Why you cannot find it
Most searches for a cylinder thermostat start with someone in an airing cupboard looking at a white or beige tank and seeing nothing resembling a control. On a modern cylinder the thermostat is underneath the insulation, not on top of it.
A cylinder thermostat is a bimetallic or capillary switch in a small plastic housing. It has to touch bare metal to read the water temperature, so it is held against the cylinder wall by a spring band that goes right round the tank, and the factory foam is either moulded with a recess for it or cut away where it sits. On an older copper cylinder with a quilted jacket, unclip the straps and it is underneath.
Height varies by manufacturer — some instructions say a quarter of the way up from the base, some a third — but it always sits low enough to sense the coldest useful water in the store, and below the top of the primary coil.
Two exceptions worth knowing before you go looking:
- Unvented cylinders normally do not use a strap-on stat at all. The control thermostat sits in a pocket or faceplate, alongside a separate non-self-resetting overheat cut-out. Approved Document G requires an unvented store to carry at least two independent safety devices in addition to any thermostat, so what you see on the front of the tank is a cluster, not one control.
- Combi boilers have no cylinder and no cylinder thermostat. No tank, no problem here — the hot water temperature is set on the boiler.
What it is actually doing
The stat is not wired to the boiler directly. On the two most common UK layouts it switches a motorised zone valve, and the valve switches the boiler.
- The store falls below the dial setting and the stat closes a contact, sending live to the hot water zone valve.
- The valve drives open and, at the end of its travel, closes a second switch inside the actuator head.
- That switch calls the boiler and pump. The primary coil inside the cylinder heats the store.
- The store reaches the setting, the stat opens, the valve closes, the boiler stops.
That chain is why a hot water fault is so often blamed on the thermostat and so often is not. Four separate things in it can fail, and only one of them is the stat.
The three ways a cylinder stat fails
1. It stops calling for heat
Contacts fail open, or the bimetal loses its travel. Heating works normally, hot water never arrives from the boiler, and an immersion heater will still fill the tank — the clue that the cylinder is fine and the control is not.
2. It calls for heat and never stops
Contacts weld shut, or the sensing face stops reading the real temperature. The second is more common and is almost always installation rather than component failure: the spring band has slackened, foam or a scrap of quilting has worked behind the housing, or the stat was refitted onto paint or lagging instead of bare metal. It reads cold, the boiler keeps firing, the store keeps climbing. This is the failure that produces water where you did not expect it.
3. It is simply wrong
The dial says 60 and the water is 48, or the reverse. Strap-on stats drift, cheap ones faster. The dial is a label, not a measurement. To know the store temperature, run the hot tap furthest from the cylinder for a minute and put a thermometer in the stream.
How a thermostat turns into a leak
Water expands as it heats — roughly four per cent by volume between cold and boiling. A cylinder that keeps heating has to put that extra volume somewhere, and where it goes depends on the kind of cylinder.
| Cylinder type | Where the expansion goes when the stat sticks on | What you see |
|---|
| Vented (tank in the loft) | Up the open vent pipe, back into the cold water storage cistern | A hot loft cistern and a warning pipe running outside. |
| Unvented (sealed, mains pressure) | Into the expansion vessel; when that is exhausted, out of the expansion relief valve | Water through the tundish and out of the discharge pipe, usually low on an outside wall. |
Both get reported to us as leaks. Neither is one. Approved Document G requires a hot water storage system to stop the stored water ever exceeding 100°C and to convey any discharge from a safety device somewhere visible and harmless. Visible is the point — you are meant to see it. What you are looking at is the last line of defence working, which means something upstream has already failed. The trap is that it is easy to dismiss as "it always does that", so it runs for months. More on what a leaking overflow pipe is telling you.
The slower route, which is the expensive one
Thames Water describe all the water in their region as hard, and their own advice for limiting limescale is to bring hot water down to 60°C. Above that, calcium carbonate drops out of solution faster, and in a cylinder it does so on the two hottest surfaces: the primary coil and the immersion element.
A scaled coil transfers heat badly. The boiler runs longer for the same tank and cycles more often, so the primary pipework spends more of its life expanding and contracting. Long copper runs that move repeatedly work loose at compression joints and open soldered joints that were marginal to start with, and scale nucleates pitting under the deposit — one route to pinholes in copper pipe. A stat left at maximum in London is not causing a leak this week. It is quietly shortening the life of the pipework around it.
The temperature question, answered honestly
There is a genuine conflict here, and anyone who gives you a single number without qualifying it is skipping half of it.
Too low is a bacteriological risk. Legionella multiplies in the warm range. HSE's control guidance is that hot water should be stored at at least 60°C and distributed so it reaches 50°C within one minute at the outlets — 55°C in healthcare premises — with cold water kept below 20°C. Those duties fall on employers, landlords and anyone in control of premises. They are not a legal requirement on you in your own home, but the microbiology does not know who owns the building.
Too high is a scalding risk. HSE put the scald threshold at water leaving a tap above 44°C, concentrated on the very young, elderly and disabled people, and anyone with sensory loss. Building Regulations deal with this at the outlet, not the cylinder: where a dwelling is newly built or formed by a material change of use, the hot water supply to a fixed bath must be limited to 48°C by an in-line blending valve or equivalent that occupants cannot easily alter.
And too high is a scale risk, per the section above.
The resolution the industry settles on is store hot, blend cold: keep the store hot enough to be safe biologically, then limit what reaches a bath or basin with a valve. Which is why your setting depends on things we cannot see from here — vented or unvented, whether a blending or thermostatic mixing valve is fitted, whether anyone in the property is in one of the at-risk groups, and what your cylinder manufacturer specifies. With no blending valve between cylinder and taps, turning the store up has consequences at the tap. That is the plumber's call on the day, not a number off a web page.
What is definitely not the thermostat
The stat is a dry electrical switch clamped to the outside of a tank. It holds no water and cannot leak. If there is water at the cylinder, look at these instead:
- The immersion heater boss. A weeping gasket runs down the side of the tank and collects under the lagging, so by the time you see it the insulation is soaked.
- The primary coil tappings and draw-off connections. Compression fittings on a tank heated and cooled for twenty years.
- The cylinder itself. A corroded copper cylinder pinholes low down, where sediment sits.
- No hot water, heating fine. As often the zone valve, its microswitch or the programmer as the stat. All are cheap; guessing between them is not.
- An occasional tundish drip after a long heating cycle on an unvented system, which can be normal expansion relief. Daily or continuous discharge is not.
Where we come in
Replacing a cylinder thermostat is a plumber's half hour and does not need us. We get called to the version where the symptom has run long enough to do damage: a cupboard floor wet for months under the lagging, a ceiling staining below an airing cupboard, a discharge pipe everybody assumed was condensate.
From below, those look identical to a pipe leak, and they are told apart by isolating the cylinder and pressure-testing rather than by looking at a damp patch. If it is the cylinder, the store or a primary connection, that is boiler and heating leak repair, and our own handymen put the floor, ceiling or plaster back afterwards.
Detection is £150 an hour, agreed before we attend, with no call-out fee, and if we do not find the leak you do not pay for the time spent looking. The breakdown is on our pricing page. If your cylinder simply is not getting hot and nothing is wet, call a plumber, not us.
