What is a condensing boiler and why should I care?

How do condensing boilers work?

When your boiler burns natural gas, carbon dioxide and water vapor are created as a by-product of the burning, or combustion process. In total, about 10% of the fuel's energy is tied up as water vapor. In a standard non-condensing boiler, that water vapor, along with the carbon dioxide, exits through the chimney, essentially becoming wasted energy.

A condensing boiler uses not just the heat from the combustion of natural gas, but also from this otherwise unused energy. Inside a condensing boiler, the vapor is converted back into water droplets, which surround the heat exchanger, which in turn extracts the heat and converts it into additional heating energy. This water, along with acidic byproducts of the fuel combustion process, are then drained out of the boiler and into a neutralizer.

With a condensing boiler like the Viessmann Vitodens series, the heat exchanger and the home's heating design play a part in how this energy is most effectively used. Let's look at the heat exchanger first.

Condensing Essential 1:  The heat exchanger

The heat exchanger in a Viessmann Vitodens boiler is shaped like a spring, but with flat tubes coiled together. Each coil is spaced precisely 0.8 mm apart thanks to small embossed pins, which look like small dimples. The water vapor created by the natural gas burning in the boiler is condensed, collecting along the outside of the heat exchanger coils, and drips through the spaces in the coil down to a small drain.

By condensing the water vapor, the heat recovered is fed back into the heating system, reducing the heat wasted up the chimney (or out the vents), as well as reducing the fuel required to heat the water being drawn into the boiler.

It's important to note that the water vapor which condenses around the heat exchanger, and the water being pumped through the heat exchanger and into your home are completely separated. The water that condenses around the exchanger is acidic and drips into a neutralizer, preserving the cleanliness and safety of the water being pumped throughout your pipes.

Condensing Essential 2:  The full heating system

In order to achieve this condensing effect, it is critical that the temperature of the water being pushed throughout the house is lower than with a non-condensing boiler. For ideal condensation, the water should return to the condensing boiler at around 130°F (54°C), significantly lower than a non-condensing boiler system.

To achieve this, a condensing boiler heats water to a lower temperature initially (which saves fuel), then strategically cycles it through the house to utilize the heat in as many places as possible for the lowest potential return temperature.

The best way to ensure that your boiler is only heating water to the lowest temperature necessary, is to use an outdoor temperature sensor, highlighted in red in the image below. These small devices are linked to your boiler and help it increase or decrease the operating temperature to compensate for weather fluctuations. When replacing a non-condensing unit with a condensing boiler with an outdoor temperature sensor, a homeowner can save up to 30% on their heating bills annually.*