When Should I Set My Hot Water Cylinder to Come On?

One of the most common questions homeowners ask is: When should I set my hot water cylinder to come on each day?

An unvented hot water cylinder is typically programmed to operate at specific intervals rather than continuously. We recommend setting it to heat three times a day for standard use and all day when accommodating extra guests.

Why Is Timing Important?

Each time hot water is used (e.g., for showers, washing dishes, or handwashing), cold water automatically refills the cylinder. In winter, this incoming water can be extremely cold—sometimes just a few degrees Celsius. As cold water mixes with the stored hot water, overall temperature drops, making precise programming crucial.

Recommended Cylinder Programming Examples

Here’s how different programming setups impact hot water availability:

Example 1: Standard Programming

ON: 6:00 AM
OFF: 9:00 AM
ON: 6:00 PM
OFF: 9:00 PM

At 9:15 AM, someone takes a shower, using one-third of the tank’s hot water. However, since the programmer doesn’t turn the heating back on until 6:00 PM, the cylinder remains partially filled with cold water. The hot water will still be usable but not at maximum temperature due to mixing.

Example 2: When Extra Hot Water Is Needed

ON: 6:00 AM
OFF: 9:00 AM
ON: 6:00 PM
OFF: 9:00 PM

At 9:15 AM, someone takes a shower, using one-third of the tank’s hot water. Then, at 12:00 PM, another shower is taken, further reducing the available hot water. Unless the programmer is set to come on midday, another one-third of the tank is used, leaving the remaining water tepid or cold until the next scheduled heating at 6:00 PM.

This is why it’s beneficial to set the cylinder to come on three times a day—morning, midday, and evening.

By including a midday heating cycle, any cold water that has filled the tank gets the opportunity to heat back up to the target temperature, ensuring consistent hot water throughout the day.

What Is the Ideal Temperature for a Cylinder Thermostat?

The optimal thermostat setting for an unvented cylinder is 65°C. This prevents bacterial growth, ensures efficient energy use, and provides comfortable water temperatures.

Weekend Considerations

Lifestyle differences on weekends may require a custom schedule:

People tend to wake up later, so you may want your first heating cycle to start later in the morning.
Heavy hot water usage (like entertaining guests) might require an extra heating cycle in the afternoon.

Example: If you’re hosting guests on Sunday, you might want to adjust your hot water program:

ON: 4:30 PM (instead of 6:00 PM) → ensuring piping-hot water for showers after lunch.

Should You Set Your Hot Water Programmer to “ALL DAY”?

You can set your hot water to constant if you have guests using it frequently, but it does not waste excessive energy because:

Cylinders are well-insulated—they retain heat efficiently without frequent reheating.

The thermostat prevents unnecessary heating—when water reaches 65°C, the heating stops.

However, daily all-day programming is not recommended because:

Each time you use hot water, the cylinder senses temperature loss (e.g., dropping to 59°C) and fires up the boiler even if only a small amount of heat is needed.
It’s best for temporary high-demand situations (like accommodating guests).

How Long Does a Hot Water Cylinder Take to Heat?

A typical unvented cylinder heats in 60-90 minutes, but time varies by capacity:

Capacity (litres)	Estimated Heating Time
90	~60 minutes
120	~70 minutes
150	~75 minutes
170	~80 minutes
200	~90 minutes
250	~100 minutes
300	~120 minutes
400	~140 minutes