Boiler Installation Pitfalls Part 1: Never Condense a Non-Condensing Boiler

Some boilers are designed to condense--others are not. If you design, install or operate boiler systems, you need to understand the difference, because operating one as the other will either downgrade your efficiency or cut your boiler’s life short.

It all has to do with the process and byproducts of combustion. Whenever a hydrocarbon fuel such as methane (CH4) is ignited, the carbon and hydrogen combine with oxygen (O2) to form carbon dioxide and water vapor while giving off heat. In a boiler, this heat is transferred via a heat exchanger to the heating water. Approximately 90% of the heat is “sensible,” which means that it can be felt and will manifest temperature change on a thermometer. The other 10% is the “latent heat” of combustion.

Non-condensing boilers, which are not as efficient as condensing boilers, forfeit this 10% of heat, letting it escape through the boiler stack. Is this wasted energy? Yes--but the only way to safely and effectively use that remaining 10% to heat the boiler water is to condense the heat out of it by cooling the exhaust to below 130°F before it leaves the stack.

Any Boiler Can Condense But Not Every Boiler Should

While we refer to boilers as condensing or non-condensing, the fact is any boiler will condense if the return water temperature drops far enough below 130°F before entering the boiler heat exchanger. This creates a problem for non-condensing boilers because hydrocarbon flue gases are extremely acidic and when they are allowed to condense, the resulting fluid is extremely corrosive. Not only will it destroy the heat exchanger of a non-condensing boiler, but it can also eat a boiler from the inside out.

failed copper-fin heat exchanger due to operating boiler in condensing mode.

Non-condensing boilers simply are not designed to condense, which is why we recommend keeping the return water temperature to non-condensing boilers above 140°F. If you are operating a non-condensing boiler in a system that has a water temperature below 140°F we highly recommend that you install a cold water protection valve in the boiler water piping. This is a thermostatic valve that will blend hot water off the discharge of the boiler with the cold return water, thus elevating the return water temperature to above 140°F before it enters the boiler. (More on this in an upcoming article!)

Unlike the heat exchangers in non-condensing boilers, the heat exchangers used in condensing boilers, are made with materials that can withstand the corrosiveness of condensed flue gases. As such, they can be safely operated at condensing temperatures, delivering 92% or higher efficiency. This, combined with many other advantages (small footprint, lower emissions, etc.), explains why condensing boilers account for most new boilers being installed today.

Know Your Boiler

Condensing boilers may be commonplace these days, but non-condensing boilers are still in production and certainly many remain in operation. That’s why it is imperative that you know what type of boiler you have before setting your operational parameters. Always review the IOM for the boiler model to see if it is suitable for condensing applications.