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Tankless Water Heaters and Flow Switches

A flow switch in a tankless water heater is used to detect the presence of water flow and activates the system to start heating water. Whenever hot water heated is needed, a reed sensor within the tankless water heater identifies the need and initiates the process of heating the water.

A “tankless” (aka “instant”) water heater provides hot water on demand, rather than storing it in a tank. Tankless water heaters are more energy-efficient and provide endless hot water, as long as you have a gas supply or other heating element to turn the unit on. Tankless gas water heaters produce between 4 and 8 gallons of hot water per minute (GPM), while electric units produce 2 to 4 GPM. Tank models are usually in the range of 40 to 80 gallons per hour (GPH). Tankless units are generally rated in terms of “energy factor,” which is expressed as a decimal number rather than a gallon-per-hour rating, e.g., 0.75 for an electric unit or 0.5 for a gas unit.

The Tankless water heater contains a reed switch which is either open or closed, depending on whether there is continuity through the switch. The reed switch detects the presence of water in its vicinity, and it can be used to turn on other devices. If the tankless hot water system is controlled by an electronic thermostat, the reed switch is used to activate the electronic system upon demand. When the water heater receives a hot water call, and it detects that no water is flowing (indicating that there isn’t a pipe leak), it will wait until 5 seconds or more after detecting flow before turning on its burner.

If this flow sensor is not working, the tankless heater may run continuously without heating water.

Can Tankless Water Heaters Run Without Flow Sensors?

The answer to this question is yes and no. Some models of tankless water heaters can be activated via their control systems (e.g., electronic thermostats), while other models require that a flow switch be present in the unit to activate it. Tankless units that have a control system – which can detect demand – typically do not require a flow switch, as they will turn themselves on and off based on demand. Tankless units without a control system require a flow sensor for proper operation. Tankless water heaters with their own controls can be activated without flow switches, as the control will detect the water flow and start it. Tankless units with manual controls require a flow switch.

  • Manual Controls: A tankless water heater with a manual on/off control cannot be started without a reed sensor, otherwise we risk burning out the heating element. Tankless units without their own controls (e.g., manual on/off) require a flow switch for proper operation, and the unit will not start without one wired properly to it. Tankless units with their own controls typically do not require a flow switch as they will turn themselves on and off based on demand.
  • Flow Switches: Tankless models that do need a flow switch to turn on typically have a manual on/off control that cannot be activated without water flowing through the heater. Tankless units with their own controls typically do not require a flow switch as they will turn themselves on and off based on demand. Most tankless water heaters will not work properly if they don’t have a flow sensor, but there are some rare models of tankless water heaters that can be activated via their control system, even if they don’t have a flow switch. Tankless units with manual controls require a flow switch.

To summarize: Tankless hot water heater models with manual on/off controls cannot be started without a reed sensor wired to the unit, and these units require a flow sensor for proper operation. Tankless hot water heater models with their own controls typically do not require a flow switch as they will turn themselves on and off based on demand. Tankless heaters with manual on/off controls cannot be started without a reed sensor wired to the unit, and these units require a flow sensor for proper operation. Tankless heaters without their own controls (e.g., manual on/off) require a flow switch for proper operation, and the unit will not start without one wired properly to it. Tankless units with manual on/off controls cannot be started without water flowing through the heater, and these units require a flow sensor for proper operation. Tankless units with their own controls typically do not require a flow switch as they will turn themselves on and off based on demand.

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