Top 10 design tips for condensing boiler systems
In addition to pumping setup, the following top 10 design items should be considered when designing condensing boiler systems.
1. Cost impacts: A condensing boiler has a more expensive initial cost than an equally sized noncondensing boiler. To gain the efficiency benefits, the system must be designed for condensing temperatures.
2. Boiler controls: Purchase and install a boiler management system dedicated to control the boilers to optimize the boiler system efficiency. Running more boilers at lower loads provides better efficiency for condensing boilers.
3. Boiler heat exchanger: Understand the heat exchanger style selected and the potential limitations of the heat exchanger selection. It may require mixing valves and extra pumps to allow condensing to occur only where desired.
4. Flue materials and locations: Individual flues are generally required due to the high variability of airflow and the boilers will require fans to remove the products of combustion out of the building. Coordinate flue materials with the manufacturer selected.
5. Drains required: Condensate drains are needed at the base of the flue stack and the heat exchanger with condensate traps. Coordinate plumbing drainage systems required with the plumbing engineer to confirm the drains are compatible with the acidic condensate.
6. Hot water return temperature: The lower the hot water return temperature to the boiler, the better the system efficiency. Design the system for the lowest possible hot water supply temperature but be careful of aggressive hot water reset schedules that assume energy savings. (Many times reset schedules use more energy due to higher flow requirements at lower temperatures.)
7. Minimum boiler flow: Generally, some type of minimal flow is required for the boilers. Provide a minimum flow bypass valve or a few three-way valves in the system design to maintain the minimum flow required by the boiler.
8. Pumping arrangements: Some boiler manufacturers prefer to pump into the boiler, and some prefer to pump away from the boiler. Provide variable primary pumping when possible, which always provides lower initial and operating costs. Make sure to include two-way, two-position control valves at each boiler to prevent flow to a boiler when it is not in use.
9. Larger heating coil requirements: Larger, deeper coils will be required with lower hot water supply temperatures. Pay close attention to pressure drops.
10. Follow the manufacturer’s instructions: Every boiler manufacturer is different, and not following the installation guidelines could void the warranty. Always confirm the design requirements with the specific boiler manufacturer selected.
David Grassl is a mechanical engineer at Ring & DuChateau, a consulting engineering firm based in Milwaukee, and an adjunct professor in the Civil & Architectural Engineering & Construction Management Department at the Milwaukee School of Engineering. He has analyzed and designed multiple heating plants including steam boilers, standard boilers, and heat recovery/condensing boiler systems.