Energy Efficient Hot Water Boiler Plant Design Part 1: Combustion Process and Combustion Efficiency

By Chad Edmondson

It takes approximately 1000 Btus of energy to convert one pound of water into vapor.  Think of that as the price you pay as part of the combustion process in a typical non-condensing hydronic boiler.  That’s a good starting place for this next series on Energy Efficient Hot Water Boiler Plant Design. 

So much of boiler efficiency rests in what happens during the process of combustion – and how much energy escapes through the chimney in the form of water vapor.  If the water that is produced during the combustion remains in a gaseous state, it is lost through the flue, taking precious Btus with it. If the water vapor condenses inside the boiler, these Btus stay within the system – for better or worse depending on how the boiler is designed. 

Where Does the Water Vapor Come From?

Three major thingsare produced during the process of combustion inside a boiler:  Water vapor (H20), Carbon Dioxide (CO2), and Heat.  This is what occurs when fuel and air are ignited inside a boiler.  About 90% of the energy produced by natural gas-fired boilers is in the form of sensible heat and about 10% is latent heat.  The latter is contained within the water vapor that is produced during combustion. 

If the boiler is designed for non-condensing operation, and the system return water temperature is sufficiently high (above 130°F), the water that is produced during combustion will remain in the form of gas and is ultimately vented from the boiler.  With it goes about 10% of the energy you have put into the system.  This is the “downside” of non-condensing boilers – the physics that keeps them from ever achieving more than about 83 - 87% efficiency.  (Note: More modern design non-condensing boilers utilize fans and smaller flues, which increases efficiencies up to about 87%). 

Of course, any boiler can condense if supply water temperatures drop below a certain point.  But if the boiler is not designed with materials that can withstand the effects of the carbonic acid that is produced when water and CO2 mix, it won’t be in service very long.

Understanding Condensing Boiler Efficiency

Non-condensing boilers once dominated the marketplace and thousands are currently in service today. Operators are careful to keep system return water temperatures above 130°F to avoid “rain in the boiler”.  However, for reasons of efficiency, more and more condensing boilers are being selected for replacement and new construction.  At JMP, we estimate that as much as 90% of new boilers purchased in our market are condensing. 

Although condensing boilers are more expensive, the amount energy they save makes them a financial no-brainer for most applications.  Remember -- a 1-million Btu/hr condensing boiler produces approximately 93 pounds of water per million Btus.  Since each pound of water contains about 1000 Btus of energy that equates to 93,000 Btus.  If this water were to remaining in a gaseous state as it presumably would in a non-condensing boiler, all that energy would be lost up the chimney.   If the water condenses, than that energy remains within the system in the form of latent heat.  Therein lies the increasing popularity of condensing boilers! 

Click here to watch our YouTube video on Combustion Basics!

Download this document on Condensing & Non-Condensing Boilers/Water Heaters.  This is a simple 2-Page "cheatsheet" that covers important topics like condensation, temperatures, venting, piping, and boiler materials.

Next up, we’ll take a more in-depth look at condensing boiler technology.