Gas-fired Boilers - High-efficiency and Standard-efficiency

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Gas-fired Boilers - High-efficiency and Standard-efficiency

Over the last 20 years, a new generation of higher efficiency gas boilers has come to market. An essential difference in the design of these units is how they are vented, eliminating the need for dilution air. The newer designs have been modified to reduce the amount of heated air that escapes during the on and off cycles and by extracting more of the heat contained in the combustion by-products before they are vented.

Residential gas boilers for sale in Canada are required to have an annual fuel utilization efficiency (AFUE) rating of at least 80 percent. A boiler's energy efficiency performance over a heating season is called the Annual Fuel Utilization Efficiency (AFUE). This AFUE is expressed as a percentage. The higher the percentage, the more efficient the boiler. ENERGY STAR qualified, high energy efficient boilers have an AFUE of 85 percent or more.

List of models: gas boilers
Manufactures of ENERGY STAR qualified models

Manufacturers have improved efficiency levels in many 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 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. It draws household air into the 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 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 before the burner lights again.

Some boilers 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 perform better than those using draft hoods and vent dampers.

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 to help develop and maintain an adequate draft. These burners, like those 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.

Condensing Gas Boilers
Condensing gas boilers employ either an aspirating burner with an induced draft fan, or a power burner, similar to the units described previously. However, they have an additional heat exchanger made of corrosion-resistant materials (usually stainless steel) that extracts latent heat from water in combustion products by condensing the water before it is exhausted. A chimney is not needed, reducing the cost of installation. Because the flue gas temperature is low, the gases are 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. In practice, however, condensing boilers in hydronic (hot water) heating systems can have difficulty achieving this efficiency. For the condensing boiler's heat exchanger to extract the latent heat effectively, the system has to run with the lowest possible return water temperatures, preferably not exceeding 45–50°C (113–122°F). Unfortunately, most radiator systems are designed to operate at significantly higher return water temperatures, which 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 help reduce the return water temperatures include:

radiant floor heating
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 to get efficiencies up during these periods (this method is not effective in the peak heating season)
using radiator systems that have sufficient heat exchange surface to operate effectively at lower temperatures
using the return water to preheat water used in combined space and water heating systems
Oil fired boilers
An oil-fired boiler uses the same type of burner as an oil-fired, forced-air furnace, although a boiler is often somewhat smaller and heavier. There is no circulating fan and filter housing as with a forced-air system. Instead, most boilers require a circulating pump to push heat around the house through the pipes and the radiator system, as shown in Figure 5. The seasonal efficiency of old conventional hydronic systems is similar to that of conventional forced-air systems, which is around 60 percent.

A boiler's energy efficiency performance over a heating season is called the Seasonal Energy Utilization Efficiency (SEUE). This SEUE is expressed as a percentage. For boilers with no standing pilot light, the SEUE is equivalent to the Annual Fuel Utilization Efficiency (AFUE). The higher the percentage, the more efficient the boiler. ENERGY STAR qualified oil-fired boilers must have an AFUE rating of 85 or higher.