Optimizing hot water systems with condensing boilers
Hot water system initial cost
There is no doubt that condensing boilers are more expensive than traditional noncondensing boilers, but the increase in initial costs varies based on heat exchanger construction, configurations, and manufacturers. To illustrate the cost differences between noncondensing boilers, partial condensing boilers, and full condensing boilers, a comparison of boiler costs from three modular boiler manufacturers was obtained based on contractor pricing for Milwaukee. A summary of the boiler characteristics is provided, which show a summary of cost data for each boiler at five equal boiler input sizes. Based on this analysis, it was determined the cost for a full condensing boiler and for a partial condensing boiler is relatively equal, while a full condensing boiler is approximately 25% to 30% more expensive when comparing equally sized boilers.
The initial cost increase for a condensing hot water boiler system doesn’t necessarily stop at the boiler, as equipment selection is different based on using lower hot water supply and return temperatures. The most common change is the physical size and associated cost of the hot water coils in the system. Hot water coils will require deeper coils with more heat transfer surface area to accommodate the lower hot water supply temperature, increasing initial equipment cost and potentially operating costs as well if the designer is not careful. Higher operating costs occur when traditional heating coils are selected with lower hot water supply temperatures resulting in higher air and water pressure drops.
Hot water coils in air handling units are typically not an issue as deeper coils in air handling units can easily be accommodated with different coil selections, but some variable air volume (VAV) box manufacturers are unable to provide low temperature heating coils factory mounted on VAV boxes to sufficiently accommodate this issue. To solve this issue, the engineer should select loose coils to ensure the capacity desired can be provided, with low air pressure drop and sufficient supply air temperatures, instead of using standard factory-mounted VAV box coils. This allows the flexibility of obtaining coils in any size and with a low enough air pressure drop to not use excessive fan energy.
The use of loose coils will cause an increase in labor and possibly ductwork costs due to the requirement to transition from the VAV box outlet to the size of the loose coil and back to the duct size required by the system. However, if system efficiency is the goal, the engineer should pay attention to final equipment selection pressure drops.
System design tips
Full condensing boiler systems are simpler to design than partial and conventional noncondensing boiler systems. Some condensing boiler systems can be set up in a variable primary flow (VPF) arrangement with a minimum flow bypass control valve to maintain boiler minimum flow to allow a piping and pumping configuration that minimizes pumps and maximizes system ΔT. In a VPF system, the flow varies throughout the system, including through the boilers, which is a shift from the previous mind-set that boilers always require a constant flow. These systems eliminate the need for secondary distribution pumps and use multiple pumps in parallel to serve the entire hot water system. Full condensing boilers also are not susceptible to thermal shock, so there is no need for mixing valves, primary-secondary pumping, or high return water temperatures.
VPF systems will always have fewer pumps compared to a primary-secondary system, which will save initial costs from less piping and valves, fewer electrical connections, less controls work, and eliminated vibration isolation at the additional pumps. Similarly, one set of pumps will save on mechanical room space requirements.
The one item that must be addressed is the more complex staging controls and ensuring minimum flow in the system at all times, but this will still maintain an overall net savings in initial cost. For operating costs, VPF systems will always have lower operating costs as there is less pressure drop in the system due to fewer pumps and accessories at the pumps, but also because the pumps will be more efficient pumps. The VPF pumps allow the designer to select larger, more efficient horsepower pumps instead of smaller, constant volume, low efficiency circulator pumps typically found on the primary side of the primary-secondary system. Also, the use of variable speed in the entire system will allow the entire system flow rate to vary, which will provide operating cost savings as energy will vary approximately with the cubed power of the flow rate.
One exception to using VPF pumping systems is that partially condensing boiler systems that use the a secondary heat exchanger to condense flue gases will require constant flow through the heat exchanger. To achieve this, the primary-secondary system is used with two sets of pumps, each with a dedicated function to the hot water system. The primary pumps serve as production pumps and serve only the boilers in the system.
These pumps are typically high flow, low head constant flow pumps staged on with a boiler to provide available hot water to the secondary pumps. The secondary pumps serve only the hot water coils in the system, which are equipped with two-way control valves and are high flow, high head variable speed pumps that vary the system flow in response to the load. The primary and secondary loops are connected via a common pipe or decoupler that is a shared portion of the piping circuit in each loop and hydraulically separates the two loops so flow in one loop does not affect the flow in the other loop.
However, the disadvantage to this configuration is that mixing will occur, either blending excess primary hot water with secondary return water and increasing the hot water return to the boilers (which decreases efficiency), or blending excess secondary water with the hot primary supply water and reducing building hot water supply. In any case, partially condensing boiler systems are still more efficient than noncondensing boiler systems and should not be a basis for ignoring the benefits of condensing flue gases.
Heating hot water systems are forgiving systems when they are designed within the constraints of the equipment being used. Condensing boilers can offer efficiency gains above that of a noncondensing boiler, but if careful consideration isn’t given to the type of boiler being used, premature failure of equipment and increased operating costs are likely to occur. Understanding the differences between full condensing, partial condensing, and noncondensing boilers, and hybrid systems will be useful to maximize overall system efficiency along with the specific requirements for each manufacturer to include during design.
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.