Understanding True Operating Efficiency of Condensing Boilers

As a late follow-up to my initial post Can Boilers Really Achieve 98% Efficiency?on March 29th 2016 I attended an event, that 32BJ hosted in NYC, where condensing boilers were discussed. There were multiple speakers but Asit Patel, President at ANP Energy Consulting Services Corp., focused on understanding true operating efficiency of a condensing boiler.

I must commend him on his presentation, in a room filled with over 100 people Asit brought the conversation back to fundamental principles of heat transferring from high to low temperatures and core concepts of sensible and latent heat transfer to understand BTU’s and energy content. With an audience of all educational levels on this topic, I believe everyone in the audience followed and kept up with the presentation.

In his time allotted, he discussed the misconception of the marketing material most condensing boilers state. It’s sometimes misleading that their boilers can achieve efficiencies of up to 95 - 98%. These high efficiencies can be achieved, but only under certain circumstances. Unfortunately many in our industry take higher efficiency number and apply in their savings calculation. Don’t get me wrong; Are condensing boilers much cleaner and efficient than the old time #6 oil burning hunk of cast iron and steel in the basement of old buildings?… Absolutely! However a better understanding of what actually happens in a condensing boiler is necessary.

What condenses in condensing boilers is the water vapor in exhaust gases which otherwise goes up the chimney and into the atmosphere. Water vapor is one of the by-products of the combustion process. Water vapor has heat content of 970 btu/lb of vapor. When we condense water vapor (gas) back into water (liquid) latent heat (970 but/lb) is released. Condensing boilers’ are designed to be able to extract and reuse this heat that otherwise would have gone up the chimney. The fact that they are designed to do this without compromising the heat exchanger is what makes them more efficient than a conventional boiler.

The heat exchanger of conventional boilers, made of cast iron or steel, cannot withstand the acidic solution that results from flue gas condensation. Heat exchangers of condensing boilers are designed to drain the resulting condensate through them and are made from materials (stainless steel, cast aluminum, and even thicker cast iron) that can withstand acidic solutions (note that drains aren’t, but should always have a neutralizer). In order to be able to condense the water vapor in flue gases, the exhaust temperatures have to be below the dew point which is ~ 132F.

True operating efficiency of the condensing boiler depends on return water temperature and firing rate. If we look at the graph above, efficiencies of over 95% can be achieved if the water entering the heat exchanger is below 80F.

When Asit brought up questions such as:

  • Is my system capable of providing adequate service with entering water temperature of 70F?
  • How often is my system going to operate in that mode – all the time or fraction of the time?

The answers to these questions made the room realize that efficiencies depend on the time of year and additional parameters that would allow the manufactured stated numbers to be realistic. To list a few of the many considerations: firing rate, jacket losses, flue gas temperature and cycling are some of what we need to know.

Creative examples were presented on how higher efficiencies were achieved by carefully engineering and modifying the piping. As he explained, the goal was to get the boilers to condense! In addition to strategically piping the system to dictate the return water temperature, we must take other factors into consideration that assist with condensing such as: oxygen levels in flue gas, utilizing a VFD if necessary and/or optimizing the control settings.

What was great about the presentation is that we were all left with the understanding and emphasis that anything is possible if properly engineered, highly stated efficiencies are dependent on specific conditions, and when determining savings a realistic approach must be taken into consideration.