4-1-13
Two kinds of efficiency are of concern for boilers. The first is combustion efficiency. If the condensing boiler is properly sized, tuned and loaded, it will achieve 86% combustion efficiency regardless of the operating temperature. This is the very best any non-condensing boiler can achieve. If the condensing boiler is properly loaded and controlled by a properly programmed out-door-reset function, the combustion efficiency will rise with a drop in return water temperature.
Thermal efficiency reflects how well a particular boiler transfers heat from fire to water--shown in Dana'a graph. Though it is generally true, it is not absolute. When building condensing boilers, we found the smaller the heat exchanger surface (among other factors) in relation to the fired output, the more likely thermal efficiency will suffer. The attached combustion efficiency tests (not the same thing) shows the combustion test results of a recent IBC boiler we installed in Minneapolis and a Buderus "mid"-efficiency atmospheric, non-condensing boiler installed in an Eastern suburb (Woodbury). Both boilers tests were were recorded with the boilers at high-fire and steady state.
Combustion efficiency has more to do with a properly tuned burner and the complete conversion of fuel to heat and water vapor, including the recovery of that water vapor in a secondary heat exchanger. (The lower the return water temperature, the lower the stack temperature). This is why we use condensing boiler for almost all of our driveway snow/ice melting designs, which are normally at full fire for hours and
always measure 94% combustion efficiency or higher.
To sort it all out one need only measure the stack temperature of a condensing boiler in the CH or space heating mode. A typical gas boiler will fire with a steady state stack temperature from 350-500° Fahrenheit. Note the 250°F stack temperature of the "mid"-efficiency Buderus boiler
and the 100°F stack temperature of the condensing IBC boiler
.
What we can do about it? Well first, we have to tune the boiler on initial start-up to make sure we are burning gas cleanly. This can only be done by a trained technician with a calibrated combustion analyser. Secondly, we design heat emitters (radiators, radiant floors, wall and ceilings) to operate at a maximum 140°F supply water temperature and assuring return temperatures will help the boiler recover nearly 100% of the available heat converted in combustion.
As people throw "efficiency" around, I just want to measure the stack temperature, since what goes up the stack is money lost.
Morgan M. Audetat
www.BadgerBoilerService.com