Optimizing hot water systems with condensing boilers
Exhaust gases from a condensing boiler will always be at a lower temperature than those of a noncondensing boiler because the water vapor in the flue gases has condensed, assuming the condensing boiler is operating in a condensing application. Flue temperatures for condensing boilers are typically around 100 F, as opposed to 250 to 300 F from a noncondensing boiler.
Although each manufacturer’s requirements must be followed, the exhaust flues suitable for a reduced temperature, such as polyvinyl-chloride (PVC), chlorinated-polyvinyl-chloride (CPVC), and polypropylene, may be used with condensing boilers. In applications where plastics are not used, a corrosion-resistant flue such as stainless steel or aluminum may be allowed, but under no circumstances should other metal flues be used due to the corrosiveness of the flue gases. Again, if a condensing boiler is not in condensing mode, the flue gases will be similar to those of a noncondensing boiler and must have a stack rated for such high operating temperatures.
Materials like PVC and CPVC will release toxic fumes when overheated, so polypropylene or corrosion-resistant metal vents are the preferred materials. In addition to flue stack materials, manufacturers have specific requirements about the allowable equivalent length of venting allowed. In any case, all venting shall slope back to the boiler to properly drain all the condensate in the system. Figure 2 shows an example of three sealed combustion condensing boilers with intakes from an adjacent area well and flue stacks routed up to the roof.
Condensate traps and acid neutralization
Two new issues that must be addressed with condensing boilers are condensate management and neutralization. To separate the condensate and steam, a condensate trap is provided by the boiler manufacturer to separate the exhaust flue gases from being released back into the building (Figure 2). As the water condenses and mixes with the CO2, the pH drops to approximately 3 to 4, so proper disposal of condensate is required.
The condensate shall also be piped through an acid neutralization trap of marble, limestone, or alkaline chips, which will neutralize the condensate to more acceptable limits. Furthermore, it is important to coordinate the drain piping for the condensate drain with the plumbing engineer on the project as drain piping should be PVC or cast iron piping to protect the building’s sanitary sewer system, and not copper or steel piping, which will quickly corrode over time. Noncondensing boilers do not have any condensate or require any drainage.
Hot water system efficiency
When designing a hot water system, it is important to ensure the system is designed for condensing applications. If the system is not designed for condensing applications and the hot water return temperature never falls below 140 F, a more expensive boiler was purchased without receiving the benefits of a condensing boiler as the water vapor in the flue gases will not condense. This will limit the maximum thermal efficiency to that of a standard, noncondensing boiler at 88%. Conversely, if the hot water return temperature falls below 140 F and the boiler is a noncondensing boiler, the heat exchanger will be unable to withstand the effects of the acidic water vapor and will fail sooner than its useful life.
With condensing boilers, boiler efficiency is driven by boiler load and hot water return temperature. By designing hot water supply and return water temperatures lower than conventional designs, it is possible to increase the efficiency of the boiler as more water vapor is condensed from lower hot water return temperatures and more energy is recovered that would otherwise be discharged out the flue, providing an efficiency increase of approximately 10% to 12% compared to noncondensing boilers.
Equally important as return water temperature to the boiler is the number of boilers in operation. As opposed to noncondensing boilers, condensing boilers’ efficiency also increases as the boiler load decreases. Most condensing boilers are provided with a high modulating gas burner capable of modulating down to 20:1 ratios, which are much more efficient than staged or stepped control with noncondensing boilers. The boiler burners shall be controlled by a boiler management system to control the burner output directly to the requirements of the building load and to maximize system efficiency by operating the appropriate number of boilers.
The burner’s ability to fire at low loads allows more time for the flue gases to remain in contact with the heat exchanger and in return provides greater energy transfer and more precise load matching. For this reason, it is typical for condensing boilers to run multiple boilers at low loads to increase efficiency. Similarly, condensing boilers can modulate output temperature lower as heating demand decreases, whereas noncondensing boilers will always have restricted temperature limitations to avoid condensation. This all translates to the boiler’s ability to stay on at low loads and avoid cycling and all the associated losses with post-purge, pre-purge, and warming the heat exchanger back up to temperature to maximize system efficiency.
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