The cold-water control set on an unvented cylinder

Note 9 of 10 · free to read

The inlet controls in order, what each does, why the order matters, and how the expansion relief valve differs from the temperature relief valve.

Key points

Usual order from the main: line strainer, pressure-reducing valve, check valve, then expansion vessel and expansion relief valve.
The pressure-reducing valve limits the cold inlet to the specified operating pressure; the cylinder’s maximum pressure rating and the relief-valve settings are different values.
The check valve stops heated water expanding back into the main, so dedicated expansion provision is essential.
The expansion relief valve opens on excess pressure; the temperature relief valve opens on excess temperature.
Every part can be present and still be defeated if the order is wrong.
To clean the Megaflo Eco strainer, isolate and relieve pressure before removing the pressure-reducing cartridge that carries it.
The expansion relief valve responds to pressure alone: it is the backstop if the vessel fails or the pressure-reducing valve lets pressure climb.

Start at the cold main and follow the inlet-control group towards the unvented cylinder. Each device prepares or protects the supply before it reaches the vessel.

Line strainer: catches grit and debris so the finer controls downstream are not damaged or held open by dirt.
Pressure-reducing valve: limits a varying mains supply to the regulated inlet pressure set for the cylinder and control set. Take that operating setting from the manufacturer’s data: it is not the cylinder’s maximum design pressure or a relief-valve setting.
Check valve: a single check valve blocks reverse flow through the cylinder’s cold inlet towards the main. It does not protect every other connection: a faulty mixer can still allow hot-to-cold crossflow elsewhere.
Expansion vessel and expansion relief valve: with the route back to the main closed, the vessel, or an internal air gap, takes up the expansion. The expansion relief valve is the pressure backstop if the vessel fails or the pressure-reducing valve lets pressure climb.

Why the order matters

Each control sits where it can do its job: pressure control protects the parts downstream, the check valve guards the supply, and the expansion connection sits where it can take up and relieve expansion. Rearranged, the same parts can trap expansion or bypass protection, even though every component is present. Adding a non-return valve to a supply closes the route that expansion used to take, so review the expansion provision whenever one is fitted.

Settings on three cylinders

Each cylinder's data sets its own figures, and they vary:

Megaflo Eco: operating pressure 3 bar from its pressure-reducing valve, expansion relief valve 8 bar (the cylinder's maximum design pressure), temperature and pressure relief valve 10 bar and 90 °C, and mains of up to 16 bar at the inlet set. Its strainer comes out with the pressure-reducing cartridge once the supply is isolated and the lowest hot tap opened.
Premier Plus: operating pressure 3.5 bar, expansion relief valve 6 bar, temperature and pressure relief valve 10 bar and 90 °C, and an expansion vessel pre-charged to 3.5 bar to match the reducer, checked with a tyre gauge under the cap opposite its water connection.
UK Cylinders direct with an external vessel: where mains pressure may exceed 10 bar, a further reducing valve goes upstream of the supplied inlet set. The inlet set sits above the T&P valve so it can be serviced without draining the cylinder, nothing may restrict the pipe to the expansion vessel, and the cold main is at least 22 mm throughout. Showers, bidets and monobloc mixers need balanced cold pressure to avoid back-pressurising the hot circuit. Use the inlet set's balanced connection; if that cannot be used, the instructions permit an independent 3.0 bar reducer on each affected mixing fitting's cold supply, positioned so it does not compromise flow to the cylinder.

Two relief valves, two jobs

The expansion relief valve responds to pressure. It does not sense temperature.
The temperature relief valve, in the T&P valve on the cylinder, responds to dangerously high water temperature.

They guard against different failures and are not interchangeable. If the expansion relief valve weeps, check both the vessel charge and the pressure-reducing valve. Temperature itself is controlled by the thermostat and the non-self-resetting cut-out, not by any part of the cold-water set. Follow the manufacturer's arrangement and settings for the specific unit.

Safe delivery temperature

A cylinder must be stored hot enough for water hygiene, while very hot water delivered to taps can scald. Set the storage thermostat high enough for hygiene, then limit the temperature people get with thermostatic mixing valves at outlets used by children, older people or anyone less able to react. Turning the storage temperature down is not the answer.

Some stores, such as heat stores, solar cylinders and solid-fuel systems with no control between the heat source and the vessel, can hold water above 80°C in normal use. Where that can happen, fit an in-line tempering valve at the vessel outlet so the supply to the hot water distribution pipework does not exceed 60°C. A bath's supply in a new dwelling is separately limited to 48°C by a blending valve.