Energy Source Builder

Line-voltage Thermostat Study

It's no secret that electric line-voltage thermostats are inaccurate. In some models, even the dial doesn't dare indicate temperatures. Instead, it gives you a vague idea of what to expect with "Colder" printed on one end of the dial and "Warmer" on the other. In between is an uncomfortably large range called "Comfort Zone."

The inaccuracy isn't surprising. The typical line-voltage thermostat contains a bimetallic strip that bends as the temperature changes. Unlike low-voltage thermostats that carry only enough current to flip a switch, line-voltage models must carry the full current drawn by the heater. This can be up to 20 amps and generates considerable heat in the bimetallic element.

Builders like the bimetallic thermostats because they are rugged and inexpensive. Where they fall short is performance. Poor calibration, slow reaction and wide temperature swings are common.

As usual, technology comes to the rescue. Several years ago electronic line- voltage thermostats (ELVTs) appeared on the market. In these units, a lightweight thermistor senses the temperature while semiconductors control current to the heater. ELVTs are more precise, better calibrated and have lower temperature swings (technically called hysteresis). The greater accuracy is reflected in the dial, which indicates temperature settings with actual numbers.

Theoretical Savings

People tend to set a thermostat so that they are comfortable at the lowest part of the temperature swing. The high hysteresis of bimetal thermostats requires a setting two to three degrees higher than the actual comfort temperature. Of course, the high end of the swing means the actual temperature could be up to six degrees higher than necessary.

With more accurate electronic thermostats people should find a setting much closer to the actual comfort temperature. It stands to reason that the electronic thermostats should reduce average room temperature and save energy.

Research conducted by the Portland Energy Office in Portland, Oregon, set out to verify this assumption. The results contain a couple of surprises.

Real World Sample

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The study focused on multi-family buildings because many of these units contain electric baseboard, ceiling cable or wall-mounted heaters. Twenty-seven units were selected. In half the units, existing thermostats were replaced with electronic ones. The other half kept their existing thermostat. After two months, the groups switched. The original thermostats were returned to the first group and the second group received ELVTs. None of the participants was told the new thermostats were supposed to save energy.

Data was collected and analyzed by Lambert Engineering of Bend, Oregon. They used data loggers to track total energy use and demand, zonal electric use and zonal temperatures. Researchers were able to capture one key factor: the temperature at which individual baseboard heaters were turned on and off. Capturing each zone's set point and hysteresis was essential in understanding the unexpected results.

Mechanical Factor

Monitoring of their performance in the field confirms that electronic thermostats are very accurate. In this study, the temperature swing was just 0.61°F. The first surprise was that bimetal thermostats were not as bad as expected, although still considerably worse than the electronic ones. The average temperature swing for all bimetallic thermostats was 2.4°F. However, a group of 19 units in one apartment building showed an average hysteresis of only 1.4°F.

Thermostat settings were 0.9°F lower in apartments with ELVTs. Based on this difference, the savings from using electronic thermostats should have been 222 kWh per year or about 14 percent. However, the average room temperature unexpectedly increased by 0.29°F. No actual energy savings could be measured.

Human Factor

Human behavior played a role in the lack of energy savings. These apartment dwellers had been using the bimetal thermostats as on/off switches, instead of temperature regulators. This was especially true at night when the bimetal control was turned all the way down, so the heater never came on.

When the more accurate ELVTs appeared, the occupants continued to set the temperature back at night. However, they didn't set it back as much. The data loggers captured almost as many thermostat adjustments, but the amount of each setback was lower.

A researcher speculated that occupants faced with the uncertain choices on the bimetal thermostat's dial simply shut down the heat. When the ELVTs appeared with actual numbers on the dial, they may have selected specific--but higher--temperature settings.

Lower Electrical Demand

Despite the lack of measurable energy savings, monitoring did show savings in peak electrical demand. Demand is the instantaneous draw of power from the utility's system. On average, apartments with ELVTs showed a peak demand 447 Watts (six percent) lower than units with bimetal thermostats. The likely reason for lower demand is the shallower night setback. The heaters did not run as long in the morning to return the apartments to their daytime temperature.

Customer Preference

After using both types of thermostats, 91 percent of the occupants liked the electronic models. Only 36 percent said they liked the bimetal thermostats.

This real world research confirmed that ELVTs control temperature more precisely, provide greater comfort and offer more logical operation. The measurable reduction in electrical demand benefits the utility and possibly the building owner, who may pay a demand charge. However, this study fails to show energy savings that might benefit the occupants.

Although bimetal thermostats performed better than expected, there is still reason to be cautious. Many are still poorly calibrated and inaccurate. Telling the good-quality units from the poor ones could be difficult. The ELVTs may be a good choice because of greater reliability and better comfort.

This article appeared in Energy Source Builder #54 December 1997,
©Copyright 1997 Iris Communications, Inc.

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