equally impressive.

An efficient forced-air system also enables you to control humidity, filter indoor air, introduce fresh air from the outside and quickly change the temperature of an area. However, improperly installed and maintained forced-air systems can be noisy and distribute unwanted fumes and particulate matter.

a breath of fresh air

In furnaces, fuel is sprayed into a combustion chamber, mixed with air and burned. A flue pipe vents combustion products out of the home. Furnaces with sealed combustion use outside air for combustion instead of indoor air from the home by means of outside vents directed to the appliance jacket.

Sealed-combustion furnaces are better because they don’t create combustion byproducts or negative air pressure, which can suck cooking and laundry fumes into a living space. They also don’t compete with occupants for oxygen, and won’t leak carbon monoxide into the home in negative-pressure conditions.

filter facts

Forced-air heating and ventilation systems often transport a tremendous amount of dust, mold spores and other irritants that circulate inside a house. Most filters on forced-air furnaces remove only the larger particles. Therefore, they somewhat protect the furnace, but not your health.

Higher-efficiency MERV (minimum efficiency reporting value) filters with a rating between 6 and 10 have a tighter pleat that removes a higher percentage of particles. They’re easy to install and fit the standard inch-wide filter slot located in a furnace’s air handler. You can also invest in a larger 4- or 6-inch filter box to reduce replacement frequency.

In some cases, a higher-efficiency air filter can slow airflow enough to reduce furnace efficiency. To prevent this, don’t purchase a filter with a rating above MERV 10.

size up the situation

With the exception of steam systems, where the boiler is sized to the radiator, the best way to determine the proper size of a new heating system is through a Manual J heat-loss analysis performed by a heating contractor or energy auditor. The system should be sized to no more than 25 percent over the peak hourly demand. A heating system that is too large wastes fuel and money because it keeps cycling on and off. It also only runs at peak efficiency for short periods and spends most of its time either warming up or cooling down, similar to a car in city- versus highway-driving conditions. Many existing furnaces installed in the 1950s and ’60s are two to three times larger than necessary. A properly sized unit saves you significant amounts of energy and money.

ducting the issue

In forced-air systems, air flows through ducts. These metal or plastic-and-fiberglass conduits branch off the furnace into a home’s walls, floors and ceilings. Unfortunately these ducts often leak, which affects heating bills, indoor air quality and combustion equipment safety, and increases the risk of mold and decay. Nationwide, more than 3 percent of the hot air a furnace generates never reaches any room. Each year, U.S. residential duct leakage costs consumers $5 billion, with an energy loss equivalent to the annual energy consumption of 13 million cars.

Homeowners often repair duct leaks with duct tape, which typically loses effectiveness within three to five years. Alternatively, duct mastic maintains a seal for decades. Proper sealing reduces dust entry, excess humidity, automotive exhaust, radon gas, fumes from stored paints, solvents, pesticides and combustion gases. It’s particularly important to seal return ducts.

Ductwork along exterior walls and non-insulated spaces, like attics, garages and crawl spaces, loses significant amounts of warm air because of insufficient insulation. To improve efficiency, move ductwork to an insulated space or insulate the space surrounding the duct-work. Also, situate supply and return registers on walls and ceilings to prevent debris and dust from falling into ducts. Don’t locate a wall return too close to the floor because it acts like a vacuum cleaner, eventually coating heating and cooling coils with dust, dirt and pet dander.

With soaring natural gas prices, furnace efficiency should be a priority in your home. Otherwise, you’re burning money, as well as natural gas.

Kim Master is operations manager for Boulder Green Building Guild and co-author of Green Remodeling: Changing the World One Room at a Time. For information, visit www.greenbuilding.com.

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