Condensing boilers can be a highly efficient means of generating and circulating heat, as they recover latent heat from water produced during combustion and minimize cycling losses.

FIGURE 1. Typical condensing-boiler-efficiency.

Figure 1 illustrates the relationship between heating-water return temperature (HWRT) and boiler efficiency. Low HWRT is required for high efficiency. This article will discuss how low HWRT can be achieved with industrial-grade pressure-independent control.

By optimizing heat-transfer performance at coils with pressure-independent control and matching thermal-energy production to load through better hydronic-system design, it is possible to heat more space with less energy, equipment, complexity, and cost.

RETROFITS

Any boiler will condense if the HWRT is low enough. A condensing boiler, however, is built to withstand the acid in combustion gases.

Although tens of thousands of facilities are likely candidates for condensing boilers, virtually none has the heating-water-distribution-system performance necessary for high efficiency. Figure 2 shows condensing-boiler-system performance — over a wide range of outside-air-temperature (OAT) and load conditions — in a laboratory facility in St. Louis. Note for how little of their operating time the boilers receive return heating water at a temperature low enough to condense.

FIGURE 2. Condensing-boiler-system performance, laboratory facility, St. Louis.

In the system in Figure 2, the heating-water supply temperature (HWST) is on a typical linear reset schedule with the OAT. The distribution is designed for 30°F delta-T (ΔT) with 180/150 split coils, but achieves only 20°F ΔT at best. Pump speed varies from only 50 to 54 Hz. This means heating-water flow does not vary significantly over the range of load conditions. As a result of low ΔT (and near-constant flow), the pump variable-speed drives do very little to save energy.

That the HWRT in this facility is so high and the boilers rarely condense largely is a function of conventional-control-valve performance. Fortunately, typical coils have far more heat-transfer capability. With better flow control, ΔT performance can be improved dramatically. Low HWRT enables boilers to condense during the majority of operating hours.

Tables 1 and 2 show the expected performance of a typical air-handler heating coil. The coil is designed for an entering-water temperature (EWT) of 180°F, a leaving-water temperature (LWT) of 160°F, an entering-air temperature (EAT) of 55°F, a leaving-air temperature (LAT) of 92.28°F, and 20,000 cfm. Table 1 assumes the system is operated with a consistent 180°F supply-water temperature and no reset. Table 2 assumes a linear reset schedule between 180 and 140°F. Both tables moderately reduce LAT as load declines. As with any HVAC system, the majority of operating hours are at part load. This system should be expected to condense any time the load is less than 50 percent.