Condensing Gas Boilers

What they are and when to use them

Why condensing boilers?

The natural gas burned in your buildings contains a small amount of water vapor. This water vapor absorbs heat during combustion and carries that same heat up the chimney into the atmosphere. To re-absorb that heat, the flue gases must be cooled enough to condense back to water and release the heat inside the boiler before entering the chimney. All boilers are designed to burn fuel, then absorb the heat from combustion.

When is it practical to use a condensing boiler?

Condensing boilers are traditionally suited for low temperature systems such as in-floor heating, snow melting and domestic hot water service. Condensing boilers also work in buildings with cast iron and copper baseboard radiators, provided there is enough square footage of radiator to heat the building at the lower more efficient temperatures. It is common in Victoria to need only high radiator temperatures for several weeks in winter while during most of the heating season the buildings can be satisfied at or near condensing temperatures. Applying a condensing boiler to an existing high temperature building is possible if the conditions are correct. Low temperatures are important for keeping the boiler cool enough to condense the flue gases and recover that heat before it enters the chimney. Operating at lower temperatures burns less gas and when in the condensing range can recover 8% of the heat trapped in the water vapor.

Why not just run a conventional boiler cooler?

Low temperatures can actually destroy a conventional high temperature boiler through thermal shock and condensation corrosion. The condensation is acidic with a pH value of approximately 3.5. This is near the acid level of red wine. These small levels of acid combined with low water temperatures will literally eat a conventional cast iron or copper boiler. To achieve low system temperatures with conventional boilers requires the installation of 3-way or 4-way mixing valves. These valves allow the systems to run at cooler energy-saving levels, with the conventional boilers operating at their designed high temperatures.

Designed for low temperatures

The condensing boilers are constructed of stainless steel and use stainless steel or plastic chimneys. The mildly acidic condensation runs through a limestone filter to neutralize it before it enters the city sewer. These boilers generally hold less water than a standard boiler and usually operate with fully modulatinggas burners. The fully modulating burner allows the boiler to more closely match the rate of combustion with the building heat load. This helps create longer run times at lower inputs to help keep the condensation process going. The combination of low temperatures and modulating burners can show combustion efficiencies of 96%. This is generally 10% higher than good quality well controlled conventional boilers. The extra 10% in savings comes at capital cost for equipment that can be 25% to 50% higher. Application and payback are serious considerations for a condensing boiler. When these conditions are right it is well worth using this technology.