Start with what has not changed
Under the controls, a modern cylinder is the same object it has been for years: a welded stainless steel vessel with a set of threaded openings in it. Those openings are the interesting part, because water only ever escapes a cylinder at a joint, at a boss, at a relief device, or through a hole in the shell itself. Adding a circuit board does not change that list.
On a typical unvented cylinder you have a cold feed carrying a combination inlet valve group — a pressure reducing valve, a check valve and an expansion relief valve — a hot outlet, one or two immersion heater bosses, a coil flow and return if a boiler or heat pump feeds it, a temperature and pressure relief valve, a drain point, an expansion vessel either strapped on or built in, and one or more thermostat pockets. Every one of those is a seal someone made on site or in a factory, and every one of them is a candidate.
What is genuinely new about a smart cylinder
Mixergy and the stratified cylinders that work on similar principles change how the stored water is heated and measured rather than what contains it. Instead of treating the tank as one body of water with a single thermostat, they heat from the top down and model how much usable hot water is actually charged, so a part-full cylinder is a smaller quantity of properly hot water rather than a whole tank of lukewarm. They can be told what to do by an app, a tariff, a solar PV diverter or a heat pump controller.
Doing that needs hardware a plain cylinder does not have: multiple temperature sensors up the height of the vessel, a control board and its power supply, and — depending on the model and how it is charging — an internal circulation route with a pump or a motorised valve on it. That is the honest answer to what is new. It is instrumentation and control, plus a small number of extra water-carrying components.
More components means more joints. It does not automatically mean more leaks, because the added parts are mostly factory-fitted and the ones that fail most often on any cylinder are the ones an installer made on site. But it does mean more places to check, and it means a cylinder cupboard now contains electronics that will not enjoy being rained on by a weeping fitting above them.
Heat-pump-ready cylinders: a much bigger coil
A heat pump delivers water at a far lower flow temperature than a gas boiler, so a cylinder designed for one carries a much larger internal heat exchanger to move the same heat across a smaller temperature difference. Coils sized for heat pumps are often several times the surface area of a boiler coil, and the tappings that serve them are larger.
The failure mode is not new, but its consequences are distinctive. A coil is a pipe inside a tank, with mains-pressure stored water on one side and your sealed heating circuit on the other. If it perforates, the two sides connect. Which way the water moves depends on which side holds more pressure, and both directions cause a problem:
- Stored water into the heating circuit. System pressure climbs, the boiler or heat pump relief valve lifts, and water appears at the discharge outside. It reads as a pressurising fault, not a leak.
- Heating water into the stored water. Discoloured or metallic-smelling hot water at the taps, inhibitor disappearing from the heating circuit, and a system that loses pressure with nothing wet anywhere in the building.
A perforated coil is one of the few cylinder faults that genuinely presents as an invisible leak, and it is a common reason a heating circuit that keeps losing pressure has no wet patch to show for it. It is also one of the easier ones to prove: isolate the coil, pressure-test it separately from the vessel, and watch which side moves.
The failure points shared with every cylinder
| Where | What it looks like | Smart-specific? |
|---|
| Immersion heater boss | A slow weep under the immersion cover, often only when the element is hot and the gasket has relaxed | No |
| Coil flow and return tappings | Damp on the threads, a rust-coloured trace on the insulation below | No |
| Combination inlet valve group | Water at the expansion relief outlet, usually a failed expansion vessel rather than a failed valve | No |
| Temperature and pressure relief valve | Hot water at the tundish and the discharge pipe outside. Never a nuisance to be capped | No |
| Sensor pockets and probe entries | A weep at the pocket thread, or condensation mistaken for one | Partly — more of them |
| Motorised or diverter valve on an internal charging loop | A weep at the valve body or its unions, often only under pump pressure | Yes |
| Shell or weld | Rare, and usually terminal. Pitting corrosion in stainless steel, or a failed weld | No |
| Pipework within a metre of the cylinder | Compression nuts, flexible connectors, push-fit at an awkward angle | No — and this is what it usually is |
That last row is worth sitting with. When someone calls about a leaking cylinder, more often than not the cylinder is fine and the water is coming from a fitting on the pipework serving it, or from the relief system doing exactly what it was built to do. The cylinder is simply the biggest object in the cupboard, so it gets the blame.
A fault on the app is not a leak
This is the bit that is specific to smart cylinders, and it causes real confusion. A cylinder with an app can tell you something is wrong. What it cannot tell you is that water is escaping, because nothing in it measures that. A fault notification on a connected cylinder is almost always one of:
- A sensor reading out of range, or a sensor that has stopped reporting.
- A lost network or gateway connection, so the unit is fine and the app is blind.
- An immersion element or relay fault, which stops it heating.
- A control board or firmware error.
None of those is a leak. Equally — and this matters more — a cylinder reporting no faults at all can still be weeping at a tapping, and a leak on the pipework two feet away will never appear in any app. The instrumentation watches temperature and charge state. It does not watch the floor.
So the first move on a fault notification is the same as it has always been: look. Torch, dry hand, the underside of every fitting, the floor of the cupboard, the ceiling of the room below. If everything is dry, the notification is an electrical or control matter for the manufacturer's support route or the installer, not a leak detection job, and we will say so before we charge you for a visit.
Who should touch it
Unvented cylinders in England and Wales come under Part G3 of the Building Regulations. Installing, replacing or altering one is notifiable work for a competent person holding the relevant unvented qualification, and the discharge pipework is a safety system that must never be capped, plugged or routed somewhere it cannot be seen at the tundish. Smart cylinders add a second competence on top: the controls, firmware and warranty terms are the manufacturer's territory, and unpicking a control fault without their documentation wastes everyone's time.
Our half of it is the water. We find where it is escaping, prove it, and repair what is in our scope — pipework, fittings and the joints around the cylinder. If the answer turns out to be a perforated coil or a failed vessel, we say so in writing and the cylinder goes to whoever holds the warranty. That report is written so it can support a trace and access claim on the water damage, which is a separate question from whether the manufacturer covers the cylinder. What a loss adjuster looks for is set out under insurance claims and trace and access.
Where the honest gaps are
Smart and heat-pump cylinders have not been in British houses long enough for anyone to have a credible long-term failure picture. Plain unvented cylinders have three decades of field history behind them; a stratified cylinder with a control board and a large heat-pump coil does not. Anyone quoting you a confident failure rate for this equipment is guessing.
What can be said is narrower and more useful. The water-carrying parts are conventional parts, and they fail in conventional ways. The added electronics fail as electronics do, which is visibly and without flooding anything. And the one component whose long-term behaviour is genuinely untested at scale is the large low-temperature coil, because it is newly common rather than newly invented.
If you want a figure before anyone attends: our rate is £150 per hour, agreed before we come out, with no call-out fee. If we do not find the leak, you do not pay for the time we spent looking. Most cylinder cupboards are resolved well inside the first hour, frequently by ruling the cylinder out entirely.
