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Thread: Efficiency of Modulating Boilers

  1. #1
    DIY Junior Member calebgerdes's Avatar
    Join Date
    Mar 2013

    Question Efficiency of Modulating Boilers

    Hey all,

    Need to figure out if there is any efficiency loss when a boiler modulates up or down. I believe, from just simple logic that the efficiency changes, if there are any, would be minimal. Any thoughts?


  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    New England


    It's not about the modulation, it's about matching the size of the boiler properly to the load! Some efficiency comes from the condensation - there's a huge about of heat in the heat of vaporization, and it's reversible when it condenses. So, what the modulating burner allows you to do is to better match the load, and if properly setup, you'll get condensation more frequently than with a larger burner that can only turn on or off. A properly setup burner has a burn efficiency about the same, regardless of its output.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    In the trades Dana's Avatar
    Join Date
    Jan 2009


    At the same return water temperature entering the boiler it will generally run more efficiently at the low-fire than at high fire. When the return water temps are in the condensing range that difference can be appreciable, but oversizing to always run at low fire results in short-cycling, which is even lossier.

  4. #4
    Master Hot Water Mpls,MN BadgerBoilerMN's Avatar
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    Mar 2011


    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 boilerName:  BadgerBoiler 86% Efficiency Buderus.jpg
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Size:  32.2 KB and the 100°F stack temperature of the condensing IBC boilerName:  BadgerBoiler IBC 96% combustion efficiency 2013.jpg
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    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

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