Flue Gas Condensation Could Be Killing Your Boiler

Flue gas condensation can occur in any hot water boiler. The causes of this condensation and the importance of properly mitigating it are easily misunderstood. Flue gas condensation is also part of the reason why condensing boilers can have efficiencies up to 99% whereas a typical non-condensing boiler will max out around 80–85% efficiency. However, many a boiler have been sent to the scrap yard due to failure from corrosion of flue gas condensation. Let’s dig deeper into how condensation can cause metal failure in a boiler heat exchanger.

A pit and staining on concrete where a stack was allowed to leak.

How Condensate Forms

Flue gas will form and condense on a heat exchanger and in the boiler stack when it falls below its dew point, and this occurs when the return water temps of the system are around 135°F. The vapor of the flue gas undergoes a phase change into a liquid which is now acidic with a pH typically between 2–5. The boiler now needs to be able to manage this condensate by draining it from the bottom of the heat exchanger and out to the drain in the boiler room. Since the condensate is acidic, it will corrode floor drains and concrete unless it is treated before being sent to drain. In some cases, neutralization treatment is required by code. To neutralize condensate, it must flow through a neutralizer such as magnesium oxide that will raise the pH of the condensate before it reaches the drain. 

Since condensate also forms in the stack, the stack must be made of an approved material such as stainless steel or CPVC, and it’s important that this condensate gets neutralized and drained as well. The stack drain should tie into the boiler condensate drain piping before the neutralizer, ensuring that there is a water trap to prevent flue gases from being leaked into the room. If the stack is not of the proper material or is not sealed correctly, it will leak and corrode whatever the condensate touches. If the leak is left long enough, concrete will stain and actually form pits due to the acidity of the condensate. 

Condensing vs. Non-Condensing Boilers

Manufacturers design condensing boilers to handle the acidic nature of condensate by building the heat exchangers out corrosion-resistant material such as stainless steel. As an example, Fulton uses duplex stainless steel in all of their condensing boilers. Duplex stainless steel has some of the highest tolerances to corrosion and thermal shock, making it an excellent material to handle the condensate loads of a condensing boiler. A traditional boiler that is non-condensing has a heat exchanger constructed of copper, carbon steel, and/or cast iron, which are not resistant to corrosion. Because these boilers are not designed to handle sustained condensate loads, if the system return water temps are below 140°F, the flue gas condensate will eventually wear down the metal, causing a leak in the heat exchanger. For this reason, it’s important to identify what kind of boiler is installed in a facility and make sure that the system’s temperatures are within the manufacturer’s design specifications. 

A typical condensate trap and neutralizer kit.
Testing condensate pH leaving a neutralizer that needs to be refreshed.

Flue gas condensation is a good thing when going for record efficiencies in a hydronic system, but the boiler and stack must be designed to handle the condensate. If either are incorrectly selected for the application or are not installed properly, flue gas condensation will be the silent killer. It will eat the boiler from the inside out, which will surprise most facility managers whose budget doesn’t include replacing a boiler that is only a couple years old. 

The Best Defense

The best defense against flue gas condensation is having a qualified boiler company inspect your hydronic system and maintain the boilers per the manufacturer’s instructions. This will keep your facility energy cost low through total system efficiency of running condensing temperature in the hydronic loop.

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