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Community and Q&A

Water heater standby losses

GBA Editor | Posted in Mechanicals on

Does anyone have a reference for the standby losses (in BTU/hr) for a “typical” gas-fired 40 gallon water heater?


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  1. Doug McEvers | | #1

    In my case it is what ever the pilot light uses, the water heater never fires unless hot water is drawn. The water heater is quite well insulated but is the 54% or so efficient model. My gas monitoring says we use about 6 therms per month per occupant for hot water, cooking and gas drying of clothes.

  2. Riversong | | #2

    The losses are not rated in BTU/hr because they are composed of recovery losses (fuel to hot water conversion), standby-by losses through the insulation and up the flue, and cycling losses in the piping.

    A non-condensing gas water heater has an Efficiency Factor of 0.60 to 0.65, meaning that 35% to 40% of the BTUs are lost from the system over the course of daily use.

    If you DHW consumption is 150 therms/year, then 52.5 to 60 therms/year are lost from the system (some of that contributes to space heating or becomes a liability if air conditioning).

  3. Travis Thompson | | #3

    In performing calculations for domestic solar water heating systems we use a general assumption that a tank will lose about 1 degree per hour, although actual losses are probably less in a good tank. I think this would equate to about 335 BTU/hr. Gas water heaters probably lose more due to the central flue pipe.

  4. Pete Engle | | #4


    The Energy Factor is calculated using a set of standard conditions, and I haven't found those conditions yet. For instance, the water heater installed in a room at 70 degrees, 200 gallons of hot water used during two separate periods per day, the rest of the time on standby. Once you define the standard conditions, you can use the combustion efficiency, standby losses, etc. to calculate (and test) the actual EF. I'm trying to separate out just the standby losses to figure out roughly how much gas a "typical" water heater would use if it was left on for a few months, but no hot water was used at all. The EF doesn't get me there.

    I need to find the actual BTU/hr heat loss from the tank and assembly. I can then use the combustion efficiency of the burner assembly (about 75%) to calculate therms of gas used on standby.

    Travis is on the right track, but I know that gas-fired water heaters lose lots more than 1 degree per hour. I just don't know what that number is. This is going to be something that somebody's tested for a range of "typical" water heaters in "typical houses." I'm just hoping to find my way to those test results without having to go through extraordinary work to do it myself.

  5. Michael Blasnik | | #5

    The standby losses are about 70 therms/yr for a typical tank gas water heater -- and will vary mostly with tank temperature and room temperature. You might want to check out the WHAM model ( google wham water heater) to find out some more details of how to turn an EF rating into an effective UA given the recovery efficiency.

  6. Cedric De La Beaujardiere | | #6

    Michael Blasnik has it right. A good summary document with sample values is
    Starting around bottom of page 9.
    The Standby Losses is given by the value of the equation for UA on page 13.

  7. Laura Reynolds | | #7

    Ask The Energy Wiz has something to say about this.

  8. Richard McGrath | | #8

    As of April , 15 , 2015 the typical water heater has evolved to something completely different .

    New typical may differ from old typical

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