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Community and Q&A

Water heater energy factor comparison

dfvellone | Posted in General Questions on

Im looking at a sealed combustion direct vent (non-powered) water heater with an energy factor rating of .60. Comparatively, the same maker’s atmospheric vented heater with similar specs has a rating of .62.

Is my assumption correct that the energy factor doesn’t reflect the added efficiency of  the sealed combustion unit utilizing outside air, as opposed to the atmospheric vented unit which is using heated air in the house, and also potentially encouraging infiltration of outside air? 

The direct vent/sealed combustion unit is markedly more expensive than the atmospheric vented unit, and though I’m aware of the benefits of the sealed combustion, particularly in my “pretty good” construction, I’m wondering if that added expense might realize a little payback in heating energy consumption. 

Thanks, Daniel

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  1. Expert Member
    BILL WICHERS | | #1

    The efficiency numbers are only related to how many BTUs you get out of the burner in the form of useful heat compared to how much total input BTUs go into the unit in the form of fuel gas. Any losses due to interior air getting "sucked out" during the combustion process don't count, but should be relatively small.


    1. dfvellone | | #2

      So, the sealed combustion’s use of outside air doesn’t result in much efficiency? I’d assumed that the added cost of the unit - over 100% of the atmospheric-vented heater- was attributed in part to greater efficiency.

      Thanks, Daniel

  2. Expert Member
    DCcontrarian | | #3

    Code for makeup air for water heaters is 1 cfm per 2400 BTU/hr. How much energy that 1 cfm takes out of the house depends on the outside temperature; at 20F outside and 70F inside 1 cfm is 54 BTU/hr. So about 2%. At more moderate temperatures the energy penalty is less.

    It may well be that the fan for the direct vent uses more energy than it saves.

    The big advantage of direct venting is you have a lot more flexibility in where you locate them.

  3. paul_wiedefeld | | #4

    These kinds of water heaters (not a heat pump, not even condensing) are all low efficiency. No real payback to speak of.

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