Optimizing hot water systems with condensing boilers
Selecting a condensing boiler has the potential for high efficiency gains, but understanding special considerations for these boilers could cost you if you design the way you have in the past.
- Understand the differences between condensing and noncondensing boilers
- Understand the cost implications of boiler initial investment costs for various boiler configurations
- Learn top tips for designing a hot water system with condensing boilers.
Hydronic hot water heating systems circulate hot water throughout a building to heat its air and come in varying shapes, sizes, and configurations. Boiler selection
and traditional hot water systems have been designed to maintain high hot water temperatures, but changes in boiler technology allow efficiency gains to be achieved by using lower water temperatures and condensing boilers.
Traditional boiler system designs consist of some way to heat water from ambient conditions to a temperature suitable for conditioning a building’s air. In these older, conventional systems, the standard design was to maintain hot water supply temperatures upwards of 180 to 200 F with hot water return always above 140 F to prevent condensing in the heat exchangers. Similarly, conventional, noncondensing boilers were designed so that variable flow through the heat exchanger was unacceptable. Consequently, primary-secondary pumping configurations were used to maintain the desired flow through the boiler heat exchanger while varying the flow in the secondary loop based on the building’s demand.
Today, boiler system design has completely shifted. Hot water supply temperatures are decreasing, and condensing boilers are great choices for systems that use lower hot water temperatures as efficiency is increased. Condensing boiler systems are good for systems that include in-floor radiant heating systems, water source heat pumps, and standard hot water systems specifically designed with lower hot water supply temperatures.
Combustion used for efficiency gain
Most boilers use natural gas as the main fuel to heat water, although other options are available. Using natural gas, the process of combustion occurs, which is a chemical reaction when natural gas and combustion air are combined, producing heat that is used to increase water temperature:
CH4 + 2 O2 → CO2 + 2 H2O
In this process, the by-products of water and carbon dioxide are produced as well as NO, NO2, and NO3 (nitrous oxides, NOx) as nitrogen in the air is combined with the excess air. This process is common for all boilers, and the by-product of water is the key ingredient in boiler efficiency, which is treated differently depending on the type of boiler.
In a noncondensing boiler, the water remains in a vapor state (steam) and is removed from the boiler via the flue gases exiting the building. In a condensing boiler, the steam is allowed to condense and turn into liquid as the water is cooled below its dew point, recovering the latent heat of vaporization, expelling approximately 1,000 Btu/pound of water. This subtle form of energy recovery allows the latent heat to be converted to efficiency gains in lieu of wasting the energy out of the building. The dewpoint of the water vapor in the flue gases depends on the percentage of hydrogen in the natural gas and the excess air in the flue gases but begins to condense when the hot water return temperatures are between 130 and 140 F, as indicated in ASHRAE’s Handbook of HVAC Systems and Equipment. Although this article primarily discusses natural gas, it is important to note that some codes may require dual fuels for boilers. At this time, only one boiler manufacturer provides a condensing boiler that can operate with dual fuels, which may limit the use of condensing boilers in some applications.
Heat exchangers for noncondensing boilers are typically constructed of copper, cast iron, or steel and are not designed to handle the corrosive condensate produced by water vapor mixing with the CO2 creating carbonic acid. Over time, the acid in the condensate will destroy the metal of the heat exchanger. Because of this, condensing boilers require more robust heat exchangers to tolerate the acidic condensate and thermal shock from reduced hot water return temperatures. Heat exchangers are produced from a few different metals and configurations, depending on the extent of condensing allowed within the boiler.
A full condensing boiler uses one heat exchanger produced from either stainless steel or cast aluminum. Aluminum heat exchangers are usually cheaper and use thicker metals, but are more sensitive to water conditions such as pH, alkalinity, and chemicals, whereas stainless steel heat exchangers are very resistant to corrosion and much more forgiving to various water conditions. Both materials are specifically designed withstand the effects of condensate and built for years of operation. Note that the pH of the water in the closed loop system will remain the same as that in a noncondensing boiler. The optimum pH in the condensate is 3 to 4 in condensing boilers.
A secondary alternative, the partial condensing boiler, uses primary and secondary heat exchanger surfaces where the primary heat exchanger will never see condensing temperatures and always operates in a noncondensing temperature range. The dual heat exchangers allow the primary heat exchanger to be made of standard construction, typically copper, whereas the secondary heat exchanger is made of the more robust material such as aluminum or stainless steel. When the flue gases exit the primary heat exchanger, they are directed to the secondary heat exchanger where condensing of the flue gases is allowed to occur. This type of boiler heat exchanger configuration typically requires an internal pump and/or mixing valves to protect the primary heat exchanger for operation within a safe temperature range as shown in Figure 1.
Another alternative is a system with both condensing and noncondensing boilers operating together as a hybrid system. In these systems, the designer must consider which boiler is operating to protect the heat exchangers as discussed above. Generally, condensing boilers are initiated as the first boilers that would operate on the loop to attempt to maximize efficiency.
However, there are times of year when using condensing boilers may provide minimal efficiency gain compared to using noncondensing boilers depending on how the system is designed and the water temperatures used on the project. By operating the condensing and noncondensing boilers in a lead-lag sequence based on outdoor air temperature and hot water temperatures, a hybrid system provides the combined benefit of operating at maximum system efficiency using condensing boilers while significantly reducing the initial investment of an all-condensing boiler plant.
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.
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.