Count the connections and the answer stops being surprising
A plain indirect cylinder has a cold inlet, a hot outlet, one coil with a flow and a return, an immersion boss, a drain, a thermostat pocket and — if it is unvented — an inlet control group and a relief valve or two. Call it ten places water can get out.
A twin-coil cylinder adds a second coil with its own flow and return, a second sensor pocket, and usually a whole second primary circuit behind it: pump, expansion vessel, filling arrangement, air vent and safety valve. A heat pump cylinder adds a much larger coil, a sensor pocket for the controller, and in most retrofits a buffer vessel or low-loss header nearby with four more connections of its own.
Nothing about that is exotic. It is more joints, and joints are where systems leak. If you have been told a solar or heat pump cylinder is "more likely to leak", that is the honest version of the claim: more connections, more circuits, more filling points, more things a fitter had to get right on the day.
The connections a plain cylinder does not have
The second coil's flow and return
On a twin-coil cylinder the lower coil takes the solar or heat pump primary and the upper coil takes the boiler. Both sets of tappings are welded or brazed into the vessel and then connected with compression or press fittings. A weep at the coil tappings behaves exactly like a weep at the primary coil connections on any cylinder: a slow wet track down the case, insulation that stays damp, and a drip that appears on the floor a surprising distance from the fitting. The tell is that the water is primary water — cloudy, sometimes black, sometimes dyed — rather than clean hot water.
The solar or heat pump primary's own filling arrangement
This is the part people forget exists. A solar thermal primary has its own expansion vessel, its own pressure gauge, its own safety valve discharging into a catch vessel, and its own filling point. A heat pump primary has the same set of parts, usually inside or beside the indoor unit. That circuit can lose pressure entirely independently of your boiler, and on a lot of installations nobody ever looks at its gauge.
If your boiler holds pressure fine but the solar or heat pump side keeps dropping, the leak is on that circuit — which is a different search, on different pipework, often running up into a loft or out to an outdoor unit. Our page on a boiler losing pressure covers the single-circuit version of the same logic.
The extra sensor pockets
Every pocket is a hole in the vessel with a gland sealing it, and these cylinders have two or three where a plain one has a single thermostat pocket. They rarely fail, but when one does the water runs along the sensor cable before it drips, so the source is never where the puddle is.
The blending valve and the high store temperature
Solar cylinders are often allowed to run far hotter than a boiler-only cylinder would, with a thermostatic blending valve downstream keeping the tap temperature safe. Hotter water is harder on seals, on the immersion flange gasket and on the blender itself. A weeping blender on the hot outlet is a common find, and gets mistaken for a cylinder failure because the water is hot, clean and right at the top of the vessel.
The buffer or volumiser on a heat pump install
Many UK heat pump retrofits include a buffer vessel, a volumiser or a low-loss header to guarantee system volume for defrost cycles. It is a pressurised steel vessel with four or more connections, usually in the same cupboard, and it is a common source of the puddle that gets blamed on the cylinder. Look at it before condemning anything.
Glycol leaks do not behave like water leaks
Most solar thermal primaries, and some heat pump primaries where pipework runs through cold spaces, are filled with a glycol antifreeze mixture rather than plain water. That changes what a small leak looks like, which is useful, because it tells you which circuit failed without opening anything.
| What you see | Clean water | Glycol mixture |
|---|
| Colour | Clear, or rust-brown if it is old primary water | Usually dyed — pink, red, blue or green depending on the brand |
| When it dries | Evaporates and leaves a faint tide mark | Leaves a sticky, slightly tacky film that stays sticky |
| Dust and grit | Washes away | Sticks to the residue, so the track darkens over time |
| On plaster and timber | Water stain, dries out | A stain that keeps drawing moisture back in and often will not dry properly |
| Smell | Metallic if it is system water | Faintly sweet |
A sticky coloured film on the underside of a fitting, or a dark tacky track down the cylinder case, is a glycol loss. That puts the fault on the solar or heat pump primary rather than on the domestic hot water side, and it also means the circuit is losing its freeze protection. Topping that circuit up with plain water dilutes the mixture, and a diluted mixture is a circuit that can freeze in a loft or an outdoor run the following winter. It needs refilling with the correct concentration, not with a hose.
One practical point: glycol is not drinking water and must not be able to get back into your mains. The Water Supply (Water Fittings) Regulations set out the backflow protection required where a non-potable fluid can meet the supply, which is why a glycol primary is filled through a break or a dedicated filling station rather than through a braided loop off the cold main.
Where the honest answer is "nobody knows yet"
You will find pages online quoting failure rates and expected lifespans for heat pump cylinders. Treat them carefully. The UK installed base of domestic heat pumps is recent, grant-driven and still small relative to the boiler stock, and the cylinders that went in alongside them have mostly not been in service long enough to produce meaningful long-run failure data.
Specifically, these things are not established:
- How long a heat pump cylinder's coil lasts in UK water. The coils are much larger in surface area than a boiler coil because they run at lower flow temperatures. Whether that larger area ages better or worse in hard London water is not something anyone can answer from field data yet.
- Whether twin-coil vessels fail earlier than single-coil ones. More welds is a reasonable theory. It is a theory.
- What proportion of early heat pump cylinder leaks are installation faults rather than product faults. Our own impression, which is an impression and not a statistic, is that a large share of what we attend on these systems is at the connections rather than in the vessel — but a leak specialist sees leaks, not the installations that are fine, so that impression is biased by definition.
What we will tell you on the phone is what we can see and what we cannot. If you want a number for how long your cylinder will last, we do not have one, and anyone who gives you one for a heat pump cylinder in 2026 has made it up.
Things that are not a leak
- Condensate from the heat pump or its pipework. Indoor units and cold primary pipework shed condensate, and in a cupboard that looks exactly like a small leak. It is clear, appears only when the unit runs, and forms across cold surfaces rather than tracking from one point.
- Discharge from a relief valve. Water at the tundish or outside is a valve responding to a fault elsewhere — expansion vessel, thermostat or pressure control — not a hole in anything.
- Condensation on the cold feed. An even film on a cold surface, not a track with a source.
What we do and what we do not touch
We find the leak and repair the pipework and fittings. On these systems that usually means isolating each circuit separately, pressure-testing them one at a time to establish which one is losing, then tracing that circuit — thermal imaging follows warm primary runs through floors and walls well, and tracer gas finds what thermal cannot on a drained circuit.
What we do not do: work on the sealed refrigerant side of a heat pump, which is F-gas work for the manufacturer or installer, and replacement of an unvented cylinder or its safety components, which is controlled work requiring the relevant qualification. If your system is still inside its installation warranty, call the installer first — opening their work can affect it, and a leak at a connection they made is their problem before it is yours.
What it costs
£150 per hour, agreed before we attend, with no call-out fee. If we do not find the leak you do not pay for the time we spent looking. The pricing page has the full breakdown, and boiler and heating leak repair covers what a primary-circuit job involves.
These systems take longer than a plain cylinder because there are two or three circuits to isolate and test rather than one, and because the buffer vessel and the indoor unit have to be ruled out. Budget accordingly. If water has damaged the building and you intend to claim, the visit includes a written report prepared to support a trace and access claim — what an insurer-ready report contains sets out what goes in it. Your insurer decides what the policy covers; we do not.
