Thermostat: Definition & Professional Guide
Call Now (855) 557-9600
A thermostat is a temperature-sensing control device that regulates heating or cooling equipment by activating or deactivating the system when the measured temperature deviates from a set point. In plumbing, thermostats control water heater temperature, mixing valve output, radiant floor heating zones, and recirculation pump operation, ensuring that water is delivered at safe, consistent, and energy-efficient temperatures throughout the building.
Feedback Loop, Set Point Control & Dual-Element Sequencing in Electric Heaters
Thermostats operate on a simple feedback loop: measure the current temperature, compare it to the desired set point, and send a signal to turn heating equipment on or off. In water heaters, the thermostat is mounted directly on the tank wall or integrated into the gas valve assembly. When the water temperature drops below the set point, the thermostat activates the burner or heating element. When the target temperature is reached, it shuts off the heat source.
Water heater thermostats are typically set between 120 and 140 degrees Fahrenheit. The Consumer Product Safety Commission recommends 120 degrees to prevent scalding, particularly in households with children or elderly residents. However, temperatures below 120 degrees increase the risk of Legionella bacteria growth in tank-style water heaters. This balance between scald prevention and pathogen control makes proper thermostat setting a safety-critical plumbing decision.
In electric water heaters, two thermostats work in sequence. The upper thermostat controls the upper heating element and has priority, heating the top portion of the tank first to provide hot water quickly. Once the upper tank reaches temperature, the upper thermostat transfers power to the lower thermostat and element to heat the remaining water volume. This sequential operation prevents both elements from running simultaneously, which would overload a standard residential circuit.
Mechanical, Electronic, Programmable & Mixing Valve Thermostat Types
Mechanical thermostats use bimetallic strips or wax-filled capsules that physically expand and contract with temperature changes to open and close electrical contacts. They are simple, reliable, and found in most standard tank water heaters.
Electronic thermostats use thermistors or resistance temperature detectors (RTDs) for more precise temperature sensing and digital set-point control. They are used in tankless water heaters and advanced mixing valve systems.
Programmable thermostats allow scheduled temperature changes, such as reducing water heater temperature during unoccupied hours. Smart versions connect to Wi-Fi for remote monitoring and adjustment.
Mixing valve thermostats are integrated into thermostatic mixing valves (TMVs) that blend hot and cold water to deliver a precisely controlled output temperature, preventing scalding at point-of-use locations.
Water Heater Diagnosis, Zone Thermostat Wiring & Radiant Floor Services
Thermostat diagnosis and replacement are core components of Bonded Plumbworks’ water heater services. When a water heater produces water that is too hot, too cold, or inconsistent, the thermostat is one of the first components tested. Bonded Plumbworks technicians verify thermostat accuracy using calibrated thermometers and replace faulty units to restore safe, reliable temperature control.
For homes with radiant floor heating, Bonded Plumbworks installs and configures zone thermostats that independently control each heating loop through the radiant heating service, optimizing comfort and energy efficiency room by room.
ASSE 1017, UL 174 & ASHRAE Scald Prevention Temperature Guidelines
ASSE 1017 defines the performance requirements for temperature-limiting devices on hot water systems. The State building codes require anti-scald protection on all new plumbing installations, which thermostatic mixing valves fulfill. UL 174 covers the safety standards for household electric storage tank water heaters, including thermostat performance. ASHRAE guidelines recommend 140 degrees Fahrenheit for water heater storage with TMVs limiting delivery to 120 degrees.
Honeywell, Watts IntelliStation, Rinnai & Ecobee/Nest Smart Controls
Honeywell manufactures water heater thermostats and gas valve assemblies for major water heater brands. Watts produces the IntelliStation series of digital thermostatic mixing valves. Rinnai integrates electronic thermostats into their tankless water heater control systems. Ecobee and Nest produce smart thermostats used for hydronic heating zone control.