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HEM March/April 1993 - Home Energy Magazine Online March/April 1993

Home Energy Magazine Online March/April 1993

METERING TOOLS

`Read Me Your Thermostat':
Short-Term Evaluation Tools

by Laurence F. Kinney

Laurence F. Kinney is president of Synertech Systems Corp., an energy research and services organization in Syracuse, N.Y.

Installing and using run-time meters the right way, in concert with well-planned weekly calls to meter reading clients, can provide effective evaluation of weatherization work.

It's not easy to measure savings from energy conservation work, yet it is crucial for a job well done. Many of the problems facing evaluations based on fuel savings result from the time required to collect adequate consumption data. Evaluation designers generally believe it's important to know household fuel usage for as much of the heating season (and non-heating season, to establish baseline consumption) as possible prior to weatherization and for as long as possible after that. They consider 18 months an adequate time span for gathering data. In fact, data is seldom collected over an 18-month period. To save time, the crucial before-weatherization data that provides the base from which savings are calculated is usually gathered from records kept for other purposes, such as meter-based utility bills.

An approach which shows particular promise makes extensive use of simple and inexpensive instrumentation to measure quickly--over weeks, not months--how weatherization changes the efficiency of dwellings. This approach lends itself to large numbers of dwellings and is quite suitable for agency staff and others who may not have technical backgrounds. This approach does not measure "savings" at all, but rather enables one to project what savings can reasonably be expected barring major changes in occupancy or behavior. Utilizing short-term consumption data can, in turn, help estimate appropriate levels of weatherization investment. The tools must fit the approach (see accompanying article, "Keeping A Running Score on Weatherization,").

Elapsed Timers

Most furnaces and boilers used for residential heating are either off or on, and when on, they consume energy at a constant rate. Measuring the rate of consumption is fairly straightforward. In the case of natural gas, watching, with a stopwatch in hand, the dials turn on the gas meter while the furnace (and only the furnace) is firing yields sufficiently accurate data on firing rate. For oil furnaces, that rate can be determined with sufficient accuracy by pulling the burner and examining the flow rate etched on the nozzle in gallons per hour. (The Btu in fuel oil or natural gas can vary by up to 3%, so precision is quite difficult to achieve.)

If one knows the rate of consumption of a furnace or boiler, then by measuring the time it is operated over a given period in which outside temperature is tracked, it is possible to produce a simple but useful weather-adjusted index of consumption, Btu per heating degree-day (Btu/HDD). This is calculated by the elapsed time multiplied by the firing rate of the furnace (or boiler), all divided by the heating degree-days for the appropriate reference temperature over the same period. Dividing by the heated area gives the energy intensity (Btu/ft²-HDD).

Over the last 12 years, Synertech has used elapsed timers to determine savings due to both building shell and HVAC system retrofits. Under most circumstances, we prefer simple electro-mechanical devices that use 24VAC (alternating current voltage) synchronous motors to drive mechanical wheels calibrated in hours and tenths of hours. These are widely available for $10-$40 (see box, "Short-Term Metering Equipment Makers," p. 37).

Gas Furnaces

In the case of natural gas or propane-fired furnaces or boilers, the elapsed timer is wired in parallel with the gas solenoid valve (see Figure 1). When the current flows in the gas solenoid, the valve opens, gas flows, and the timer begins. When the thermostat is "satisfied," the gas solenoid valve closes and the timer quits registering time. Since the timer we recommend is a mechanical device rather than digital, power outages will not cause a loss of data.

Oil Furnaces

In the case of oil-fired furnaces, it is sometimes possible to work at 24VAC , but we prefer to measure when the motor that pumps oil (and air) is actually running. We use a transformer, housed in a small box with a fuse, as an intermediary between the oil burner and the timer. The 120 VAC primary (input) of the transformer is wired in parallel with the oil burner motor, and the 24VAC secondary (output) is wired to the timer. In practice, the transformer is located 6-10 ft from the burner--frequently mounted to a solid joist above the furnace--and is connected to the burner with a flexible, armored BX cable. The timer, on the other hand, is located where the client can read it conveniently--usually in the living area, close to a telephone. We use ordinary thermostat wire to connect the timer to the transformer box (or the furnace in the case of gas). Office staff then can call the client weekly to check readings and calculate the week's consumption for that house using a simple worksheet.

The Thermostat As Data-Logger

A new generation of set-back thermostats have several advantages over elapsed timers. Chief among the advantages is the ability to store the cumulative time that the thermostat "calls" for heat. Several models produced by the Hunter Fan Co. feature a review function that provides the total elapsed time the thermostat calls for heat "today," "yesterday," "this week," and "last week," where a week ends at midnight Sunday. A "filter review" command totals the time the system has run, up to 1,000 hours of firing time--a feature intended to prompt the homeowner to change the furnace filter, but useful for our purposes as well.

Installing a thermostat is easier than installing an elapsed timer, the readout is more accurate, and the recorded time periods are identical for all users (midnight Sunday to midnight the following Sunday). For weatherization programs, it may be counted as a material expense, thereby reducing the material expense for "evaluation" to zero. (For disadvantages of using the Hunter thermostat this way, and alternative wiring methods, see box "The Hunter Thermostat As Logger--Special Cases.")

Filling Out The Audit Forms

Before The Calling Begins--and In Between

Several things need to be in place before taking the weekly client readings of the elapsed timers or thermostats. Let's look at a typical house assuming that the crew installed Hunter's Auto Temp or Energy Monitor II Plus thermostat (or equivalent) during the audit and that the client agreed to receive weekly phone calls to report the previous week's consumption. (In the case of clients without phones, using a pre-addressed and pre-stamped postcard is adequate.)

On that initial visit, the auditor will begin filling out the form (see Figure 2):

The firing rate (Btu/hr) for furnaces using natural gas is determined by measuring the time for a full rotation of the 1 ft³ or 2 ft³ dial on the gas meter while only the furnace is firing. Assuming that 1 ft³ of natural gas has an energy content of 1,000 Btu, the auditor determines the cubic feet per hour of gas flow which then is multiplied by 1,000 to yield the firing rate in Btu per hour. The auditor compares the firing rate, determined by the timing method, to the label on the furnace. If there is a significant difference, the procedure is repeated.

The fuel dispersion nozzle in an oil-fired heating system is inscribed with the rate at which the nozzle will allow oil to flow in gallons per hour (for a pump pressure of 100 psi). The flow rate multiplied by the Btu capacity of the fuel (approximately 140,000 in the case of No. 2 grade of fuel oil) gives the Btu/hr firing rate of the system.

Firing rates for furnaces using propane are read directly from the label on the furnace itself.