This article was originally published in the September/October 1997 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 1997



DESLog Delivers Timely Answers about Home Energy Use

DESLog software can help weatherization agencies collect utility information quickly and evaluate weatherization strategies for cases where the problem is not as obvious as this thermostat located above a heating register.
Blower doors, pressure tests, and other diagnostic tools are useful for assessing home performance before and after weatherization. But what does sealing bypasses or blowing in wall insulation really mean in terms of energy use? Diagnostic tools do not measure energy use directly; and until recently, the only way to measure true energy savings from weatherization was to track utility bills over the course of a year.

A new software program, DESLog, now makes it easy to predict annual energy savings using heating system run-time data. The program was developed by the Center for Energy and Environment (CEE) in Minneapolis for the Minnesota Department of Economic Security (DES).

The DESLog software correlates daily fuel consumption, based on furnace run times, with average daily outside temperatures collected from the nearest National Oceanographic and Atmospheric Administration weather station. From this data, DESLog computes house-specific annual savings and provides space heating system cycling trends. All of this is done in as little as one month after weatherization work is completed.

The one-month turnaround time comes without much sacrifice in accuracy. In a Minnesota DES comparison of DESLog with PRISM--an energy consumption analysis program that uses longer-term utility bill data--DESLog overpredicted average PRISM results by only 3%. And DESLog has other advantages over long-term methods.

  • It eliminates the need for baseload estimates since only the heating system is monitored.
  • It monitors centrally heated homes that use natural gas, propane, oil, or electricity.
  • It provides insights on improper system performance by analyzing heating system cycling behavior.
  • It reduces lost cases due to incomplete utility meter readings or occupancy changes.
  • It can obtain separate savings estimates for client education, shell measures, and mechanical system improvements.
The DESLog program uses regression analysis to compare pre- and postweatherization energy use. The program presents the analysis results in a chart like the one shown in Figure 1.

To accurately depict the impact of weatherization, DESLog requires a minimum of six weeks of run-time data--three weeks before and three weeks after the weatherization work is completed. Longer monitoring periods are necessary in climates where day-to-day outside temperatures are fairly constant.

Weatherization Agencies Pilot the System Nine voluntary weatherization agencies took part in a Minnesota DES pilot study of the DESLog system during the last heating season. The agencies installed self-contained, battery powered run-time loggers in a random group of single-family homes, collecting a total of six weeks of data, three weeks before and three weeks after weatherization. A few of the agencies coordinated the run-time logger installation and data retrieval with their energy audits and inspections. This avoided creating any additional demands on their production schedules.

Some of the data loggers used in the pilot were installed incorrectly or were not reinstalled after weatherization. One data logger was rescued from a discarded furnace after a mechanical contractor sent an old furnace--logger included--to a landfill. Luckily, an auditor found the logger and its data intact. The unusual winter weather also took its toll: one logger was submerged in the Red River's floodwaters.

In spite of the mishaps, the agencies found the DESLog analysis useful. They could quickly assess the effectiveness of their weatherization work, and in some cases they returned to houses that showed lower-than-expected savings. The monitoring also helped to identify short-cycling problems in the heating systems.

Community Action of Minneapolis (CA of Mpls.), one of the participating agencies, has since begun using the DESLog analysis in a regular production-based protocol that identifies the need for reinspections. According to Vicki Carey, a CA of Mpls. auditor, At one house the data logger alerted us to a short-cycling problem we wouldn't have otherwise found. Some of the causes were more apparent than others.

Figure 1. Pre/post-weatherization gas use as a function of outside temperature for a Minneapolis site.
As part of their quality control efforts, CA of Mpls. used logger data to identify and relocate a thermostat that was above a supply register. The changes improved the distribution system and cycling of the furnace. Short-cycling problems in other homes were eventually linked to oversize furnaces, undersized return ducts, and incorrect anticipator settings. Resident Behavior Must Stay Constant Like any diagnostic tool, DESLog does not always produce reliable results for every house. During the pilot study, the weatherization agencies followed suggested monitoring protocols at 50 houses, and 78% of these had conclusive pre- and post-savings estimates.

In general, the short-term monitoring method does not work well early in the heating season, when occupants are still occasionally opening windows or making other changes in the home. Predictions are also thrown off by occupants who adjust their thermostats sporadically; the DESLog models are based on the heating system's reaction to outside temperature, and other variables must remain fairly constant.

Despite the limitations, agencies now have a quality control tool that allows them to improve the cost-effectiveness of weatherization in the same heating season and with the same field technicians performing weatherization.

The DESLog software is available from the Minnesota Department of Children, Families and Learning's Office of Energy Programs. For information on receiving the DESLog software, contact David Miller, Minnesota Department of Children, Families and Learning, Office of Energy Programs, 390 N. Robert St., St. Paul, MN 55101. Tel:(612)297-3406; E-mail: dmmiller@

--Karen Linner
Karen Linner is a policy analyst at the Center for Energy and Environment in Minneapolis, where she provides DESLog training and technical assistance.


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