Do I actually need a power flush?

A power flush costs £300 to £600 and it appears on a lot of quotes. Sometimes it's essential. Sometimes it's the easiest £400 an installer will make that week. Here's how to tell.

What it actually does

Over years, rust and sludge build up inside your radiators and pipework. That black gunk restricts flow, makes radiators cold at the bottom, and, if you fit a shiny new boiler onto a filthy system, it gets pumped straight through the new heat exchanger and blocks it.

That's the real risk. Most manufacturers will void the warranty if the system wasn't cleaned properly, which is why installers push it.

When you need one

Radiators cold at the bottom and warm at the top. Black water when you bleed them. An old system that's never been cleaned. Any system where you're replacing a boiler that failed with sludge in it.

In those cases it isn't an upsell. It's the difference between a boiler that lasts 15 years and one that fails in three.

When a chemical clean will do

A newer system, clean water when you bleed, radiators heating evenly. A chemical clean plus a magnetic filter is often enough, and it's a fraction of the cost.

Ask the installer directly: is my system dirty enough to need a full power flush, or will a chemical clean satisfy the warranty? A straight answer to that question tells you a lot about who you're dealing with.

The bit nobody mentions

A magnetic filter costs about £100 fitted, catches the sludge before it reaches the boiler, and is required by most warranties anyway. If it isn't on your quote, ask why not.