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How to Get the Most Out of Your Mixed Boiler Plant

Gain System Efficiency by Selecting Multiple Boilers


During much of the heating season, the heating load is small, usually less than 25% of the boiler plant capacity. It is only during the coldest days of the year that the full capacity of the boiler plant is required. By selecting multiple boilers over a single boiler, a more efficient system can be achieved. There are major savings in maintenance costs, service costs, and fuel efficiency. The largest gain in efficiency results from changing from a single boiler system to two boilers. Another significant gain can be made when switching from two to four boilers. However, the marginal gains in efficiency decrease as the number of boilers is increased beyond four.

Not only is each boiler's capacity smaller and more matched to the heating load, but the boiler itself is much smaller in physical size. In the past, installing a single large capacity boiler required several cast iron boiler sections to be moved through doorways, and then the boiler and burner assembled by technicians in the boiler room. Once assembled, the boiler had to be tested and commissioned using specialized combustion analysis equipment. Now, by installing multiple small-size low mass boilers, this labor-intensive process can be avoided. The multiple small-sized low mass boilers come fully assembled and tested from the factory and can be moved into the boiler room by a single installer. This offers the installer a huge cost savings.

Consider This...
In order to achieve the highest possible energy efficiency, it is common to install multiple high-efficiency condensing boilers as the heating plant. To gain these high-efficiencies, these boilers must operate at low temperatures in order to extract the latent heat out of the combustion flue gases. This occurs when operating below the condensation point of the boiler. Using an Outdoor Temperature Reset control, the multiple boilers are operated to maintain low operating temperatures during mild outdoor air temperatures, which are also the same periods when the heating load is small. During cold outdoor temperatures, the heating load is large and the boilers are often operated at high temperatures, which are above the condensation point of the boiler. Once operating above the condensation point, the high-efficiency boilers no longer recover the latent heat of the flue gases so that their efficiencies are equivalent to a mid-efficiency non-condensing boiler.

The drawback to installing multiple high-efficiency boilers is the large initial capital cost. Often a high-efficiency boiler is two to three times the cost of a similar capacity mid-efficiency boiler. In many cases using a combination of high efficiency and mid-efficiency boilers in a boiler plant can achieve the same efficiency performance as a system with all high-efficiency boilers.

Get the Most Out of Your System
The key is to ensure that the high-efficiency boilers are always operated first and only operate the mid-efficiency boilers during peak heating loads. The result is similar efficiency performance at much lower installation cost. Another important consideration is to operate mid-efficient non-condensing boilers above the condensation temperature to prevent damage to the heat exchanger. This is accomplished by requiring the boilers to maintain a minimum boiler target temperature when the mid-efficiency boilers are firing.

The tekmar Boiler Control 275 provides many cost saving options to heating designers and installers. This control offers Outdoor Temperature Reset, Domestic Hot Water generation, boiler Equal Run Time Rotation, and the ability to mix-and-match among the following boiler combinations:

  • Up to four high-efficiency boilers
  • Combination of up to three high efficiency boilers and one mid-efficiency boiler
  • Combination of one high-efficiency boiler and up to three mid-efficiency boilers
  • Up to four mid-efficiency boilers

While operating mid-efficiency boilers, the control applies a minimum boiler target temperature to prevent flue gas condensation. When choosing boilers, be confident that tekmar boiler controls are compatible with all major boiler manufacturer equipment available on the market today. Whether you are designing a new boiler plant or retrofitting an old one, tekmar offers you the very best in system flexibility and performance.