Commercial boilers generate heat through the combustion of fuel and the transfer of thermal energy to water. The heated water is then distributed throughout a commercial building for use in HVAC, processing, manufacturing and other applications. In North America, natural gas is the most common fuel type for commercial boiler systems, and the hot water they produce is frequently used for radiant or forced-air space heating.
Understanding the difference between conventional boilers and condensing gas boilers is imperative for optimizing building efficiency, thermal comfort and HVAC equipment selection.
Conventional boilers (figure 1) burn gas to heat water to a high supply temperature (160°F to 180°F). The exhaust from this combustion process has a remarkably high temperature (greater than 300°F) and needs to be exhausted out of the building via a chimney or flue. The high-temperature exhaust gas illustrates the challenge with conventional boilers in modern commercial buildings: there is significant thermal energy losses and, therefore, a limitation to the boiler’s efficiency of approximately 80%.
| Figure 1. A conventional boiler |
Condensing boilers (figure 2) utilize hot exhaust gas to transfer additional energy to the water being heated. This occurs via both sensible and latent heat transfer – the hot exhaust gas cools to a point where its vapor condenses, leading to an accumulation of acidic liquid condensate that must be drained and managed correctly. Compared to conventional boilers, the by-product of this is exhaust gas of a much lower temperature (around 100°F), less thermal energy losses and an increase in heat transfer and boiler efficiency (up to 98%). The boiler heat exchanger is also constructed using noncorrosive materials, such as stainless steel, to manage this application.
| Figure 2. A condensing boiler |
A significant consideration for boilers and their operation is the temperature of the building water being returned to the boiler – most notably, the temperature of the water compared to the dew point of the exhaust gas, which begins below 130°F. Condensing boilers should be below this temperature to leverage latent heat transfer, whereas conventional boilers must be above this temperature to avoid latent heat transfer.
Because of this, conventional boilers are traditionally designed for a 20°F temperature difference, or delta T, at design day (180°F supply, 160°F return). This ensures that acidic condensate liquid doesn’t accumulate at full- or part-load operation, preventing the boiler from becoming damaged over time.
Conversely, condensing boilers perform optimally when the temperature difference between the supply and return water is maximized. The closer the return water gets to 60°F, the higher the boiler efficiency becomes (figure 3). Because of this, condensing boilers are often seen operating at larger water-system temperature differences of 30°F to 50°F and are installed with lower design-day supply-water temperatures of 120°F to 140°F.
| Figure 3. The relationship between boiler return-water temperature and boiler efficiency that comes from additional latent heat transfer |
More buildings are using condensing gas boilers to improve their energy efficiency and variable energy costs. As this trend continues in the HVAC industry, it is essential that equipment manufacturers develop products that can effectively heat with lower-temperature water while also creating higher waterside temperature differences.
Key takeaways:
| Conventional Boilers | Condensing Boilers |
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If you’re looking for more ways to improve the efficiency of the HVAC systems in your projects, reach out to our Air Moving team at airmovement@priceindustries.com.