Higher turndowns influence stability of hydronic systems
Hydronic heating systems have come a long way since the one-pipe gravity-system design. As building technology has advanced, so has the capacity to improve building climate control. The ability to more accurately modulate equipment input has helped meet building heating demand more efficiently.
This article will focus on boiler hydronic heating systems. There are many different types of hydronic systems; however, they all are made up of the same basic components, such as pumps, coils, control valves, and boilers. This article will discuss how boiler turndown can improve the interactions among many of these components to create a more stable and reliable heating system.
Modern control valves, pumps, and boilers have greater modulation abilities than past models. Contemporary pump variable-frequency drives (VFDs) can adjust pump impeller speed from 20 to 100 percent. Modern control valves can have turndown ratios of up to 100-to-1. Current boilers can have a variety of modulation capabilities, from a step-burner control to a boiler with a 20-1 turndown ratio. Each of these improved components interacts with and influences the others' operation. A heating system is like an orchestra: All of its components need to be in tune with each other to create music; otherwise, it just makes noise.
The Goal of a Heating System
To properly understand the advantages of higher turndown, let's examine a heating system’s ultimate objective. No matter the building type, a heating system's purpose is to create a constant and comfortable climate in which occupants can live and work. The more consistent the room temperature, the more content and productive the occupants. Research from the U.S. Department of Energy shows that, "Buildings with higher indoor temperatures (above 73.4˚F, even though that is near the middle of the recommended temperature range) were associated with approximately a 30- to 80-percent increase in building-related nose, eye, and skin symptoms as well as headaches."1 Therefore, improperly controlled living spaces directly correlate to health issues.
Because a heating system's ultimate objective is to create a comfortable climate, equipment that can maintain precise control under all conditions is essential to a well-designed system. A heating-system design should guarantee that a building will have heat on the coldest day of the year, which, on average, is less than 1 percent of the heating season. Traditionally, these designs incorporate redundancy in boiler heating plants to ensure buildings have heat when required.
Sustaining Stability
Higher-turndown boilers help maintain heating-system stability by controlling a building's supply-water temperature accurately. As days get warmer and building heating load decreases, higher turndown allows a heating plant to operate longer without cycling. Once a heating plant is below its minimum input, its boilers will cycle to maintain temperature. The more a unit cycles, the lower its seasonal efficiency. The only way to minimize cycling in this type of system is to allow the boiler plant to deviate from the required boiler set point, which can be as much as ±15˚F. This swing in a system's supply-water temperature also creates a fluctuation in its energy supply. As the energy supply varies, significant instability is created when the control valve, VFD pumps, and coils try to compensate for the lack of boiler turndown and control.
Table 1 demonstrates a basic building load. It shows how load decreases as weather becomes warmer and the point at which a heating system might begin cycling. The table represents a 15-1 turndown plant using three 2,000-mbh boilers with 5-1 turndown burners. The table does not take into consideration makeup air or internal heat gain from people, lighting, computers, etc.
When possible, equipment redundancy is beneficial. Like other equipment, boilers need to be maintained and sometimes require unscheduled repairs. If a plant’s single boiler requires repair, the building will be without heat until servicing is complete. However, most commercial boiler systems have redundant boilers. In the previously mentioned example, the three boilers only need to fire at 77.5 percent to handle the load on the coldest day of the year. Therefore, if one of the boilers required repair, the other two could handle 86 percent of the designed building load.
Table 1 also shows that at 65˚F the system load is less than the minimum firing rate of one 2,000-mbh boiler (400,000 Btuh). Because the boiler plant cannot run continuously, the boiler would cycle. Bin data reveals the heating plant would cycle for 742 hours at 65˚F, or 12 percent of the heating season. During this time, the boiler plant would be operating inefficiently and creating system instability (Figure 1). If the boiler plant was utilized year-round for building reheat and/or domestic-water heating, it could cycle for up to 30 percent of the year.
Attaining Results
A higher-turndown system can be achieved in several ways. A system could utilize a greater number of lower-British-thermal-unit- (Btu-) capacity 5-1 turndown boilers or depend on a smaller number of boilers with greater turndown capabilities.
For example, plant represented in Table 1 used five 1,000-mbh boilers with a 5-1 turndown ratio, the overall turndown ratio would be 25-to-1. Therefore, the heating plant could modulate down to 200,000 Btuh without cycling. Based on Table 1, the plant could handle seasonal heating loads, greatly reducing system cycling, improving system stability, and increasing seasonal efficiency.
The downside is that additional units must be installed (five vs. three). When more units are installed, installation costs increase. If costs are a concern, installing three 10-1 turndown boilers—creating a 30-1 turndown plant in which two boilers can handle 86 percent of the load—could be cheaper. This solution maximizes system efficiency and meets budget requirements. Both options satisfy system-efficiency requirements and greatly improve system stability by reducing cycling.
Reference
1) Study: Energy efficiency practices good for building occupants' health. (2009, Feb. 25). Retrieved from http://ohsonline.com/articles/2009/02/25/energy-efficiency-practices-good-for-building-occupants.aspx
Peter Rimassa is the boiler business segment manager for AERCO International (http://www.aerco.com ), a manufacturer of high-efficiency boilers and water heaters. He has more than 10 years of experience in the HVAC industry and is a member of the American Society of Heating, Refrigerating and Air-Conditioning Engineers.