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EF for Indirect Storage Water Heaters

Nick Sisler | Posted in Energy Efficiency and Durability on

Is there a standard method for estimating/calculating the EF of a boiler-powered indirect storage water heater?

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  1. GBA Editor
    Martin Holladay | | #1


  2. GBA Editor
    Martin Holladay | | #2

    Indirect water heaters are rated with a metric called the combined appliance efficiency rating. I'm not sure exactly how this number is determined.

    According to the ACEEE, "The efficiency of a combination water and space heating system is indicated by its combined appliance efficiency rating (CAE). The higher the number, the more energy efficient. Combination appliance efficiency ratings vary from 0.59 to 0.90. Look for CAE of 0.85 or higher."

    According to Keystone Energy Efficiency Alliance, "The CAE typically ranges from 0.59 to 0.90. The higher the CAE, the more efficient the model."

  3. Nick Sisler | | #3

    Thanks Martin. Unfortunately, it doesn't look like the manufacturers publish the CAE. I'm looking at a Buderus boiler with a SuperStor Indirect Water Heater (I've included product links for each below). Do you know if there's another place to find the CAE?

  4. GBA Editor
    Martin Holladay | | #4

    I was frustrated by my Google searches using the term "combined appliance efficiency rating." Usually, if a rating is based on a standard test method, a Google search will turn up a definition. Something smells odd -- so I'm guessing that the rating method has been proposed but not universally adopted.

    Perhaps a GBA reader who knows more than I do about this metric can chime in with more information.

  5. Expert Member
    Dana Dorsett | | #5

    The efficiency of heating with indirects cannot be reduced to a single EF type number, since the "as-used" efficiency varies with the space heating load and the oversizing factor of the boiler to both the heating & hot water loads. This has been tested & modeled fairly extensively by Thomas Butcher of the Brookhaven National Laboratories in NY:

    (There is extensive writeups on the tested systems and their performance in the appendices.)

    Clearly right-sized boilers with heat-purge controls (if the boiler is high-mass) is a lot more efficiency in water-heating-only mode than 3x oversized boilers, or most high-mass boilers with embedded "tankless" hot water heating coils.

    But the quality of implementation counts too. An indirect plumbed-in without insulating the pipes for the boiler loop and for the near-tank potable plumbing will be a DOG compared to one with all near-tank and near-boiler plumbing insulated to R4+, even using the same equipment running at the same temperatures.

    In general the additional load of the HW improves the duty cycle and thus average performance of any non-modulating boiler during the heating season. With modulating condensing boiler it's less clear, but it's still a pretty efficient way to heat water, and the recovery rate is higher than standard gas-fired standalone hot water heaters.

    The CAE is fuzzy number calculated via ANSI/ASHRAE Standard 124 test procedure (updated most recently in 2007) that is probably even less useful than the DOE EF number for determining the true as-used efficiency. Butcher also points out that CAE numbers overstate the true performance of real-world systems based on his modeling, in this article:

    But EF numbers are also bogus, overstating the real-world performance of tankless water heaters, and that of tank-type heaters when the daily volume uses are low, and understates that of tank type heaters in high-volume water use applications.

    Ultimately it all boils down to idling/standby losses, and raw combustion efficiency, and the fuzzy-numbers are only valid within the water heating paradigm- you can't really just compare EFs to know if a tankless will beat a tank- you have to know the application and dig deeper. And you can't really compare CAE to EF, or even between different boilers, without knowing the oversizing factors & boiler types, etc. But it's possible/easy to improve the real-world performance of any of them with the appropriate plumbing insulation, and that improvement alone may exceed the performance delta between an indirect that scores high on CAE vs. a lower scoring version.

  6. Nick Sisler | | #6

    Thanks a lot Dana!

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