THE COMBUSTION process has taken a major leap forward with the introduction of smaller, easily controllable, variable-speed high-pressure blowers. Although standard atmospheric burners are still available at a lower cost, high-efficiency appliances incorporate this new blower technology, along with solid-state control logic that can monitor load parameters and outdoor weather conditions and then adjust burner power based on actual loads. This improves the overall efficiency of the appliance to the point that AFUEs approaching 94% are the norm instead of the exception.
AFUE is a subjective term. It is supposed to put all like-fired appliances on an equal footing so that the consumer has the opportunity to compare appliance efficiency on a one-to-one basis. I could spend a lot of time talking about the fallacies of AFUE, and some day maybe I will.
We contractors understand one thing: Simple thermal efficiency, which is similar to steady state efficiency. The higher the thermal efficiency, the lower the cost of operation to the end user.
Along with the leap forward of controlled, sealed combustion brought about by the introduction of the variable-speed fan and pre-mix gas technology, burner technology has also taken a major step forward. The burners used on many of these appliances today are a mixture of ceramic, stainless steel and some space-age cloth materials made of a fiberglass type of material. That old picture of the three-section, tear-drop-shaped flame is a thing of the past.
Today’s high efficiency burners have more of a glow than an open flame. They resemble a catalytic burner more than anything else.
In the process of operating at relatively low operating temperatures, the appliance is subjected to the potential of flue gas condensation production. Only a few metal compounds have a high resistance to the corrosive tendencies of the acidic condensation. Stainless steel and certain other aluminum alloys have this characteristic.
Some of the earlier designs of high-efficiency pulse boilers used thick cast iron for their combustion chamber material. Some boiler designs have the burner placed above the heat exchanger to keep the condensation from dripping directly onto the combustion process. Others have a circular heat exchanger that is designed such that condensing takes place outside of the combustion zone, thereby avoiding fouling of the combustion process. One design actually sprays a fluid directly into the flame, thereby extracting as much energy as is possible.
In every case, the condensate produced is acidic and needs to be neutralized prior to discharge into the sanitary sewage disposal system. Follow the manufacturer’s recommendations as they pertain to condensate handling.
As of the writing of this article, 362 individual condensing boilers are listed at the www.sedbuk.com Web site for condensing boilers that are rated and approved for use in the United Kingdom. I’m sure this is a partial list because it doesn’t include the dozen or so U.S. companies that currently have condensing technology on the market.
The market for condensing boilers is about 20 years young in Europe. That is partially due to the fact that many countries in Europe are participants of the Kyoto Protocol, and, by decree, they must reduce their air pollution discharges by a significant amount. It is obvious that they work quite well.
If you have not yet taken the initiative to research this technology, you would be wise to align yourself with a good, reputable manufacturer and do so. If you think this is a passing fad, think again. It’s being considered as the “standard” in the European market. And, what usually happens over there first will eventually work its way over here. You’ll be playing catch up with the technology. It is here to stay.
I have personally witnessed actual, working units in the field that were pushing 100% thermal efficiency. Combine their thermal efficiency with the efficiency of waste heat recovery and solar DHW and you’ll have a very efficient system. Guaranteed.
Over the next few columns, we will look at futuristic systems such as waste heat recovery, ground source heat pumps, hydrogen fuel cell generators, solar and co-generation systems. These futuristic systems are here today!
Until then, Happy Extremely Efficient Hydronicing!
Mark Eatherton is a Denver-based hydronics contractor. He can be reached via e-mail at [email protected] or by phone at 303/778-7772.