Guides to the cylinders we install and service – vented, unvented, thermal store and heat pump – covering coil sizing, the unvented control set, discharge pipework and the faults that recur in hard water.
The types
| Type | Fed from | Pressure at outlets | Notifiable |
|---|---|---|---|
| Vented copper | Loft tank, by gravity | Low – determined by tank height | No |
| Unvented stainless | Mains directly | Mains pressure everywhere | Yes, under G3 |
| Thermal store | Varies | Mains pressure hot water from stored primary | Depends on configuration |
| Heat pump cylinder | Mains or gravity | As the system | Yes if unvented |
| Twin coil | As above, with two heat sources | As the system | As the system |
The distinction that matters most is vented versus unvented, and it is decided by the incoming mains flow rate rather than by preference – see unvented or vented.
The coil, which determines performance
The single most overlooked specification. The indirect coil is the heat exchanger between the heat source and your stored water, and its surface area determines the reheat rate.
Two cylinders of identical volume can perform completely differently. A cylinder that reheats fast effectively has more capacity over a busy morning than a larger one that reheats slowly.
- Boiler cylinders have a modest coil, because a boiler delivers at high temperature and the temperature difference does the work.
- Heat pump cylinders need a far larger coil – frequently several times the area – because a heat pump delivers at much lower temperature. Fitting a heat pump to a standard boiler cylinder is a common cause of disappointment.
- In hard water, coils scale, and a scaled coil transfers progressively less heat. The symptom is a cylinder that takes an age to heat and never quite gets there.
The unvented control set
Six layers, each taking over when the one before it fails:
- Cylinder thermostat – the normal operating control.
- Expansion vessel or internal air gap – absorbs expansion as water heats.
- Pressure reducing valve – limits incoming mains pressure.
- Expansion relief valve – opens if pressure rises beyond its setting.
- High-limit thermostat – manually resettable, cuts the heat source if the control stat fails.
- Temperature and pressure relief valve – the last line.
All six are checked at the annual service, and a system where one has silently failed is running with a layer missing. The expansion vessel is the one that fails most often, because its charge falls gradually over years – see expansion vessel failure.
The discharge pipework, which most installations get wrong
A concealed tundish, and a D2 discharge pipe the same size as D1. Between them they mean the system cannot discharge safely and nobody would notice if it tried. It is a five-minute visual check – find the tundish, compare the two pipe sizes. See D1 and D2 discharge pipework.
The requirements in brief: D1 no more than 500mm from the valve to a vertical, visible tundish in the same space as the cylinder; D2 at least one size larger, falling continuously, within the maximum equivalent length that every bend reduces, in metal pipe, terminating somewhere nobody can be scalded.
What recurs in hard water
- Scaled coils, reducing reheat rate. The most common performance complaint.
- Immersion elements scaling, overheating inside their own deposit and burning out – see immersion heaters.
- Thermostat pockets scaling, so the stat reads the pocket rather than the water.
- Expansion vessels losing charge, which is universal rather than hard-water-specific but it is what we find most.
- Relief valves seized from never being operated, or failing to reseat after repeated discharge.
Each model page below is populated from what we actually find on the job – common faults, part numbers, service quirks and access notes – rather than from manufacturer literature. That takes time to accumulate and it is worth more than a rewritten brochure. Pages appear as the records support them.
Cylinder never serviced? The expansion vessel has almost certainly lost its charge, and that is a modest job.
Questions we get asked about this
How long does a hot water cylinder last?
A copper vented cylinder typically fifteen to twenty-five years, a good stainless unvented one longer. In hard water the coil’s performance frequently degrades before the vessel itself fails.
Why does coil size matter so much?
It determines reheat rate, and reheat rate determines how much hot water you effectively have over a busy morning. Two cylinders of the same volume can perform completely differently.
Do I need a different cylinder for a heat pump?
Almost certainly – a much larger coil area, because a heat pump delivers at a lower temperature. A standard boiler cylinder will cripple a heat pump.
How often does an unvented cylinder need servicing?
Annually. It is a manufacturer’s requirement and a warranty condition, and the practical reason is that the expansion vessel loses charge and relief valves seize if never operated.
Water is discharging from a pipe outside. What is it?
A relief valve opening, and the usual cause is a failed expansion vessel. Not dangerous in itself and it should not be left, because a valve that keeps discharging will eventually fail to reseat.
How do I know if my discharge pipework is compliant?
Find the tundish – it should be visible, in the same space as the cylinder. Compare the pipes going in and coming out; the outgoing one should be visibly larger. That five-minute check finds most problems.
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