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Personal: Residential


Maximizing Efficiencies of Existing Heating Equipment


Non-condensing Gas Boilers

Residential gas boilers sold in Canada today are required to have an annual fuel utilization efficiency (AFUE) rating of at least 80 percent. The minimum AFUE rating for boilers to be ENERGY STAR qualified is 85 percent.

Manufacturers have improved efficiency levels in a variety of ways, including the following:

  • 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 through their casing (sides) and up the chimney when they are not in use. New boilers have much better insulation to keep the water hot.

  • Better draft control methods to reduce flue losses. Many boilers use draft hoods, located downstream of the boiler. These draw household air into the gas vent along with the flue gases and stabilize the airflow through the appliance, isolating the burner from outside pressure fluctuations. But they also continuously draw heat from the boiler and warm household air up the chimney. A vent damper is 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; it opens again before the burner lights again.

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. These units have:

  • no dilution air
  • high resistance to spillage during the on cycle
  • minimal flow up the stack during the off cycle

XAs a result, they tend to perform better than those using draft hoods and vent dampers.V

Many of today’s 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, which helps develop and maintain an adequate draft. Like the burners used in advanced oil-fired equipment, they 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 aspirating burners with a powered exhaust system.

Condensing Gas Boilers

Condensing gas boilers use either an aspirating burner with an induced draft fan or a power burner. They also have an additional heat exchanger made of corrosion-resistant materials (usually stainless steel). It extracts latent heat remaining in the combustion products by condensing before the products are exhausted. Because a chimney is not needed, installation costs are reduced. The flue gas temperature is low enough for the gases to be vented through a PVC or ABS plastic pipe out the side wall of the house.

A condensing boiler can have an AFUE rating of 90 percent or higher. But in practice, condensing boilers in hydronic (hot water) heating systems can have difficulty achieving this efficiency. For the condensing boiler's heat exchanger to extract all the heat effectively, the system has to run with the lowest possible return water temperatures, preferably not exceeding 45°C – 50°C (113°F– 122°F). Unfortunately, most radiator systems are designed to operate at significantly higher return water temperatures. This makes it difficult for the flue gas to condense. If the return water temperature is too high, actual operating efficiency may be only slightly higher than that of the better models of non-condensing boilers.

For a condensing boiler to achieve its potential, the heating system must be designed to return water to the boiler below the temperature of the condensing flue gas. Residential applications that operate at low return water temperatures include radiant floor heating or pool water heating.

For radiator systems, it may be possible to lower the return water temperature with techniques such as:

  • using an outdoor reset controller to lower the supply water temperature in the late spring and early fall (this method is not effective in the peak heating season)
  • using radiator systems that have sufficient heat exchange surface to operate at lower temperatures
  • using the return water to preheat service water for combined space and water heating systems