Age is the worst predictor on the list
People ask how many years they should get. It is a fair question with an unsatisfying answer: the honest range is wide, roughly ten to twenty-five years for a domestic cylinder, and we have seen both ends of it in the same street. Two identical cylinders fitted the same week can be eight years apart at the end, and the difference is almost never the badge on the front.
These are ranges, not specifications. No one can give you a real figure for your cylinder without looking at it. What we can do is tell you what the variables are, because once you know them you can usually place your own cylinder in the range yourself.
The four things that actually decide it
1. How hard your water is
Every drop of water in the Thames Water region is hard — the company says so itself, because the supply passes through chalk and limestone before it reaches you. Hard water and stored hot water are a bad combination. Above roughly 60°C, calcium comes out of solution and forms scale, and scale goes straight to the hottest surface it can find.
In a direct cylinder that is the immersion heater element, which scales, overheats and fails. In an indirect cylinder it is the coil, which loses efficiency long before it fails, so the first sign is that the hot water runs out sooner than it used to rather than anything dramatic. Scale also settles in the base of the vessel, where it holds a damp layer against the metal and gives corrosion somewhere to start.
A London cylinder is doing harder work than the same cylinder in a soft-water area. That alone shifts the likely life down the range.
2. What temperature it has been run at for years
There is a genuine conflict here and it is worth understanding rather than guessing. Stored water needs to be hot enough to control legionella — the control guidance that landlords and workplaces work to sets storage at 60°C — but every degree above that accelerates both scaling and thermal stress on the vessel.
A cylinder that has spent fifteen years running at a scalding setting because someone turned the thermostat up once and never turned it back has aged faster than its years. If you cannot hold your hand under the hot tap at all, the storage temperature is probably higher than it needs to be. That is a thermostat and blending question for whoever services the system, not a reason to replace anything — but it tells you something about the cylinder's history.
3. What it is made of
| Type | How it tends to fail | What that means for you |
|---|---|---|
| Copper, vented (the old tank-fed kind) | Pinholes, usually low down or near a boss; sometimes a split seam | Fails slowly and visibly. Often weeps for weeks before anything else happens. |
| Stainless steel, unvented | Weld or seam failure, often accelerated by chloride pitting | Can fail with very little warning, and at full mains pressure. |
| Glass-lined steel with a sacrificial anode | Anode depletes, then the steel starts corroding | Life depends almost entirely on whether the anode was ever replaced. |
The pattern matters more than the material. A copper cylinder tends to warn you. An unvented stainless one is holding mains pressure, so when it goes it can put out a great deal of water very quickly.
4. Whether the expansion arrangement was maintained
This is the one that gets ignored, and on an unvented cylinder it is arguably the most important of the four.
Water expands when it heats. A vented system pushes that expansion back up to the loft tank. An unvented cylinder has nowhere to put it except an expansion vessel, or an internal air bubble on the bubble-top designs. When the vessel loses its air charge — and they all do eventually — the expansion has nowhere to go, so every heating cycle drives the pressure up until the expansion relief valve opens and dumps to the tundish.
The symptom is water trickling from the outside discharge pipe, usually after the cylinder has reheated. People live with that for years and treat it as normal. It is not. The cylinder is now taking a full pressure cycle every heating period, the relief valve is being worked constantly and will eventually stick, and the vessel or the internal bubble needs recharging. An unvented cylinder is supposed to be checked annually for exactly this reason. One that has never been touched is the single most common reason a cylinder fails early.