What temperature should my hot water be set to?

There are two competing risks. Storing water too cool allows legionella bacteria to multiply. Delivering it too hot scalds. The resolution is to store hot and deliver cooler, which is why blending valves exist.

Safety

What you'll need

• Access to the cylinder thermostat, on a vented system
• A thermometer
• A qualified installer for anything unvented

Step by step

1. Identify what you have.A cylinder in a cupboard with a cold tank in the loft is vented. A cylinder with no loft tank, with a pressure relief pipe and an expansion vessel, is unvented. A boiler with no cylinder at all is a combi.
2. Stop here if it is unvented.Unvented cylinders are pressure vessels with layered safety controls and legal requirements. Adjustment and servicing are for a G3 qualified installer only.
3. Set a vented cylinder to 60 degrees.That is the widely recommended storage temperature for controlling legionella in domestic hot water. The cylinder thermostat is usually a strap-on dial about a third of the way up the cylinder.
4. Do not store below 55.Legionella multiplies in the range roughly 20 to 45 degrees. Turning a cylinder down to save energy moves the stored water into that range.
5. Measure what actually arrives at the tap.Run the hot tap and put a thermometer in the flow. On many systems it will be a few degrees below the storage temperature.
6. Deal with scald risk at the outlets.Thermostatic mixing valves fitted at a bath or basin blend the stored hot down to a safe delivery temperature, typically 43 degrees at a bath. That is the correct approach with children or vulnerable adults.
7. Set a combi differently.A combi has no stored water, so legionella is not the same concern. Set the hot water temperature to what is comfortable, usually somewhere around 50 to 55, so you are not mixing in large amounts of cold.
8. Insulate the cylinder and the pipes.A well insulated cylinder loses far less standing heat, which is a much better saving than lowering the temperature. Check the jacket or the factory foam is intact.
9. Insulate the first metres of pipe.The pipe leaving the cylinder loses heat continuously by convection. Lagging it is cheap and pays back quickly.
10. Check the thermostat is reading correctly.If the measured tap temperature is far from the setting, the thermostat may be loose against the cylinder or faulty. On a vented cylinder, a strap-on thermostat is inexpensive to replace.

Tips

• Store at 60, deliver at 43 to a bath. Two different numbers doing two different jobs.
• Insulating the cylinder and the pipes saves more than lowering the temperature and carries no health trade-off.
• If you ever suspect the cylinder has been sitting cool for a long period, run it up to 60 for an hour before use.

Common mistakes

• Turning the cylinder down to 45 to save energy — It puts the stored water into the range where legionella multiplies most readily. The energy saving is small and the risk is not worth it — insulate instead.
• Adjusting an unvented cylinder's controls — Unvented cylinders are pressure vessels with statutory requirements. Their temperature and safety controls must only be adjusted by a G3 qualified installer.

If it doesn't work

Questions people ask