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Conventional control valves typically are sized by rule of thumb (5 psid, same pressure drop as the coil served, one line size smaller than the pipe, etc.). This leads to control valves that are poorly sized in the hydraulic gradient. They do not modulate well, particularly at low flow, and typically are installed with flow limiters that guarantee poor ΔT performance. HWST-reset schedules drive down ΔT and increase flow, masking poor equipment performance.

The hotter the water, the greater the heat loss at boilers and in pipes. Are reductions in boiler and pipe heat loss offset by added pump and fan energy consumption? How good is the insulation? In heating mode, does heat make it into the space anyway? In cooling mode, do heating and reheat valves leak, leading to simultaneous heating and cooling?

Lowering HWST reduces ΔT and increases flow. The greater the flow, the greater the pump energy consumption. Lowering HWST also may reduce LAT. The colder the air, the greater the airflow and fan energy that may be required to satisfy space temperature requirements.

CONCLUSION

Condensing boilers offer outstanding opportunities for conservation; however, typical distribution-system performance (and high HWRT) can adversely impact final results. With modern technology and a systems approach to optimizing the production, distribution, and use of heating water, condensing-boiler systems can be designed to realize their full potential.

There are numerous successful distribution-system projects using industrial-grade pressure-independent control, primarily in the large-commercial and institutional HVAC and district-energy markets. All are designed to maximize heating- and/or cooling-coil performance.

When properly applied, condensing boilers and pressure-independent control are easy for operators to understand. Components can be selected to deliver high performance for the life of a facility, even as it expands. High ΔT generates savings and creates opportunities for additional conservation and performance enhancement.

Whether for a new facility or an existing one, condensing-boiler systems with pressure-independent control can achieve high performance. These should be seen as investments for facility owners seeking to save energy, minimize costs, and reduce emissions.

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Eric Moe, MSE, MBA, has 17 years of experience in mechanical-system design and integration. He is an active member of the International District Energy Association and the Association of Energy Engineers. Formerly, he served as director of business development for Flow Control Industries Inc. He can be contacted at ericmoe40@gmail.com.

For past HPAC Engineering feature articles, visit www.hpac.com.

Condensing-Boiler Tips

Following are best practices to help you get the most out of your condensing boilers:

Get out of the boiler room

A key to very high efficiency is low HWRT, which enables boilers to operate the majority of time in condensing mode. Optimization takes a systems approach that considers production, distribution, and demand for hot water.

Know your heating and reheat coils

Get familiar with a common coil-rating and selection program. Assess the capability of the distribution system to deliver low HWRT. Be sure to consider the effect of HWST reset and lower LAT. When coils and control valves are operating together properly, higher HWST produces lower HWRT. Lower LAT increases ÄT, but also increases airflow.

Use industrial-quality pressure-independent control valves at coils

Industrial-quality pressure-independent control valves maximize heat-transfer performance at coils and deliver high ΔT for the life of facilities. High-temperature capability, debris tolerance, and availability of parts are a few reasons why industrial-grade pressure-independent control valves are so cost-effective in the long run. No balancing valves are required, and Y-strainers are not needed at every coil.

Carefully consider whether HWST reset is necessary

Take into account total-system energy use when planning a control strategy. Be sure the boiler will condense, but minimize total energy consumption. Consider driving up HWST at peak load to maximize system efficiency and available capacity.

Design new projects for at least 60°F ΔT

Be sure to consider boiler capabilities when choosing a piping configuration. Use pressure-independent control at heating coils, and eliminate reverse return and balancing valves. Bear in mind that 60°F-ΔT coils generally cost no more than 20°F-ΔT coils, while the pumping and piping expense can be considerably less.

Specify and verify performance

Conventional two-way valves, balancing valves, and variable-frequency drives do not optimize performance. After installation, startup, and commissioning with industrial-grade pressure-independent control valves, verify that the distribution system is performing as intended by trending data as shown in Figure 2. Boilers should condense, space temperatures should be satisfied, and total-system energy consumption and emissions should be minimized.

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