Twice a year when the clocks change between daylight and standard time, public service announcements and fire departments remind people to change the batteries in their smoke detectors. While this ensures the smoke detectors are ready to detect a fire, there is another battery in the house that is overlooked and could lead to the destruction of a home or office through water damage.
Digital thermostats that have replaced the original mercury switch thermostats offer scheduled heating and cooling, providing improved efficiency. But they all rely on a pair of “AA” size 1.5v batteries to maintain the heating schedule.
When those batteries fail after a few years, the thermostat stops working and shuts off, disabling the furnace. The result is a house or office that can drop to freezing temperatures inside after only one day. As the water pipes inside the building freeze and crack, the water pours out, damaging everything from the roof to the basement.
While it may be tempting to replace the standard alkaline batteries in a thermostat with longer lasting Lithium batteries, this is even more likely to lead to unexpected failure.
Unlike Alkaline batteries, Lithium batteries have a different discharge profile. Lithium batteries provide a steady voltage until they are completely discharged, resulting in almost no time for a low battery warning. Since the voltage on Alkaline batteries drops as they discharge, the thermostat will provide a warning for a week or two that the battery voltage is low and requires replacement.
Rechargeable Ni-Mh or Ni-Cd batteries should never be used in a thermostat, since they individually provide only 1.2 volts (2.4v for two batteries in series) at full charge, well below the 1.5/3 volts required. This will trigger a low battery warning even when fully charged. Rechargeable batteries also self-discharge and will not last more than a few weeks in a thermostat.
Regardless of the battery chosen for a thermostat, the outside of the thermostat should have a label indicating when new batteries were installed, and they should be replaced every year, preferably when changing smoke detector batteries.
Recommendations on an improved digital thermostat design:
- include a piezo speaker chirp and flashing red LED for the low battery alarm, powered by the 24-volt furnace transformer.
- use a replaceable Li-Ion battery as a backup battery and power the thermostat from the 24-volt transformer, so that battery power is only used when utility power is disabled.
- track the last and next battery change date on a calendar for reference.