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Thread: Disappointing efficiency of alpine condensing boiler?

  1. #1
    DIY Junior Member richard_b's Avatar
    Join Date
    Feb 2013

    Default Disappointing efficiency of alpine condensing boiler?

    Hi. I recently had an ALP150 (AFUE 96) installed, air intake and exhaust piped up the chimney. They did flue gas analysis and the combustion efficiency of 87% seemed rather disappointing. Here are the numbers, from a Wohler A 400

    Cal date 9/21/2012

    Low fire
    TS.....148.4F (stack temp)
    TA.....64.7F (air temp)
    EA....25% (excess air)
    SL.....12.1% (stack loss)
    Dpt....131.1F (dew point)

    High Fire
    TS.....163.4F (stack temp)
    TA.....64.7F (air temp)
    EA....39% (excess air)
    SL.....12.6% (stack loss)
    Dpt....127.9F (dew point)

    Is this the expected efficiency on a condensing boiler w/ AFUE 96? Anything that can be tweaked based on these numbers to improve efficiency? Thanks in advance for your input.

  2. #2


    Can you give us more details? I have the same unit and I've spent a lot of time tweaking it.

    How many zones?
    What type of radiant heat?
    How many feet of rad on each zone?
    Do you have an indirect tank? What size?
    Details from your current config on the 'Adjust' menu?

  3. #3
    DIY Member Gordan's Avatar
    Join Date
    Feb 2013
    SE PA


    Your stack temperatures indicate that this boiler is far from running in condensing mode. A condensing boiler will only condense with A) return water temperature of below 130F (lower is better), and B) adequate flow through the heat exchanger.

    Having said that, the efficiency calculations on combustion analyzers do not take condensation into account and should not be relied upon to give you the true efficiency of the appliance.

  4. #4
    In the trades Dana's Avatar
    Join Date
    Jan 2009


    If the return water entering the boiler from the system was over 140F during the test (which seems likely, given the dew point numbers on the exhaust product) 87% would be about what you could expect. If it was getting 90-100F water back from a radiant slab you'd see much better combustion efficiency.

    AFUE tests are run with 120F return water, at a bit under 25% duty cycle for fixed-fire boilers, or at low-fire for mod-cons, but how they're used in the real world will vary considerably (since running a cast iron boiler with 120F return water will destroy it in one heating season, whereas mod-cons need lower water temps to run at highest efficiency.)

    A primer on AFUE can be found here:


    With low-mass modulating boilers like mod-cons the as-used AFUE pretty much hits it's steady-state combustion efficiency as long as the burn times are longer than a couple of minutes and the return water temps stay in the right range. An ALP150 would be WAY oversized for most residential applications (even for 5000' houses in the Adirondacks) and would need either massive radiation (concrete slab radiant floors) or large buffering thermal mass (like a tank) to keep it from short-cycling.

    If the house is cut up into a gazillion zones with low mass radiation it DEFINITELY has as-used AFUE issues that will impact both efficiency and longevity, but there's usually a fix.

    What drove the boiler size decision here?

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