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
Targeted Investment

by Fairlie Firari

An analysis of fuel consumption can aim weatherization crews toward the best retrofits and assist multifamily building owners and tenants in selecting appropriate investment levels.

Any type of energy audit in multifamily buildings should begin with the analysis of fuel consumption and cost. This analysis is vital for targeting investments in energy-saving measures for different end uses.

Gather the Bills

Since most apartment dwellers are renters, the first goal is to sort out who pays the bills for what fuels. Sometimes the owner pays, other times the tenants, and often the bills are split, so that residents pay for electricity and natural gas but not for heating oil, or some other combination. Both owners and tenants often balk at collecting consumption information. The trick is to find a contact in each utility or fuel provider who can assist you in getting this information. Then the owners and tenants need only sign a release form to grant you access to the data.

Determine End Uses

Once the auditor has collected and weather-normalized the utility bills, the next step is to determine the fuel source of the heating, cooling, water heating, cooking, and laundry equipment. This is not always as easy as it seems.

There are sometimes obscure relationships between different fuels in multifamily buildings, which can derail an otherwise straightforward allotment of specific fuels for different end-uses. While the breakdown between electricity and gas or oil usage depends in large part on the types of systems, it is also significantly influenced by choices made by the occupants. For instance, a tenant with electric resistance heat, who pays the electricity bill but not the natural gas bill, may use a gas oven for heat. A casual analysis of the utility bills will show a low electric heating cost, and a high gas cooking cost. Unfortunately for those of us who make building science decisions, these economic and ofttimes unhealthy human decisions (see Carbon Monoxide from Ovens, p. 18) can thwart our most cautious Btu-driven decisions.

Most building fuel use follows a pattern of peaks and valleys. Base usage is the valley where energy consumption is consistently low for periods of time and reflects minimal use of appliances. In many cases, base use is easy to identify. For instance, consider a gas-heated home with gas oven, stove, and hot water. In the summer, when gas is only used for domestic hot water and cooking, the baseline (in Btu per day) can be assumed to be the amount of fuel that is used typically for those appliances throughout the year. Most additional gas usage during the heating season can be attributed to heating, and should increase proportionally with more severe weather.

Similarly, an all-electric building in an area that requires both heating and cooling should have fuel usage valleys in the spring and fall. The lower valley is the base electric usage. This base accounts primarily for household electric appliances and lighting.

After allotting the usage peaks to heating and cooling, and determining the area's most recent heating and cooling degree-days, standardize consumption despite weather differences by calculating the building's Btu per ft2 per heating or cooling degree day (Btu/ft2/HDD or CDD). Nationally, the average total energy usage for apartment buildings is about 16 Btu/ft2/HDD, yet construction in severe weather areas tends to be more energy-efficient. In especially cold climates, insulation levels are higher, as the alternative can be expensive, or even potentially life threatening.

A recent study of New York City multifamily buildings (averaging 35 apartments per building) found a relatively high average energy consumption of 22 Btu/ft2/HDD. All of these buildings had central heat and DHW systems. More disturbing in these buildings was the fact that 48% of the fossil fuel usage went to heat water. This discovery encouraged greater implementation of water conservation measures, as well as retrofits to make the generation of hot water more efficient. The analysis helped us to both understand average usage in multifamily buildings in the city and provide a basis for comparison between similar buildings.

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.

Formula A:
(Heat Btu/ft2/HDD x 60 x $/therm) +
(Cool Btu/ft2/CDD x 60 x $/therm)
= Heating and Cooling investment in $/unit

DHW Investment

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.

Formula B:
[DHW Btu/(heat+cool+DHW Btu)] x 300 x $/therm
= DHW investment in $/unit

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.

Formula C:
Base electric kWh/unit x 5 x $/kWh
= Electric Appliance investment in $/unit

Examples from the Field

Example 1:

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
B. 0.35 x 300 x 0.80 = $84 DHW investment
C. 250 x 5 x 0.15 = $190 electric appliance investment
Total proposed investment = $980/unit

Example 2:

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
B. 0.30 x 300 x 0.50 = $45 DHW investment
C. 400 x 5 x 0.07 = $140 electric appliance investment
Total proposed investment = $2,000/unit

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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