There’s been a steady improvement in the efficiency of hot water heaters over the years for both tankless direct vent water heaters and conventional water heaters. This increase in efficiency is largely due to the fact that the design of these new high efficiency water heaters is what’s known as a condensing design. And if you’ve been doing any research into hot water heaters, you’ve likely run across this term. Like most people you’re probably wondering just what condensing means relative to a hot water heater. Well wonder no more, the magic is about to be revealed!
Combustion is by definition, the oxidation of a fuel. What this means in simple terms is that oxygen is combined with a fuel source in a controlled environment to produce an exothermic reaction. An exothermic reaction, in case your wondering, is a process that releases heat. Along with producing heat, the combustion process, at least in ideal situations, also produces water vapor and co2 as byproducts.
So to recap, a fuel such as natural gas or propane is combusted or burned in the the presence of oxygen. This reaction then releases heat which is transferred to the water within the heater, and water vapor (h20) and carbon dioxide (CO2) as byproducts of combustion. And the end result is that you now have hot water for use in your home. Simple, and ironically a lot like rocket science.
The older, less efficient style of water heater was not a condensing unit. Fuel was burned, heat was released, and some hot water was made. Unfortunately, a large amount of the heat produced during the combustion process was wasted and sent up and out the chimney in the form of hot gas and water vapor.
In order to increase efficiency and lower the amount of heat that was wasted up the chimney, engineers designed the new condensing style of water heater so that more heat is removed from the exhaust gases. So much more heat in fact, that the water vapor that is a byproduct of combustion is cooled to a point where it condenses back to a liquid before it leaves the chimmey.
As you can probably guess, since it takes a fairly large amount of heat to produce water vapor, there’s a fairly substantial amount of extra heat that can be removed from the exhaust gases and used to heat water. The effect on the efficiency is quite large, and condensing hot water heaters can have thermal efficiency values of well over 90%. This is the reason that condensing hot water heaters are so much cheaper to operate, and why you’ll save a lot of money when you install one.
And it doesn’t matter whether it’s a conventional tank style or a tankless direct vent water hear, the savings are similar for any type of condensing unit.