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A little experiment with indirect HW heater

gusfhb | Posted in Mechanicals on

SO there was some discussion about the relative efficiency of indirect water heaters some time back.

It made me wonder, and I had the opportunity to check out the costs associated with standby losses in my system. The family was away for 28 days, so I could shut off the boiler, but that would be boring, so I decided to see how much energy it wasted creating no hot water.

The system:
Buderus GB125BE condensing oil boiler
Superstore SS 60 60 gallon tank

The buderus has the Logamatic control which tracks boiler hours, and I have kept track of hours vs oil deliveries for nearly 2 years to where I have a high degree of confidence that the boiler uses .76 gallons per hour.

The results: The boiler ran approx 8 hours in 28 days using approx 6 gallons of heating oil.

Now there was some usage in this time, as I came back occasionally for work, and I did try to track this a bit, and it is pretty much down in the noise.

My conclusion is that ‘if’ you are heating with a boiler and ‘if’ that boiler is a cold start type boiler then using it to heat water is not a ridiculous thing to do. A tank type gas HWH is a big hole in the house which is bound to have higher standby losses. An electric tank might have lower standby losses but at a much higher energy cost.

Caveats: My boiler is pretty much the state of the art in oil boilers, condensing direct vent, with a computer control. Grandma’s converted coal boiler is not going to work well in this application. The hour meter is digital and in one hour increments, over a year it is accurate enough, but trying to parse out small time increments is dicey.

If you could cut your heat load enough to get rid of the boiler, do it, no matter what you use for hot water.

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  1. GBA Editor
    Martin Holladay | | #1

    Thanks for sharing your findings. If fuel oil costs $4 a gallon, it sounds as if the standby losses from an indirect hot water tank like yours cost a little less than a dollar a day.

  2. Expert Member
    Dana Dorsett | | #2

    And the standby losses were probably almost all from the boiler-tank loop + near-tank potable plumbing, all of which can be reduced further (and VERY cost-effectively with $4 oil) with R6 pipe insulation. Insulate everything, including the temperature & pressure outflow, and the potable cold feed within 10' of the tank. You may have to use fiberglass pipe insulation on the boiler loop, but for the potable 5/8" or 3/4" wall closed cell foam stuff is just fine. (Don't cheap out with the 3/8" goods from box stores. If you can't find it at plumber's supply house, Graingers will have it or order it via internet stores.

    BTW: The Brookhaven Nat'l Labs modeling of heating water with different types of boilers & indirects is pretty decent, a summary of which can be found here:

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