Hot Water Too Hot or Too Cold: Thermostat or Stuck Valve?

What temperature should a hot water cylinder be?

Guidance from the Health and Safety Executive on legionella, written for managed buildings, says stored hot water should be held at 60 degrees Celsius or higher and reach the outlets at 50 degrees Celsius within a minute of running. The reason is bacterial growth: legionella grows in warm, stagnant water and the HSE gives 60 degrees as the stored temperature that kills it. The guidance is aimed at employers and those who manage premises, including landlords, rather than owner-occupiers, but the bacteria behave the same way in any cylinder, which is why cylinder thermostats are commonly set around 60.

That leaves a tension. Water stored at 60 degrees can scald at the tap. The usual answer in larger buildings is a thermostatic mixing valve at the outlet that blends stored water down to a safe delivery temperature. In a home, the practical advice is not to drop the cylinder thermostat to a low figure to avoid scalding, because that moves the stored water into the temperature range where bacteria grow. If scalding is a concern, for children or elderly people, ask an engineer about mixing valves at the outlets.

So the number on your thermostat dial is not the temperature at the tap. Pipework loses heat, mixer showers blend, and a thermostat reads the water beside its own pocket, not the whole cylinder.

Why is my hot water too hot?

Water that is hotter than usual means the heat source is running past its set point. There are three usual reasons, listed in order of how easily you can check them.

• Thermostat dial raised: someone has turned the cylinder thermostat up. Check the dial where one is visible and compare it with the usual setting.
• Two-port valve stuck open: on a boiler-fed cylinder, a valve that will not close lets the boiler keep heating the coil after the cylinder thermostat has called for it to stop. A hot flow pipe to the cylinder when the programmer is off is the clue.
• Thermostat failed closed: the cylinder thermostat or the element thermostat no longer opens, so the heat never stops. The high-limit cut-out then trips as the second line of defence.

Why is my hot water too cold?

Too cold has more causes than too hot, because the heat has to arrive and stay. A thermostat set too low is the simplest: the cylinder holds only lukewarm water by design. A failed element or a coil that is not delivering fully means the water never reaches the set point. A cylinder that is too small for the household runs out and then recovers slowly, which feels like cold water rather than a fault.

Separate these with timing. If the water is hot for a short while and then goes cold, the cylinder is losing its stored volume or the recovery is slow, which is a capacity or heat input issue. Our guide to hot water that runs out quickly covers that test. If the water is never properly hot even after a long heat-up, the heat source is failing, and why a cylinder is not heating up explains how to find which one.

A mixer or shower can also fool you. A thermostatic shower that is set low, a blended tap or a pressure imbalance between hot and cold can make a cylinder that is hot appear cold. Test at a plain hot tap away from the shower.

How do I tell a thermostat setting from a stuck valve?

Use the heat source off test. If the water is too hot, switch the boiler hot water channel off and the immersion off, then watch the cylinder over a few hours. If it keeps rising, one heat source is not switching off. If it cools normally, the issue was a setting that has now been interrupted.

Then separate the sources. With the boiler off and the immersion on, a cylinder that stays too hot after the thermostat should have stopped it points to the element thermostat. With the immersion off and the boiler on, a cylinder that overheats points to a stuck two-port valve or a failed boiler-side thermostat. The test order is set out in our two-port valve guide.

What an engineer adds is a measured reading. A thermometer on the outlet pipe or the cylinder surface compares the real stored temperature with the dial. If the two disagree widely, the thermostat is reading wrongly or has lost contact with the cylinder, which is a replacement-part fault, not a cylinder fault.

When is it a repair, and when is the cylinder at fault?

Almost always a repair. A thermostat, a valve head or an element is replaced and the cylinder stays. The cylinder itself becomes the issue only where repeated overheating has damaged it, or where scale has reduced the working capacity so far that it cannot hold or heat enough water.

The next step depends on the symptom. For a cylinder that overheats or delivers scalding water, treat it as a safety fault and have it diagnosed before it is used. For one that is simply not hot enough, work through the checks above and then arrange a hot water cylinder diagnosis, which tests each control and heat source in turn and names the part at fault.