Cylinder thermostats are energy-saving, safety devices. They control the heating element of your hot water cylinder by measuring the temperature of the water inside and switching the heat supply on and off to ensure water temperature stays within the acceptable range.

If the water temperature falls below the required minimum temperature, the heat supply (boiler or electricity) will start up. The New Zealand Building Code requires the minimum temperature inside a hot water tank to be 65^oC. This is hot enough to inhibit the growth of harmful bacteria like Legionella. Once the desired temperature is reached, the thermostat will turn off the heat supply again.

On the other hand, if the temperature exceeds the maximum setpoint, the cylinder thermostat will switch off the heat supply to avoid unnecessarily overheating your water. This saves a lot of energy and obviously money on your energy bills.

As the water in the tank sits unused, it will start to lose some heat. If the temperature drops below the low limit, the thermostat will kick in to reheat the water and maintain the temperature.

Some hot water tanks will have either a manual or an auto-reset thermostat. So what’s the difference?

Manual-reset thermostat

Standard domestic electric water heaters’ control circuit consists of a manual-reset high limit switch, a lower thermostat, an upper thermostat, two heating elements and wires.

The upper element is energized when power is initially turned on to the unit. This heats the water in the upper third of the hot water tank, where hot water is drawn when you open a tap inside the house. When the desired temperature is reached in the upper third of the tank, power is switched to the lower heating element which continues to heat the water in the lower part of the tank until the lower thermostat setting is reached.

If the temperature inside the tank reaches 85^oC (170^oC), the high limit switch will be tripped. To ensure the over-temperature and energy cutout is set, you need to press the ‘reset’ button on the thermostat. See samples below:

Auto-Reset Thermostat

Just as the name suggests, an auto-reset thermostat has the capability to reset itself.

This is often offered as an upgrade feature for solar and hot water heat pump applications. Generally, solar water heaters generate tons of heat and have a tendency to heat water beyond the thermostat settings. This would result in frequent tripping of the high-limit switch, hence the need for an auto-reset thermostat.

Manual-reset thermostats may be used in split solar hot water systems, but it would need replacing every few years.

Some modern hot water cylinders also provide auto-reset thermostats as an option, allowing the water to remain within the ideal temperature range without any intervention.

The ‘reset’ button

The thermostat isn’t the only part of your electric water heater that can trip the reset button. If you are finding yourself having to reset the water heater too often or if you run out of hot water frequently, one of the following might be to blame:

• Loose wiring. A loose wire in the heating element cause excess heat that will activate the high-limit switch.
• Faulty high limit switch. If this device starts malfunctioning and consequently overheats the water, the switch will trip.
• A short in the heating element. The thermostat may still work if there’s a short in the heating element, but the water will continue to be heated beyond the recommended temperature and trip the high-limit switch.
• The problem may also be the electrical setup in your home.

If you suspect any of these, you should get a professional in to inspect your water heater.

More information about thermostats

If your hot water cylinder doesn’t have a thermostat or has a faulty one, do not attempt to install or replace it yourself. This is a job for a licensed and qualified tradesperson. There are electrical wiring and plumbing regulations and standards that must be followed. Also, it can be dangerous to an untrained person. Be sure to have the minimum limit set to 65^oC to disinfect your water supply of harmful bacteria.

Please remember that setting the thermostat to a higher setting does not make the water heat up faster. Your heating system is designed to heat up water at a predefined rate, and raising the setting too high will simply result in wasted energy and even risks of scalding.

Since a temperature of 65^oC may be too hot for most people, it is advisable to install a thermostatic mixing valve. This device mixes hot and cold water so you will get the right temperature from your taps and showers.

For more information about hot water cylinder thermostats, call the experts at Hot Water Cylinders NZ on 0800 429 546.