Technical bulletins
October 2018

Staying out of hot water - The basics of the control systems for hot water heating appliances

By Ryan Hazlett P.Eng., CFEI,

We use hot water for showering, cooking, clothing and dishware sanitation and space heating, so it is likely no surprise that water heating appliances are used in almost every household and business in the country. We generate hot water from a variety of appliances including hot water tanks, instantaneous hot water heaters, and conventional boilers. These appliances use a variety of energy sources including propane, natural gas and electricity. Despite the large variation in styles and methods used to heat water, the control systems, which make them operate safely, are actually fairly similar.

Below are descriptions of some of the major control systems in a hot water heating system:

  1. The Gas Regulator: Obviously only used in gas fired appliances, this device uses a diaphragm and spring to ensure gas pressure is taken from the utility supply pressure and dropped to the specific pressure acceptable to the appliance. The regulator will generally have an adjustment screw which adjusts the compression on the spring and allows the outgoing pressure to be altered. An increase in gas pressure can cause overheating, dry firing and incomplete combustion as well as flame impingement (when flames directly touch metal surfaces) and pre-mature failure of combustion chambers.
  2. The Gas Valve: The gas valve takes an electronic signal from the thermostat or temperature controller and allows gas into the appliance to begin the heating cycle. Generally, these valves are failsafe. In other words, they will shut when they lose power in order to prevent gas from inadvertently flowing through the appliance. Sometimes, they can be built as a unit with the regulator and are then referred to as a combination valve.
  3. The Flame Sensor(s): The flame sensor is a device which sits directly in contact with either the pilot (if applicable) or burner flame in order to verify the presence of a flame. It frequently contains a thermocouple and when heated, will send an electrical signal to the gas valve or circuit board to indicate a flame is present.
  4. The High Limit Switch: Usually consisting of a temperature measurement device placed at the water outlet from the appliance, this device will open the electrical circuit to the gas valve if a pre-determined temperature is reached. Since the gas valve closes when it loses power, the open high limit will shut the gas flow to the appliance causing it to stop heating. If the appliance's normal set temperature is higher than the high limit temperature you have a condition known as "high-limit cycling" where the boiler is being turned on and off by the high limit switch rather than the thermostat. This is hazardous and can cause pre-mature failure of the high limit switch.
  5. The Low Water Cut-Off: The low water cut-off is an electrical sensor which relies on the contact of water to pass a signal through to the gas valve. Similar to the high limit, if water is no longer touching the cut-off sensor, the electrical signal will open and the gas valve should close.
  6. Flow Switch: A flow switch can be thought of as a small paddle inserted into the flow of water running through the appliance. When there is adequate flow, the pressure exerted on the paddle closes electrical contacts which allows the electrical signal to pass through to the gas valve. If the flow stops at any point, the paddle will move backwards and open the electrical contacts, closing the gas valve.

Boilers, hot water tanks, and other water heating systems are generally equipped with a large number of safety devices which control their operation. Typically, when a catastrophic failure occurs one or more of these safety devices is compromised and allows the appliance to run well outside of specifications. CEP-Sintra has a team of multidisciplinary engineers including electrical, mechanical and fire and explosion specialists to diagnose the cause and contributing factors of water heating appliance related losses.

If you have any questions or would like to learn more about this topic, please contact our Mechanical Engineering team at 877 686-0240 or info@cep-sintra.ca

Water Heater Controls