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Boiler Plate heat exchanger: What It Does and Signs It's Failing

Combis heat your tap water through a compact stack of thin metal plates fitted low down in the boiler. Hot heating water from the main heat exchanger flows through alternate channels, and mains cold water flows through the ones in between. The two never mix.

How it works

Heat passes through the thin plates from one water to the other as both flow past. Because the gaps are narrow, they clog with limescale in hard-water areas or with sludge from the heating side. Less flow through the plates means weaker, less stable hot water.

Signs it is failing

• Hot water running hot and cold during a shower
• Hot water that takes much longer to warm up than it used to
• Boiler cycling on and off or making kettling noises on hot water only
• Good heating but weak or lukewarm hot water

Hot water problems that come with normal heating usually point here or at the diverter valve. If the heating side is full of sludge, the radiators often suffer too.

Can you replace it yourself?

Swapping or descaling the plate exchanger means draining the boiler and opening the casing, which is engineer work. If you live in a hard-water area, ask the engineer about a scale reducer on the cold feed. Tell them whether the problem is temperature, flow rate or both.

Buying the right part

Plate exchangers come in different plate counts and port layouts, so they're ordered by boiler model. Fresh O-rings should go in with the new one.

Last reviewed 2026-10-21.