Condensing Boilers: Efficient Boilers for Radiant Heating

Condensing boilers are the most efficient boilers available, and require special attention during installation.

The vast majority of radiant heating systems use a condensing boiler as the main heatsource.  A condensing boiler is any boiler that is over 86% efficient. The truth is that most condensing boilers are over 90% efficient.

Savio Condensing Boiler

Savio Condensing Boiler

In order to understand the condensing boiler, we first have to understand the non-condensing boiler, which is less than 86% efficient. In a non-condensing boiler, the exhaust gasses leave the boiler at about 180 degrees. These exhaust gasses are immediately expelled outside through the use of vent pipe or chimney. The reason that these types of boilers are less than 86% efficient is that there is significant heat loss through exhaust.

Non-condensing boilers are less expensive because they are less efficient.

A condensing boiler, on the other hand, incorporates a secondary heat exchanger to remove excess heat from the exhaust gasses. This improves the efficiency of the boiler due to the fact that the heat lost from the exhaust gasses is then “recycled” by the boiler — helping to raise the water temperature. This process actually reduces the temperature of the exhaust significantly. This significant reduction in temperature, from about 180 degrees down to about 55 degrees, condenses the gasses, most of which is water vapor, forming droplets that collect and need to be drained.

This is where the condensing part of a condensing boiler comes in.


The main issue with a condensing boiler is that the vapor is comprised of water and other elements, such as sulfur and nitrogen, which make the condensate slightly acidic. The ph of the condensate is between 3 and 4, which poses installation challenges. First, the acid content of the condensate affects the type of material you can use to vent the unit. Due to the lower temperature and acid content using a chimney is out of the question.

Installation of Condensing Boilers

The water vapor and condensate will deteriorate the integrity of the brick and mortar holding the chimney together. The most popular material used for venting is stainless steel or PVC. These materials are resistant to the corrosive nature of the condensate. Also, when installing the vent piping, it is important to ensure the horizontal sections of the pipe are at a 1.5-degree (0.3 in per ft) slope away from the boiler. This will ensure that no condensate will make its way back inside the boiler. Not installing the vent pipe correctly will allow the condensation to re-enter the boiler, eventually damaging the main heat exchanger.

Don´t Forget the Condensate Drain

Another important aspect of installation that you will not find in a non-condensing boiler is the need for a condensate drain. This is essential to removing the acidic condensate away from the boiler. The drain can flow through limestone chips, as this will alter the ph so that the condensate can travel through municipal water systems or it will need to be pumped to a suitable drain location.

Another essential part of the vent configuration is a fan to help expel the vent gasses. This is due to the fact that the cooler exhaust is denser than hotter exhaust and it needs assistance to exit the home. There are multiple benefits to incorporating a condensing boiler in your home. The increased efficiency will help reduce fuel cost. There is no need for a chimney for venting purposes, and most condensing boilers can be vented in inexpensive PVC. However, due to the complexity of installation not every installer is well versed on this equipment. It is important to use an installer familiar with condensing boilers to avoid any issues in installation.

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