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Thread: High efficiency condensing boilers not so efficient

  1. #1
    Mechanical Engineer experinced in HVAC denering's Avatar
    Join Date
    Feb 2012

    Post High efficiency condensing boilers not so efficient

    I been working over the last 10 years mostly with condensing boiler and I have the believe that becouse of the way they are connected to the commercial buffers they just ocasionally achieve condensation. The way I see it, the problem comes from the fact that hot water returning from the boiler mixes all the time with cold water coming from the system. This mixing of let's say 180F water from boiler with 110F from the system, resulted in the water mix temperature return to the boiler close to 145F. We all know that water returning to the boiler have to be below 135F (CO condensation temperature) to allow for condensation. The lower the better to secure condensation.

    The question is how can I minimized the degree of mixing in the buffer to allow 110F cold water from the system to go to the boiler where it is needed cold and hot water from boiler 180F to go to the system at that temp without mixing?.


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


    First thing I'd have to ask is do you really need that 180-degree water in the buffer? Maybe on a really cold day, but how often do they really occur? Maybe a lot in Calgary, but still. If that were allowed to cool off on a mild day, you'd reach better efficiencies. Does the thing have an outdoor reset control input? Is it being used? Adjusting the buffer tank based on the heating needs would be better. Then, you'd have to decide, do you really need the buffer tank at all? Often, they're added to help fix an oversized boiler and improve its efficiency.
    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


    On my home system the buffer is plumbed as the hydraulic separator between the boiler & radiation loops, and max temp I run in my buffer is 130F, settable with an aquastat on the buffer. (I could set it lower and the heating system would still keep up, but since the buffer is the thermal mass for the domestic hot water via a heat exchanger, I don't.) The system return water is distributed to the bottom of the tank via dip tube, and the entering water at the boiler during any significant call for heat is ~122F, somewhat lower on start up from cold radiation. And that is with ~160F boiler output (high delta-T/low flow by design, to keep pumping requirements modest on the boiler loop.) YMMV.

    It's neither the boiler nor the buffer that determines the efficiency of the system so much as the system DESIGN, and yes, many boiler-replacements (and even new-installations) with condensing boilers are done without the requisite adjustments to the system to achieve condensing efficiency. But just because it's possible to set it up to condense only rarely doesn't mean you have to. Mine is not the most-efficient design possible (I could arguably squeak a few percent more out of it with an outdoor reset approach and running the hot water as a separate zone with more hardware) but it runs at condensing temps 100% of the time(!).

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