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Purchasing Energy-Efficient Boilers

Information and databases to help in selecting smaller boilers (under 300 000 Btu/hr and generally designed for residential use) can be found on the ENERGY STAR® and the American Council for an Energy Efficient Economy Web sites. No comparable database exists for larger boilers.

Financial and other consequences of buying a new boiler will be felt for more than 25 years – the normal life of a boiler. Several factors should be considered for making a wise choice.

Timing

The best time to purchase a high-efficiency boiler is when new plant is being built, when new capacity is planned, or when a boiler has failed.

For new systems and capacity changes, the type and size of the boiler are determined in the planning. Failed boilers must be replaced or repaired quickly. This is why it is always best to have a boiler management plan in place to ensure that a suitable model is available. A boiler management plan can be as simple as having written-out criteria – boiler type, size, supplier and so on – for both repair and replacement.

Following are some other important points to consider when choosing the right boiler.

Seriously consider condensing boilers. Condensing boilers extract heat from the water vapour as it condenses out of the flue gas. These boilers can be 95 percent energy efficient or higher. New, non-condensing models are only 70 to 80 percent efficient.

Where low-temperature hot water is wanted – for example, for space heating – a condensing boiler is the most efficient. Where high temperatures are wanted and there is no means to cool the return below the flue gas dewpoint, condensing is unlikely, so consider a mid-efficiency boiler.

It is crucial to thoroughly discuss existing and potential needs with suppliers because of the higher cost of a condensing boiler. Long-term fuel costs and other factors must also be considered.

Use a high-modulating boiler with excess-air control. The simplest method of controlling the flame is on-off firing. The fuel flow is either fully on or fully off. In a modulating boiler, the burner firing rate is matched to the required load. This is similar to an automobile accelerator controlling speed.

Modulated firing results in fewer heating cycles and lower standby losses than on-off firing. On-off firing can cause short-cycling (frequent firing) and is inefficient. Short cycling occurs when a boiler quickly satisfies demand and shuts down its burner until more heat is needed. Modulating the firing rate results in fewer heating cycles and lower standby losses.

If the boiler is firing at a 25 percent rate, the fuel will be in the boiler four times as long as it would at a 100 percent fire rate, so the same amount of fuel has four times as long to transfer its heat, resulting in higher efficiency.

Excess-air control improves efficiency by reducing airflow to the burner when less air is needed at lower firing rates.

Use a number of integrated smaller boilers instead of oversizing. Oversize boilers are commonly installed to handle peak demand and anticipate possible expansion. This is wasteful since oversize boilers rarely operate at peak load, and their part-load efficiency can be as much as 20 percent lower.

Several smaller units can be more efficient and economical than a single large one. They can be staged (or “sequenced”) to operate at or near peak efficiency if piped and controlled appropriately. An added up-front benefit is lower installation costs since small boilers do not require a crane to be installed.

For retrofitting, a multiple boiler approach can improve the seasonal inefficiency of large, old boilers. A small boiler can supply basic heating, and the large boiler fires only when necessary to supplement the heating during periods of high heating load.

Consider cogeneration. The most energy-efficient system for supplying both heat and electricity is a cogeneration system. These systems generate both reliable electricity and heat while reducing emissions and saving money. A cogeneration system consists of a gas turbine or reciprocating engine and a heat recovery steam generator, which is a type of boiler. If an old boiler requires major improvements, this may be the time to replace it with a cogeneration system. To learn more about cogeneration, visit the Canadian Advanced Buildings Web site or the U.S. Department of Energy Web site.

Assess heat cascading. When steam heat is needed for one process and hot water for another, the heat exhausted from one process can be input to the next process. The heat finally exhausted should be at the lowest temperature that can be economically achieved.

Assess the life-cycle costs, not just the “first” or up-front cost. Fuel costs are by far the most significant expenditure over the life of a typical boiler. For example, fuel costs would be more than 50 times the capital cost of the conventional gas-fired boiler in the example given in the “How Much Will I Save?” section. Considerable savings can be realized by taking into account all costs over the boiler's entire life cycle.

Rated Thermal Efficiency is a good starting point, but it does not tell the whole story. Thermal Efficiency is a steady-state measurement that applies only to specific operating conditions. Real-world conditions are rarely the same as test conditions and can have a great impact on efficiency. This is especially true for condensing boilers whose efficiency is a function of the operating temperature.

The Annual Fuel Utilization Efficiency (AFUE) rating may apply. It measures the average efficiency of a system over a year and takes into account the cyclic on/off operation and associated energy losses of the heating unit as it responds to changes in the load. This in turn is affected by changes in weather and occupant controls.

Boiler suppliers can help conduct life-cycle cost analyses to meet specific needs.

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