Heating 28 August 2026

What Temperature Should a Hot Water Cylinder Be Set To?

A hot water cylinder should normally be set to around 60°C. That is the figure our engineers set cylinders to across Kent, and it exists because stored water has two competing problems: too cool and bacteria can multiply in the tank, too hot and the water coming out of a tap can scald someone in seconds.

Everything else about cylinder temperature — the dial on the side, the boiler setting, the immersion, the shower that has gone lukewarm — follows from that one balance. Here is how it actually works on a real system.

Why 60°C and not lower

Water sitting in a cylinder is different from water running through a combi. It is stored, sometimes for hours, at whatever temperature the thermostat holds it at. Legionella bacteria thrive in warm, still water, and the accepted way to deal with that in a domestic system is heat: keep the stored water hot enough that the bacteria cannot establish, and make sure the whole cylinder gets up to temperature rather than just the top of it.

Turning a cylinder down to 45°C or 50°C to save money is the single most common mistake we find. It does save a little energy. It also leaves a large tank of tepid water sitting in an airing cupboard, and it gives you noticeably less usable hot water, because the hot water you actually use is blended with cold at the tap. Drop the stored temperature and you have to draw more of the tank to fill the same bath.

Why not higher, then?

Because water at 60°C and above will scald, and it does not take long. That is why the guidance is to store hot and deliver cooler — the cylinder stays at around 60°C, and the temperature at the outlet is brought down where it matters.

In practice that means:

• Baths and basins in homes with young children, elderly or vulnerable occupants should be protected by a thermostatic mixing valve, which blends the stored hot water down to a safe outlet temperature. In new bathroom installations this is a requirement for baths, not an optional extra.
• Kitchen taps are usually left unblended, because you want genuinely hot water for washing up and the risk profile is different.
• Showers with a thermostatic mixer are already doing this job for you.

Running the cylinder hotter than about 60–65°C also accelerates scale. Across most of Kent the water is hard, and the hotter you store it, the faster limescale drops out onto the coil and the immersion element. You get a shorter cylinder life and slower reheats in exchange for water you cannot safely use anyway.

Where is the thermostat on a hot water cylinder?

On most cylinders it is not on the front where you would expect it. Look for a small plastic box strapped to the side of the cylinder, roughly a third of the way up, usually under the insulation foam with a cable running out of it. Inside there is a dial marked in degrees, adjusted with a small screwdriver rather than by hand.

That stat does one job: it tells the boiler or the pump when the stored water has reached temperature and the cylinder no longer needs heat. It is not a timer and it is not a control you use day to day.

Some systems have a second temperature control you can also see:

• The immersion heater has its own thermostat inside the element boss, set separately. On a system with both, the boiler normally does the work and the immersion is the backup.
• The boiler’s hot water dial sets the flow temperature going out to the cylinder coil. If that is set too low, the cylinder can never reach its own stat setting no matter what the stat says, and the boiler will sit there running for a very long time trying.
• The programmer or timer decides when the cylinder is heated at all. On an unvented or system setup, hot water is a scheduled event, not something that happens on demand.

If your hot water has gone lukewarm, work down that list before assuming the cylinder has failed. More often than not the answer is in one of those four settings rather than in the tank.

The signs a cylinder stat has drifted

Thermostats age. The ones we replace most often are not dramatically broken — they have simply started switching off early, so the boiler stops heating before the tank is genuinely up to temperature. The pattern looks like this:

• Hot water that runs out sooner than it used to, with no change in how much anyone is using.
• Water that is hot at the tap but not properly hot — fine for washing hands, disappointing in a bath.
• A cylinder that reheats far more often than the schedule suggests it should.
• The bottom of the cylinder staying cool for a long time after a reheat.

That last one matters. A stat mounted too high, or one that has come loose from the cylinder wall under the insulation, only ever sees the top of the tank. The top comes up to temperature quickly, the stat is satisfied, and the bottom two-thirds of your stored water never gets there.

We wrote separately about the related problem of hot water fading away over months rather than years — scale, expansion and pressure faults — in our piece on unvented cylinders running out of hot water. If the temperature is right and the volume still is not, that is the one to read next.

Does it matter whether the cylinder is vented or unvented?

The target temperature is the same either way. What changes is what else is involved in getting there and who is allowed to touch it.

A vented cylinder is fed from a cold tank in the loft and is not under mains pressure. An unvented cylinder is at mains pressure and carries safety controls — an expansion vessel, a pressure-reducing valve, a temperature and pressure relief valve and a discharge pipe. Work on those controls is notifiable and must be done by an engineer holding the relevant G3 qualification. Setting the stat is not G3 work; touching the safety controls very much is. If you are not sure which you have, our guide to a vented or unvented hot water cylinder explains how to tell at a glance.

Anything involving the gas side of the boiler — the flow temperature is a control, but the appliance behind it is not something to open up — must be handled by a Gas Safe registered engineer. We are registered under number 951167.

What to do before the heating goes back on

Late summer is the sensible time to sort this. Hot water complaints climb the moment the heating comes on for the first time, because the boiler is suddenly splitting its work between the radiators and the cylinder, and any weakness in the hot water side shows up immediately.

Two things are worth doing now:

1. Check the stat is actually at 60°C and that the boiler’s hot water flow temperature is set high enough to get the cylinder there.
2. Get the system serviced while it is still under light load. A service is when we check the cylinder stat position and setting, the reheat behaviour and the controls alongside the appliance itself — not just the boiler in isolation.

A boiler service with us is a flat £95+VAT, and any additional appliance on the same visit is half that. If something needs repairing, that is £95+VAT for the first hour, always billable, then £47.50+VAT for each hour after, and any parts are confirmed and agreed with you before we fit or order them.

Talk to us

If your hot water is not as hot as it should be, or you are not sure what your cylinder is set to and would rather someone checked properly, give us a ring on 01732 792 454. We will tell you what is actually wrong before we do anything about it.

You can book a boiler service online, or see the full range of plumbing work we cover across Kent.