OSO unvented cylinder repair & installation

OSO Hotwater models and systems

Faults we diagnose

A cylinder can be blamed when the actual fault sits in the electrical supply, Economy 7 switching, immersion heater, thermostat, wiring, motorised valve, boiler/heat-pump heat source, pressure controls or discharge arrangement. We test the relevant system before recommending a replacement.

Installation and replacement

For replacement we check usable cupboard space, cylinder capacity, mains pressure and flow, discharge route, direct versus indirect heat source, electrical circuit, immersion strategy and future plans such as solar PV or a heat pump. In London flats we also plan access, leasehold constraints and compact/slimline options where relevant.

Direct electric and Economy 7 cylinders

Direct cylinders commonly use one or more immersion heaters. In an Economy 7 property the lower immersion is often arranged for off-peak heating while an upper immersion provides a boost. If the overnight hot water disappears but boost still works, the fault may be in the off-peak supply, switching, circuit, thermostat or immersion element rather than the cylinder shell.

Unvented safety and servicing

Unvented systems operate with safety controls and discharge arrangements that must remain functional. Servicing is more than checking for leaks: controls, valves, expansion provision and the discharge path are inspected in line with the system type and current manufacturer instructions. We do not bypass safety devices to keep a cylinder running.

Related pages

Manufacturer status

OSO stainless-steel cylinder ranges

OSO manufactures stainless-steel hot-water cylinders, including domestic unvented ranges and products intended for different heat sources. The model label identifies whether the system is direct electric or relies on an external boiler or renewable source.

OSO cylinders often have a distinctive compact, engineered appearance, but the same diagnostic principles apply: confirm supply/heat source, thermostat demand, immersion condition where fitted, and all unvented safety components before replacing the vessel.

When an OSO fault is electrical

If the cylinder shell is sound but the immersion trips, fails to heat or has heat-damaged terminals, an electrical repair may restore service without replacing the cylinder.