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We are modeling a net zero house in REMrate. The house scores a -6 HERS yet it still uses 3.1 MMBtu more than it produces?

capnkent | Posted in Energy Efficiency and Durability on

Isn’t a 0 HERS supposed to reflect a net zero house?

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  1. user-626934 | | #1

    Yes, this is possible. In all electric homes, a 0 HERS Index means the house is modeled to be "net-zero site energy". However, in houses with mixes of fuels (for instance gas water heater and/or gas furnace along with electricity for everything else) it's possible to achieve a 0 HERS Index (or even negative HERS Index), even while the model shows net site energy use to be positive (using more than producing on site). The reason: the HERS Index is NOT a comparison of estimated site energy use between the actual house and the reference house. The HERS Index, in fact, compares the actual house's "Normalized Modified End Use Loads" to the reference home's "End Use Loads". The main reason for the development of the "Normalized Modified End Use Load Method" (as I understand it) was to create a fuel-neutral rating system in order to mitigate the ongoing "fuel wars" between natural gas providers and electricity providers that threatened to torpedo the implementation of a national home energy rating system.

    All the fun historical details and mathematical contortions can be found here:

    Here's the abstract:
    "This paper provides much of the historical background surrounding the contentious development of a national, uniform rating method for rating the energy-efficiency of homes. A full derivation of the most recently adopted rating method, the Normalized Modified Loads Method, is provided in detail and the issues and events leading up to the development of each of the succeeding methods are described.

    Ever since the inclusion of Home Energy Rating Systems in the 1992 Energy Policy Act, the most contentious issue associated with them has been the development of a uniform rating method. To date, four succeeding methods have been proposed: 1) the "Original" Method, 2) the Equipment Adjustment Factor Method, 3) the Modified Loads Method, and 4) the Normalized Modified Loads Method. Since the beginning, the process of reaching a national consensus on an acceptable rating method has been plagued by the competing market interests of the electric and gas industry. The effort to find a solution that is equitable and "fuel neutral" has proven to be a quest not unlike that for the holy grail.

    The national discussion that has ensued from this effort has worked to clarify, and in some cases resolve, many of the issues involving the building energy efficiency marketplace, national building codes and appliance standards, and energy and environmental policy. It has led to a fuller appreciation of the complexity of building energy efficiency, its varied definitions and meanings, and to the adoption for the first time by a national association of governmental officials, a set of uniform technical guidelines for rating the energy efficiency of homes."

  2. GBA Editor
    Martin Holladay | | #2

    John Semmelhack's explanation is helpful.

    You may also be interested in reading this article on the HERS Index: How Is a Home’s HERS Index Calculated?

    One other fact to remember: once the house is occupied, there's no guarantee that the energy use of the occupants will match that of the model.

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