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.



Learning objectives

  1. Understand the differences between condensing and noncondensing boilers
  2. Understand the cost implications of boiler initial investment costs for various boiler configurations
  3. 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 H2

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 

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.

Figure 1: Three 3,000 MBH condensing boilers have galvanized sheet metal intakes and double wall stainless steel boiler flue vents. Source: Ring & DuChateau LLPA 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.

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