Efficient hot water systems

05/20/2015


The 20:1 turndown system will operate at 5% of capacity with a temperature difference of 2 F and a return water temperature of 128 F (see Table 1), which is above the condensing temperature of the combustion gases. The result is a decrease in efficiency to 88% or less. The condensing boiler investment is now functioning like a noncondensing boiler. The turndown ratio is dependent upon the boiler capacity and its anticipated part loading and must be part of the overall system evaluation.

Boiler staging also needs to be evaluated. In heating plants with multiple noncondensing boilers, it is common to load the operating boiler(s) to upwards of 90% before bringing another boiler on line. This strategy attempts to minimize the amount of time the boilers are below their most efficient firing range. When applying condensing boilers, this thought process needs to change to take advantage of their excellent part-load efficiency. Operating multiple boilers at firing rates of 50% or less can result in an efficiency improvement of 4 to 8 points compared to the full fire efficiency, depending on the hot water return temperature (see Figure 1).

All boilers lose efficiency when they cycle on and off. This is due to heat being lost through the jacket, post-purge, ambient air flowing through the boiler after the boiler turns off, and pre-purge. The lost heat must be replaced in order for the boiler to reach its operating temperature. Frequent cycling results in increased energy consumption and lower overall efficiency. Short-cycling can be minimized by avoiding oversizing of the boiler and providing multiple small boilers in lieu of one large boiler. Purge cycles are still required for safety reasons as well as boiler and flue longevity.

Pumps

Pumping schemes for condensing boiler plants vary considerably from those of traditional noncondensing boiler plants that employ constant volume pumping or primary/secondary pumping. Unlike noncondensing boilers, primary variable flow pumping can be used with most condensing boilers. The recommended minimum flow through the boiler needs to be taken into consideration, but there are typically no minimum return water temperature requirements. The minimum flow bypass should be located toward the end of the distribution system. The pipe must be of adequate size for the required minimum flow rates to also avoid excessive pressure drops and increase the pumping energy required. This will maximize temperature losses and ensure adequate mixing before returning to the boiler plant. The bypass should be provided with a modulating control valve hardwired to the boiler plant controller. This will eliminate any control system delays that could occur when the valve is wired to another controller in the control system. The bypass valve should be controlled to maintain boiler minimum flow, as measured by a differential pressure sensor across the boiler, with a slight margin of safety to avoid nuisance tripping. Bypassing too much water will raise the return water temperature, which can adversely affect the boiler efficiency.

Thorough pressure drop calculations are required to evaluate the required pump head and optimize the piping system. A reasonable safety factor should be included only after this analysis. Oversized pumps are not desirable, from an efficiency standpoint, in a variable flow application. Pumps should be selected to maximize their efficiency. Electronically commutated motor (ECM) technology makes it possible to economically provide variable pumping for systems that require less than 5 hp pump motors.

Coils

Selecting coils for new systems is relatively straightforward. The water temperature difference across the coil should match that of the boiler. In moderate climates where the heating loads are low, this occasionally may not be possible while maintaining turbulent flow through the coil. When that is the case, the coil should be selected with a leaving hot water temperature at full load that results in the boiler operating in the condensing range. Both air and water pressure drop through the coil should be minimized without oversizing the coil. The air side leaving temperature should be less than 90 F to prevent stratification in the occupied space.

Table 2: Existing reheat coils may be capable of providing required capacity at lower entering water temperatures. Evaluation requires use of a coil sizing program. In some instances an increase in airflow may be necessary. Courtesy: P2S EngineeringRetrofit applications may not require replacement of the existing heating coils just because the hot water supply temperature is lower. It is not unusual for the existing coils to be oversized, especially for zone level reheat. The existing heating coil performance at the new hot water supply temperature will need to be checked. The first example is an existing one-row coil with a heating airflow of 145 cfm at an entering temperature of 52 F, a leaving air temperature of 90.5 F, an entering hot water temperature of 180 F, a leaving hot water temperature of 141.1 F, and a heating capacity of 6.05 MBH. The zone heating requirement is 5.4 MBH. When the same coil is supplied with 160 F entering hot water, the resulting heating capacity is 4.5 MBH (see Run 1 in Table 2).

Since the resulting heating capacity of Run 1 is less than the required heating capacity, further evaluation is required. In this particular case, the heating airflow needed to be increased to get a valid coil selection. This is reflected in Run 2 with an airflow of 220 cfm and a heating capacity of 5.48 MBH. The resulting leaving water temperature is in the desired range for a condensing boiler system, while the required water flow has been reduced. The branch piping to the coil, as well as the coil, can be reused. While an increase in airflow for a single coil will not have an appreciable impact on the fan energy consumption, the air side of a system with multiple heating coils can't be ignored. Any resulting change in fan energy would need to be included in the overall energy analysis of the hot water system as well as confirmation of the heating load and coil rows/capacity.

Control valves at the coil should be two-way modulating type valves. This will allow the variable flow distribution pumps to operate at lower speeds when the building is not at peak heating load. In retrofit applications, when changing from a constant volume noncondensing system to a variable volume condensing system, all existing three-way valves will require replacement. Balancing valves are not necessary at the coil in variable flow systems using two-way valves. The use of balancing valves in this type of system results only in increased system pressure drop with no associated improvement in flow control. In the event the system serves a critical load that requires immediate hot water, use a small two-way valve and temperature sensor at the end of the run serving the load instead of a three-way valve. Modulate the valve to maintain the supply temperature in the run to the critical load at least 10 F lower than the system supply temperature.



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