The present invention relates to residential heating and/or cooling systems and other indoor comfort systems, and is more particularly concerned with battery-powered thermostats of the type that derive the power for monitoring conditions within the comfort space and for controlling the signaling to the furnace or other comfort system from a battery i.e, dry cells or alkaline cells. The invention is more specifically directed to DC thermostats with latching relays that are pulsed to change their state from OFF to ON or from ON to OFF, and remain latched into that state until pulsed into the other state. Heat may be provided from a gas, oil, or electric furnace or heat pump, and cooling may be provided from a compression/condensation/expansion/evaporation cycle air conditioner, an absorption type air conditioner, a ground-water heat exchange cooing system, or other available chilling apparatus. As used here, the term “cooling” includes both sensible cooling (reducing the temperature of the comfort air) and latent cooling (removing humidity). These comfort air apparatus may have additional functions for better control of the environmental air in the comfort space, such as multiple fan speeds, high and low heating, and high and low compressor speeds.
Wall thermostats are typically installed on an interior wall of a dwelling, business space or other residential or commercial space to control the operation of a furnace, air conditioner, heat pump, or other environmental control equipment. The thermostat continuously monitors the temperature of the room or other interior comfort space or zone, and is connected by a run of thermostat wires to the associated environmental control equipment to signal a call for heating, a call for cooling, or otherwise to keep an interior comfort space parameter, such as temperature, within some range (e.g., 68° F., ±1.0° F.). Other controls may be available, sensitive to other parameters, such as humidity or particulate level.
In the examples discussed here, the thermostats are powered by small dry-cell batteries, e.g., alkaline power cells. However, the re-pulsing action can be used favorably with any other thermostat type as well, including those powered by 24 volt AC thermostat power or any other energy source.
In many applications, i.e., in many permanent homes, and in mobile, recreational, or marine dwellings and spaces, battery-powered thermostats may be the preferred thermostat. These typically have a battery power source, e.g., a pair of AA alkaline power cells, installed within the thermostat housing to power the electronics. These supply DC energy to the internal electronics within the thermostat and power latching relays that connect thermostat power (such as 24 volts AC) to the particular thermostat wires that control heating, fan, air conditioning, and so forth. The thermostat is constantly monitoring the temperature in the comfort space and is also constantly monitoring the voltage level available on the DC battery power source. One or more controls, i.e., push buttons, rotary knob(s) etc., allow the occupant to set and adjust temperature setpoints for heating and/or for air conditioning. An LCD display screen on the housing of the thermostat allows the occupant to see the room temperature and also to see other functions, such as temperature setpoints during a temperature setting sequence. A low-battery message can also be displayed on this screen if the battery voltage drops below a level that indicates the battery is approaching the end of its useful life and should be replaced.
The latch mechanism in most latching relays relies on the magnetic force of the contact to keep the contact latched ON or latched OFF. If the thermostat is bumped, the mechanical shock can sometimes move the contact from the desired state. In that case, the microprocessor in the thermostat continues as if the relay were still in its desired state, but the furnace or air conditioning equipment does not. For example if a low temperature induces the thermostat to issue a call for heat, the microprocessor pulses the latching relay associated with heat to its ON condition, and the relay contact applies AC thermostat power to the W or heat thermostat wire, causing the furnace to commence a heat cycle. Then, if a bump or other shock knocks the contact back to the OFF condition, the furnace will shut down as if the thermostat had been satisfied. Wrong position of the relay could also occur due to a sudden or momentary power supply drop, component degradation, or other reasons that may cause the relay to fail to move to the intended position. Meanwhile, the thermostat heat set point has not been reached, and the microprocessor does not act on the heating relay. With the furnace shut down, the air in the comfort zone continues to cool down, but the thermostat does not respond because the thermostat's call for heat has not been satisfied.
On the other hand, thermal runaway can result if the heat relay contact is bumped into its ON condition when there is no call for heat, as the thermostat microprocessor acts as if the heat relay were open of OFF.
It may be the case that a the above-described erroneous condition will occur when the occupant is away for an extended period, and that the comfort space will continue to cool down until the occupant returns and can correct the thermostat. Failure of the furnace or other HVAC apparatus to cycle properly during winter conditions can result in frozen water pipes or other damage, including temperature stress to indoor plants and pets
Because of the foregoing problems, it would be desirable to provide a feature or features for battery powered thermostats that will ensure the relays are properly latched into the proper conditions, and that the heating and/or cooling conditions in the comfort space not be permitted to vary significantly during a heating or cooling operation.