Deciding whether you need one
The short version: several radiators cold at the bottom, black water when you bleed, a kettling boiler, and a system over fifteen years old are the real indicators. One cold radiator is a valve. Cold at the top is air. The full checklist is here.
Cost and time
£350 to £800 for most homes, scaling with radiator count, with London at the top of the range. A day on site for most systems, two for larger ones. Full breakdown, and the cheaper alternatives.
Risk on older systems
Flushing can expose a pinhole leak that corrosion was effectively sealing, particularly on old steel systems. A competent engineer raises this before starting and may recommend a lower-pressure chemical flush instead. It is a reason for a conversation, not a reason to avoid cleaning the system.
Before a new boiler
The system must be cleaned — BS 7593:2019 — and a permanent filter fitted, with inhibitor dosed correctly. Whether that requires a full power flush or a chemical flush depends on condition. Keep the paperwork: manufacturers ask for it on warranty claims. What the standard requires.
Afterwards
Inhibitor at the correct dose, a magnetic filter fitted and emptied annually, and whatever was letting fresh water in fixed. Without those three you will be doing this again in five or six years. What a flush does and does not achieve.
Flats and communal systems
If you are in a block on communal or district heating, you may not own the system your radiators are connected to, and an individual flush may achieve very little. Establish that before paying for anything.
Heat pumps
A cleanse before a heat pump retrofit matters more than before a boiler, because heat pumps run lower flow temperatures with no output in reserve. More here.
Landlords
There is no legal requirement to power flush a rental property. There is a requirement to keep the heating in working order, and a sludged system that fails mid-winter is an emergency repair at emergency rates. Inhibitor testing at the annual gas safety check is the cheap version of managing this.
Doing it yourself
Bleeding radiators, checking pressure and rebalancing are all reasonable DIY. A power flush is not — it needs a flushing pump you would have to hire, it involves working on a gas appliance, and doing it badly on an old system can cause leaks you then have to pay someone to fix. A DIY chemical flush is feasible on a sealed system if you are confident, but check whether it affects any boiler warranty first.
Common questions
Can I power flush my central heating myself?
Not realistically. It requires a flushing pump, involves working on a gas appliance, and carried out badly on an older system can cause leaks. A DIY chemical flush is feasible on a sealed system, but check your boiler warranty terms first.
Do landlords have to power flush rental properties?
There is no legal requirement to power flush. Landlords must keep heating in working order, so the practical approach is testing inhibitor levels at the annual gas safety check and acting before a sludged system fails in mid-winter at emergency rates.
Can a power flush cause a leak?
On older steel systems it can bring an existing weak point to light, because corrosion deposits were effectively plugging a pinhole. A competent engineer will raise this before starting, assess system age and condition, and may recommend a lower-pressure chemical flush instead.