Practical Strategies Condensing Boiler Retrofits

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Practical Strategies For Condensing Boiler Retrofits

Three retrofit scenarios and a host of design and selection tips point to the key considerations before your project heats up.

As buildings age, so do the major central equipment within the building, making it a prime opportunity for a boiler system retrofit. Older systems typically are made up of non-condensing boilers that operate in the 200ºF to 180°F range with 160ºF to 140°F returns. As boiler designs have advanced to include condensing technology, it is important to understand how to implement condensing boilers into a system to maximize operating efficiencies.

There are many types of hydronic boilers available today, both non-condensing and condensing. Non-condensing boilers are designed to operate with design return water temperatures above 140°F to prevent condensing of the moisture in the flue gases. This is because they are constructed of materials such as carbon steel, cast iron or copper, which are unable to withstand the corrosive condensate produced with condensing.

Non-condensing boilers generally operate with efficiencies less than 87%, with many well below this level. They can also have system piping limitations, both to maintain the minimum return water temperature as well as delta T or minimum flow requirements.

While condensing boilers are more efficient than non-condensing boilers, the latter generally have higher temperature limits, often with higher design-pressure capabilities. In addition, non-condensing boilers are available in larger capacities and often include the capability for a variety of back-up fuels, and their initial equipment cost is often less. 

Condensing Boiler Design  

Understanding the basics of how condensing boilers are designed and how they should be controlled are the first steps in applying condensing boilers to retrofit applications. 

Condensing boilers can achieve efficiencies that exceed 90% because they are designed to operate with cold return water temperatures, which promote condensing of the moisture in the flue gas. In a condensing boiler, the return water must be brought back to the boiler below 130°F — coinciding with the flue gas dew point for natural gas — and the colder the better to maximize condensing. The flue gas dew point is defined as the temperature at which the water vapor present in the flue gas reaches saturation and begins condensing. 

Next, the burner must be able to maintain a consistent fuel-air ratio throughout the modulation range of the boiler. This is key for condensing boilers because an increase in excess air leads to a decrease in the flue gas dew point, meaning the return water must be brought back to the boiler even cooler to allow the moisture in the flue gas to condense. The phase change from water vapor to a liquid state releases the latent energy that is to be transferred to the process, which in this case is heating water. The latent heat of vaporization is recovered within the boiler thereby increasing system efficiency. 

Finally, the boiler must have an effective heat exchanger design incorporating counter-flow heat exchange to put the coolest flue gases surfaces close to the coolest return water and also have a large amount of fireside heating surface for condensing. The condensate is produced on cold, fireside heating surfaces — the more heating surface that is available, the greater the potential for condensing. 

There are two key items that will maximize system performance in a condensing boiler system: a condensing boiler’s inverse efficiency characteristic, and maintaining cold return water temperatures.Because condensing boilers are more efficient at lower firing rates, it is advantageous to run multiple boilers at lower firing rates to improve overall system efficiency. Since the return water temperature directly impacts a boiler’s efficiency, maintaining the design delta T and keeping the return water temperature as cool as possible are important points to remember when designing a condensing boiler system and control strategy.

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