To recap quickly, water vapor is a by-product of the combustion process. By condensing this water vapor into a liquid form and utilizing the energy released, condensing boilers can operate 10% more efficiently than non-condensing boilers.
However, maximizing the benefit from a condensing boiler requires that return water temperatures be as low as possible (as we'll explain below), so they are better suited for use in some types of systems than others.
Using a Heat Exchanger
As their name implies, condensing boilers utilize the energy from condensing water vapor to increase their efficiency. For condensing to occur, the combustion gasses must be cooled below the boiling point of water, 212 °F. This cooling is accomplished by passing the gasses through a heat exchanger, where the water used for space heating grabs heat from the hotter combustion gasses.
By lowering the temperature of this water, more heat can be taken from the combustion gasses. This means the combustion gasses are cooled further, allowing more water vapor to condense and efficiency to increase. If the water inside the heat exchanger is too hot, the condensing process will be minimal or in some cases not take place at all.
Simply by lowering return water temperatures from 160 °F to 110 °F, the steady-state efficiency of a condensing boiler can be increased from 90% to over 96%. From this example, it is clear that condensing boilers are ideal for applications that use low water temperatures, such as radiant floor systems. Fortunately, other types of systems can be modified or designed to work with condensing boilers as well.
Working With Remodeled Homes
In older homes, it is not uncommon for the building envelope (e.g. walls, windows, ceilings, etc.) to be improved without modifying the heating system.
When this type of improvement is made, the radiators or baseboards left in place become over-sized for the new characteristics of the space. A common solution is to cap or cut out some radiators to reduce the heat output.
A better option is to leave all the radiators in place but install a high-efficiency condensing boiler and deliver lower water temperatures to the heating system. This will have the benefits of increasing efficiency and also providing heat in an even, comfortable fashion.
New Construction or First-Time Heating Systems
If a heating system is being designed from scratch, it can usually be done in such a way as to deliver a good combination of cost, efficiency, and comfort. By selecting larger heat emitters (radiators, coils, etc.) than would be used in a high temperature system, the water temperature in the system can be reduced and the operating efficiency increased. It is important to take into account the extra cost for larger heat emitters when deciding what water temperature to use. Typically, a supply temperature of 140 °F yields a good balance of upfront cost and efficiency.
In situations where the heating system was designed to operate at high temperatures and the design conditions have not changed, the benefits of a condensing boiler will be somewhat muted. However, because heating systems are designed for the coldest day of the year, the system will still be over-sized for the majority of the heating season. This means that most days during the heating season, the water temperature can be reduced and the benefits of a condensing boiler can be utilized. It is important to get a boiler or boiler control with outdoor reset capabilities to automatically reduce water temperature on days with mild weather to capture as much of this benefit as possible.
Regardless of the age or type of your heating system, a condensing boiler will likely save you money on your heating expenses. By choosing a system that can operate with low water temperatures, you can maximize efficiency and decrease the payback period for your boiler. In older homes, a condensing boiler is a perfect complement to insulation and window improvement. In new homes, a system designed for low temperatures can provide unmatched comfort and efficiency. In situations requiring high temperatures, an outdoor reset equipped condensing boiler can maximize savings by automatically reducing temperatures during mild weather.