Boiler Diverter Valve: Symptoms, Cost & Replacement Guide

The diverter valve is one of the most important components in a combi boiler — and one of the most common to fail. Here's what it does, how to tell if it's faulty, and what to do about it.

What is a diverter valve?

A diverter valve is a component inside a combi boiler that directs hot water either to the central heating system (radiators) or to the domestic hot water system (taps and shower), depending on what's being demanded.

When you turn on a hot tap, the diverter valve switches to send heat to the plate heat exchanger to heat the water. When you turn the tap off, it switches back to central heating.

Symptoms of a faulty diverter valve

• Heating works but no hot water — The valve is stuck in the heating position.
• Hot water works but no heating — The valve is stuck in the hot water position.
• Hot water only works when the heating is on — The valve isn't switching independently.
• Radiators get hot when you run the hot water — The valve is letting water through to both circuits.
• Intermittent hot water — The valve is sticking intermittently.

What causes a diverter valve to fail?

• Wear and tear — The valve has moving parts that wear over time.
• Sludge in the system — Black iron oxide sludge can jam the valve.
• Limescale — In hard water areas, scale can build up on the valve mechanism.
• Actuator motor failure — The electric motor that moves the valve can fail.
• Diaphragm failure — The diaphragm that senses water flow can split or perforate.

Can you fix it yourself?

No. The diverter valve is inside the boiler casing and is a gas appliance component. It must only be inspected and replaced by a Gas Safe registered engineer.

However, you can diagnose the symptoms yourself using the list above to help the engineer understand the problem before they arrive.

How much does diverter valve replacement cost?

The cost varies depending on the boiler brand and model:

• Diverter valve cartridge replacement — If only the cartridge needs replacing, it's a relatively inexpensive repair.
• Full diverter valve assembly replacement — More expensive, as the whole assembly needs replacing.
• Actuator motor replacement — If only the motor has failed, this is a cheaper fix.

On older boilers, it may be more cost-effective to consider a replacement boiler, especially if other components are also showing signs of wear.

How to prevent diverter valve failure

• Power flush the system — Remove sludge that can jam the valve.
• Add central heating inhibitor — Prevent sludge and corrosion.
• Fit a magnetic filter — Catch sludge before it reaches the boiler.
• Service your boiler annually — A Gas Safe engineer can spot early signs of wear.

Need help with a faulty diverter valve?

Sanju's Heating covers Newbury, Basingstoke and Reading. Call us on 01635 74 50 40 or book online at sanjusheating.co.uk/book