How Do Tankless Water Heaters Know When to Switch On?

Posted: Dec 04, 2012
Author: Minyard Plumbing, Inc.

Unlike storage water heaters that keep a tank full of water heated to a temperature that is controlled by a thermostat, tankless water heaters, like those made by Rinnai, work on an “on demand” basis. This means that the heater only switches on when hot water is required, heats as much water as is needed, and then switches off afterwards.

Here is a graphic showing the layout and components of a typical tankless water heater installation.

1. Exhaust

2. Heat exchanger

3. Burner

4. Fan

5. Control panel

6. Flow sensor

7. Hot water to faucet

8. Gas supply

9. Cold water coming in

10. Cold water to faucet

How does the system know when to switch on and off? The answer lies in the item numbered 6. This is a flow sensor, sometimes with a built in switch and sometimes wired to a separate switch. Below is an example of what one type of flow sensor and switch looks like.

Once the hot water faucet is opened, water begins to flow in the pipes. The flow sensor senses this movement and switches the tankless water heater on, so that it begins heating water. Once the faucet is closed and there is no longer movement in the pipe, the unit will shut down automatically.