1. It can expose a leak on an old system
The one that matters most. On an old steel system, corrosion deposits can be effectively plugging a pinhole in a radiator or a joint. Drive water through at velocity and that plug shifts — and the leak that was always there becomes a leak you can see, usually on the day, occasionally a week later.
This is not the engineer breaking your system, and a competent one will raise it before starting. But it is a real risk, it is why system age matters, and it is a reason to consider a gentler chemical flush on a system with known weeping joints.
2. It is a day of disruption
The heating is off, an engineer needs access to every radiator in the property including loft conversions and garages, there are hoses through the house, and on a larger system it runs into a second day. Not a reason to avoid it, but people are often told "a couple of hours" and plan accordingly.
3. It is expensive relative to the alternatives
£350 to £800, against £80–£150 for a seized valve, £90–£180 for a rebalance, and nothing at all for bleeding a radiator. On a system where the real fault is a closed lockshield, a flush is a lot of money for no improvement. Rule the cheap causes out first.
4. It does not fix the cause
A flush removes what has accumulated. It does nothing about why it accumulated. If fresh oxygenated water keeps entering through a leak, a failing expansion vessel or an open vented header tank, the sludge comes back. Without inhibitor and a magnetic filter afterwards you are typically back in five or six years.
5. There are things it will never fix
Undersized radiators, a failing boiler, poor insulation, air ingress, limescale on its own, or a system that was badly designed. The full list is here. Flushing a system to solve one of these is money gone.
6. Quality varies enormously
Because a proper flush means isolating and flushing each radiator in turn with the flow reversed, and that takes a day, there is a strong incentive to connect a pump, circulate for an hour and call it done. You will struggle to tell the difference on the day — which is why knowing what the job involves is worth ten minutes of reading.
7. On a very new system it can complicate a warranty
New build under a developer warranty, or a boiler under manufacturer warranty, may have conditions about who works on the system. Check before letting anyone flush it.
So when is it still the right call?
When several radiators are cold across the bottom, the water runs black, the boiler kettles, and the system is old enough to have never been treated. In that situation a flush is considerably cheaper than the radiators or the heat exchanger it saves, and the risks above are worth accepting.
The honest summary: it is a good fix for a specific problem and a poor fix for everything else. Work out which you have first.
Common questions
What are the disadvantages of power flushing?
The main ones are the risk of exposing an existing pinhole leak on an old steel system, a full day of disruption with the heating off, a cost of £350 to £800 that is often more than the actual fault warrants, and the fact that it removes sludge without addressing whatever caused it. It also cannot fix undersized radiators, a failing boiler or poor insulation.
Can a power flush cause leaks?
On older systems, yes. Corrosion deposits can be sealing a pinhole in a radiator or joint, and flushing at velocity can dislodge that seal so the leak appears. A competent engineer assesses system age and condition first, raises the possibility before starting, and may recommend a lower-pressure chemical flush instead.
Is power flushing ever a bad idea?
Yes — on a system under about ten years old with mild symptoms, on a new build still under warranty, on a flat connected to a communal system you do not own, and where the real fault is a stuck valve, air in a radiator or poor balancing. All of those have cheaper, more effective fixes.