Cylinders are replaced because they leak, because the coil has scaled up and no longer transfers heat, because the insulation is poor, or because they were never the right size. A replacement is the opportunity to fix the sizing, and in hard water it is the moment to think about what caused the failure.
Why cylinders fail
Corrosion and leaks
Copper cylinders eventually pinhole, usually at a seam or around a boss. Stainless steel unvented cylinders are far more durable but can fail at welds, particularly if water chemistry is aggressive or if the system has been run without the correct protection. A cylinder that is weeping is finished – there is no repair for a pressure vessel.
Scaled coils
This is the Norfolk one. The indirect coil inside the cylinder is the heat exchanger between the boiler water and your hot water. In hard water, scale deposits on the outside of that coil, and scale is an excellent insulator. The result is a cylinder that takes progressively longer to heat and never quite gets there, with a boiler running longer to achieve less. The cylinder is not leaking and looks perfectly fine, which is why this goes unnoticed for years.
Insulation
Older cylinders were either bare copper with a jacket or had thin sprayed foam. A modern cylinder has substantially thicker factory-applied insulation and much lower standing losses. Replacing an old, poorly insulated cylinder pays back continuously – every hour of every day, whether you use the hot water or not.
Wrong size
Very common. A house with one bathroom in 1990 that now has three, or a family of five on a cylinder sized for two. It presents as ‘we run out of hot water’, which people assume is a boiler problem.
Sizing it properly
The right size is determined by peak demand, not by average use, and peak demand means the morning.
- How many people, and do they shower at the same time? Two simultaneous showers is a very different requirement from two sequential ones.
- What kind of showers? A high-flow mains-pressure shower can use several times what a gravity one does. A body-jet system changes the answer entirely.
- Is there a bath, and is it used? A full bath is a large single draw-off.
- How quickly does it need to recover? A cylinder with a generous coil and a well-sized heat source recovers far faster than one with a small coil, which changes how big it needs to be.
- What is the heat source? An oil or gas boiler recovers a cylinder quickly. An immersion heater alone is slow. A heat pump is slower again and needs a much larger coil surface area – which is why heat pump cylinders are not interchangeable with boiler cylinders.
Two cylinders of the same litre capacity can behave completely differently depending on coil surface area. A cylinder with a small coil reheats slowly regardless of how powerful the boiler is. If you are replacing because you run out of hot water, the coil is as much a part of the answer as the volume.
Choosing the type
| Type | Suits | Watch out for |
|---|---|---|
| Vented copper | Existing gravity systems, straightforward like-for-like replacement | Still needs the loft tank; gravity pressure upstairs remains limited |
| Unvented stainless | Mains pressure everywhere, no loft tank | Needs adequate mains flow, G3 competence and correct discharge pipework |
| Thermal store | Where mains flow is poor but mains-pressure hot water is wanted | More complex, and performance depends heavily on correct setup |
| Heat pump cylinder | Air source heat pump systems | Requires a much larger coil. A standard cylinder will cripple a heat pump |
| Twin coil | Solar thermal, or two heat sources | Only worth it if the second source genuinely exists or is planned |
If you are moving from vented to unvented, the mains flow rate has to be measured first – that measurement decides whether it is viable. See unvented cylinders.
What the job involves
- SurveyAccess route, space, what the existing system is, and – if going unvented – measured mains pressure and flow.
- SizingBased on peak demand and recovery, with the coil considered as well as the volume.
- Drain down and removalOld cylinder drained and out. Access is frequently the awkward part, particularly in airing cupboards built around the original cylinder.
- Pipework alterationsNew cylinders rarely have connections in the same places. Expect some re-piping, and expect it to be itemised honestly.
- ControlsCylinder thermostat, motorised valve and wiring done properly. A great many cylinder installations leave the controls as they were and lose most of the benefit.
- Safety equipment, if unventedFull set of controls and correctly sized, correctly routed D1 and D2 discharge pipework.
- Commission and certificateFilled, vented, heated, temperatures verified, and the notification and benchmark paperwork completed.
Running out of hot water? It is worth establishing whether that is the cylinder, the coil or the controls before replacing anything.
Questions we get asked about this
How long should a hot water cylinder last?
A copper vented cylinder typically fifteen to twenty-five years, a good stainless unvented one longer. In hard water, coil performance often degrades well before the vessel itself fails – so a cylinder can be ‘working’ and still be costing you money.
We keep running out of hot water. Is the cylinder too small?
Possibly, but check the simpler things first: a cylinder thermostat set too low, a failed motorised valve, an immersion heater doing work the boiler should be doing, or a scaled coil. All present identically and none needs a new cylinder.
Should I switch to an unvented cylinder?
If your mains flow supports it, it is usually a significant improvement, especially for upstairs showers. If it does not, you will have spent money for a marginal result. The measurement is what decides, and it takes ten minutes.
Is it worth replacing a cylinder that still works?
If it is old and poorly insulated, the standing heat loss is real and continuous. If the coil is scaled, you are paying for boiler run time you are not getting back. Neither is dramatic, but both run every day of the year.
Will a water softener protect a new cylinder?
It protects the coil, which is where the damage actually happens in hard water. It is one of the strongest arguments for softening in this county – see water softeners.
Do I need a new cylinder for a heat pump?
Almost certainly. Heat pumps run at much lower flow temperatures, so they need a far larger coil surface area to transfer the same heat. Fitting a heat pump to a standard boiler cylinder is one of the most common causes of a heat pump that disappoints.
What happens when you call
Call and describe it
You speak to an engineer, not a call centre. Most jobs can be narrowed down on the phone.
We look at it properly
A real diagnosis before a price, and a straight answer if it is not worth doing.
A written quote
Itemised, so you can see what you are paying for.
The job, and the paperwork
Certificates where the work needs them, and what will need attention next.
Where we do this work
Across Norwich and the whole of Norfolk. If you are on the edge of the county, call and ask — we will tell you honestly whether we are the right people.
- Norwich
- Wymondham
- Dereham
- Attleborough
- Aylsham
- North Walsham
- Cromer
- Diss
- Loddon
- Long Stratton
- Watton
- King's Lynn
Need this doing?
Call and describe the problem — you will speak to an engineer, not a call centre. Written quotes, no estimates over the phone.