What is a Condensing Boiler?

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  • Written By: M.R. Anglin
  • Edited By: Kristen Osborne
  • Last Modified Date: 16 December 2019
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A condensing boiler is an efficient type of boiler that has the ability to use the latent heat of waste gases, especially water vapor, to do more work. Using this energy allows some condensing boilers to reach an energy efficiency level of about 90% or more, compared to about 60-75% for regular boilers. In order to capture this energy, the boiler has to have extra heat exchangers, devices that extract heat from one fluid and transfer it to another separate fluid. Extracting that heat causes water vapor to condense into liquid water, which then has to be drained away. A person can often tell when a condensing boiler is in use by the plastic cap on the flue and a visible white plume that emits from the chimney.

Many boilers work by boiling water and sending steam through the radiators to heat a building. Hot water can also be stored in a tank. When the boiler cycles on fuel, such as oil, coal, or gas, the fuel is burned in order to convert water into steam. Excess gases, such as water vapor, can drift up and escape out the flue. Instead of allowing the heat energy contained in the gases to escape, a condensing boiler uses this heat to do more work.


In order to capture that heat energy, a condensing boiler uses extra heat exchangers to extract heat from the waste gases. The latent heat from the gases is then used to help heat up the water in the boiler. When heat is extracted from the gases, water vapor condenses into liquid water. Since the gases become cooler and heavier during this process, a condensing boiler may require fans to remove waste gases and dispose of them up the flue. The boiler may also need a drain in order to remove the acidic condensate — the water that has condensed during the process.

There are typically three types of condensing boilers: combination boilers, system boilers, and conventional boilers. Each of these has a slightly different design, but all usually share higher efficiency than other boilers. Combination boilers do not have a hot water storage tank or cylinders, and so they require less space. Water for this boiler is heated on demand. In contrast, both system boilers and conventional boilers possess a water storage tank.

A system boiler has many of its components, such as hot water storage tanks, housed inside the unit. As such, there is no need for excess storage space. Regular condensing boilers are built in the same design as non-condensing boilers, but still have increased efficiency. Each of these boilers has advantages and disadvantages, and each is best suited to different circumstances. A professional plumber is able to give advice on which boiler is best suited for a person’s situation and energy efficiency needs.


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