Residential gas boilers sold in Canada today are required to have an AFUE rating of at least 80%. ENERGY STAR qualified boilers must have an AFUE rating of at least 85%. The following are some ways manufacturers have improved efficiency levels:
Elimination of continuous pilot lights. Most boilers on the market today use some form of intermittent ignition device, usually electronic ignition.
Improved insulation levels. Because boilers store more heat internally than warm air furnaces do, they are subject to greater heat losses, both out through their casing (sides) and up the chimney when they are not being fired. To reduce heat lost from casings, new boilers have much better insulation to keep the boiler water hot.
Better draft control methods to reduce flue losses. Many boilers use draft hoods. The draft hood is located downstream of the boiler proper. It draws household air into the gas vent along with the flue gases. This stabilizes the airflow through the appliance, isolating the burner from outside pressure fluctuations. But it also continuously draws heat from the boiler and warm household air up the chimney. A vent damper is now usually installed downstream of the draft hood to close off the exhaust when the burner is not operating. When the gas burner turns off, the damper is closed automatically after a short period; before the burner lights again, the damper opens.
Other boilers that use aspirating gas burners have eliminated the need for a draft hood entirely by using a powered exhaust system, usually incorporating an induced draft fan. With no dilution air, high resistance to spillage during the on cycle, and minimal flow up the stack during the off cycle, these units tend to give superior performance to those using draft hoods and vent dampers.
Today, many gas boilers have replaced the naturally aspirating gas burner with a power burner. These use a fan on the burner to improve the combustion process and ensure the development and maintenance of an adequate draft. These burners, similar to ones used in advanced oil-fired equipment, tend to have a high-pressure restriction or even close off the combustion air passage when the burner is not operating. This minimizes off-cycle heat losses without requiring a flue damper. Such units minimize dilution air, or have sealed combustion, and have performance characteristics similar to or better than the aspirating burner with a powered exhaust system.