This article was originally published in the September/October 1995 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.
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Home Energy Magazine Online September/October 1995
Using Fuel Bills for a
Setting A Draft Investment Level
Multifamily weatherization programs can calculate a draft investment level, based on the fuel use history, to target buildings with high savings potential before sending an auditor to the building. Presenting this draft investment level to the building owner can also help show him or her where in the building the savings potential lies. It is not a minimum or maximum but a ballpark estimate, which can be revised after the building has been examined.
A fuel analysis must first be done to separate heating, cooling, domestic hot water (DHW), and base electric usage in each building as a whole, or in each apartment separately (depending on whether fuel bills are available for individual units). The formulas are weighted by price of fuel, so that even when the overall Btu savings are small, a high fuel cost can make the savings-to-investment ratio acceptable. The multipliers in each formula are derived from our experience in predominantly cooling climates and studies of typical energy usage in multifamily buildings in these areas.
Heating and Cooling Investment
The heating investment is determined by multiplying the fuel usage for heating (Btu/ft2/HDD) by 60 (the multiplier) and by the fuel cost in dollars per 100,000 Btu (therm). These numbers are added together for each fuel type.
The cooling investment equals the cooling fuel usage (Btu/ft2/CDD), multiplied by 60, multiplied by the fuel cost in dollars per therm.
These are combined to get the total investment in heating and cooling measures, including both equipment and shell retrofits (see Formula A). In buildings with both heating and cooling, therefore, more funds can be allotted for shell and equipment retrofits.
The investment in domestic hot water measures is determined by first figuring out what percentage of the total fuel used for heating, cooling, and DHW is used for DHW. This percentage is multiplied by 300 (the multiplier) and by the fuel cost (in dollars per therm), as in Formula B. Note that this method puts greater emphasis on DHW investment when it is a larger percentage of the total heating, cooling, and DHW bill.
Electric Appliance Investment
To determine the investment in appliances that make up the base electric usage, multiply the average monthly base residential electric usage per unit (kWh) by a multiplier of 5, and by the electricity rate in $/kWh (see Formula C). This puts greater emphasis on electric appliance investment if the tenant has expensive electricity and a high base of electricity usage.
Examples from the Field
A building centrally heated with #2 oil at 75cents/gallon (54cents/therm) consumes 22 Btu/ft2/HDD for heat. The building has a separate gas DHW system which consumes 35% of the total heat and DHW load, at a cost of 80cents/therm. The average apartment has an electric base usage of 250 kWh per month at 15cents/kWh.
A. 22 x 60 x 0.54 = $710 heating investment
A building master-metered with electric heat at 7cents/kWh ($2.05/therm) consumes 15 Btu/ft2/HDD for heat, has an average apartment base of 400 kWh, and employs a central gas hot water system which uses 30% of the total heating. The gas cost for the DHW is 50cents/therm.
A. 15 x 60 x 2.05 = $1,800 heating investment
Decide How Much to Invest
After performing an analysis of a building's fuel bills, the next step is to decide how much money to spend making the building more energy-efficient.
Multifamily building retrofit programs have used many criteria to put an investment level together, from a basic per-unit standard to a percentage of total fuel costs. The formulas provided in Setting a Draft Investment Level are the author's method of determining a draft level of investment per unit for shell, central heating and cooling, DHW, and electric appliance retrofits. The formulas adjust investment according to both weather-related usage and cost of fuel.
Allow for Variation Among Apartments
The interaction of apartments, location, tenancy, orientation, and exposure can all dramatically affect fuel usage. Although we work with a draft investment level per unit in the building, it does not mean that all apartments will receive the same level of investment. This is particularly true in buildings where the tenants pay for all of their fuels, as the location of a specific apartment in a building can be a principal reason for odd usage patterns.
Imagine a 10-story building in a heating and cooling climate that employs through-the-wall packaged terminal heat pumps or worse, resistance heat with through-the-wall air conditioners. In the winter, warmer moist air tends to rise, warming upper floor apartments. The south- and west-facing apartments in this building would also have substantial solar gain in the winter (assuming that the building is not shaded by surrounding structures). Conversely, the lower level north-facing apartments (where colder, dryer air tends to enter the building and there is no solar gain) would be the coldest in the building and would experience the highest winter heating bills. As the seasons change, the bills would reverse; the top floor apartments would be hot and full of solar gain, while the lower level ones would be cooler.
In buildings where tenants pay for all of their energy costs, as in the example above, targeting more funds to the top and bottom floors for completely different retrofits would be intelligent. The top floor may get window films or shades, while the lower floor may get thermal curtains; both may get targeted air sealing and better controls on their heating and cooling systems.
Use Common Sense
Analyzing fuel usage in multifamily buildings is not a simple task. But it provides an important check for information gathered in a site audit and adds credibility when discussing potential savings with owners and tenants. The key is to use common sense in the application of information gained from both the fuel consumption analysis and the site audit, and use each to improve the data collected by the other.
F.L. Andrew Padian is director of Energy Audit Services for the New York City Weatherization Coalition.
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