Power Flush vs New Boiler
Clean the system, or replace the boiler?
When the heating is struggling, the real question is whether the problem is the water going round your system or the boiler heating it. A power flush costs a few hundred pounds. A new boiler costs thousands. We do not fit boilers, so we have nothing to gain from pointing you at the expensive answer, and this page is written to help you work out which one you actually need.
A power flush cleans sludge out of the system so water can circulate properly. A new boiler replaces the appliance that heats it. They fix different faults. If your radiators are cold at the bottom, the water runs black when you bleed them, and the boiler still fires and holds pressure, cleaning is usually the answer and costs a fraction of replacement. If the boiler is leaking, will not hold pressure, or its parts are no longer made, a flush will not save it. Even then the system should be cleaned before the new boiler goes in, or the sludge simply follows it into the new appliance.
What each one actually fixes
A power flush treats the circulating side of your heating: the water, the radiators and the pipework. Over years the steel inside radiators corrodes and produces magnetite, the black sludge that settles in the bottom of radiators, narrows pipework and forces the pump and boiler to work harder than they should. Clearing it restores circulation and lets the system heat the way it was designed to. What it cannot do is make a worn-out appliance new again.
A new boiler treats the appliance: the heat exchanger, fan, circuit board and gas valve. Replacing it buys you efficiency and reliability, and nothing else. It does not clean your radiators. Fit a new boiler to a system still full of sludge and the new appliance inherits every bit of it, usually making itself known within the first heating season.
Engineer noteThe most expensive mistake we see is a new boiler fitted to a system nobody cleaned. A year later the filter is packed solid, the radiators are still cold at the bottom and several thousand pounds has bought no improvement at all. The sludge does not leave with the old boiler.
How to tell which one you need
The useful question is not how old the boiler is, but which part is failing. System problems announce themselves across the radiators: cold at the bottom while the top stays hot, certain rooms that never really warm, heating slow to warm up, dark water when you bleed a radiator, or kettling and banging as restricted flow lets water boil against the heat exchanger. All of those point at the water, not the appliance.
Boiler problems sit with the unit itself: repeated lockouts across more than one season, pressure that will not hold, water leaking from the casing, a failed heat exchanger, or parts that are no longer manufactured. A non-condensing boiler fitted before the 2005 regulations change is also burning noticeably more gas than a modern condensing unit, which can justify replacement on running cost alone. Plenty of homes show signs from both lists, which is exactly where an honest diagnosis is worth having.
When cleaning is the answer, and when it is not
Most people arrive at this question after someone has told them they need a new boiler. Sometimes that is right. Often the boiler is fine and struggling against a system that has never been cleaned. The two lists below are the ones we work through on site before recommending anything.
Cleaning is usually the answer when
- Radiators are cold at the bottom but hot at the top.
- The water runs black or gritty when you bleed a radiator.
- The boiler fires reliably and holds its pressure.
- The boiler kettles or bangs when it works hard.
- The heating worked well until it gradually faded.
Replacement is the likely answer when
- The boiler leaks from its body or will not hold pressure.
- The heat exchanger has failed or a repair costs a large share of a new boiler.
- Replacement parts are obsolete and no longer available.
- It is a non-condensing boiler predating the 2005 regulations.
- It has locked out repeatedly across more than one winter.
The case most people are actually in: you may need both
These are not always alternatives. If the boiler genuinely does need replacing, the system still needs cleaning first. Most manufacturers require the system to be cleaned and dosed with inhibitor in line with BS 7593 before the warranty on a new boiler holds, and many now expect a magnetic filter fitted as well. That is not us selling you something extra, we do not fit boilers, and your installer will usually tell you exactly the same thing. Our power flush before a new boiler guide covers what that involves.
It works in your favour the other way round too. If cleaning looks likely to fix it, doing that first costs a few hundred pounds and buys you the answer. If the radiators come back and the boiler runs quietly, you have saved yourself thousands. If they do not, you have ruled out the cheap explanation and cleaned a system that the new boiler was going to need cleaned anyway. That is the part worth understanding: in the scenario where you end up replacing the boiler regardless, the flush was not wasted money, it was brought forward.
How we work out which one you need
Read the symptoms
Which signs are system-side and which sit with the appliance.
Check the water
A sample tells us how much magnetite is circulating and whether inhibitor is present.
Assess the boiler
Age, pressure, fault history and whether parts are still available.
Give you the honest answer
Including do not spend it yet, or this is a boiler job, not ours.
What each one costs
A power flush with us starts from £380, with the final price set by the number of radiators, the condition of the system and access. A straightforward combi replacement in London commonly runs from around £2,000 to £3,500, and more where the system type changes or the boiler is relocated. Those are different orders of magnitude, which is the whole reason this decision is worth getting right rather than guessing at.
That gap is also why we diagnose before quoting. If you want to check your own symptoms first, our do I need a power flush checker is a good starting point, and what affects a power flush price sets out how we arrive at a figure. If the honest answer turns out to be that your system does not need cleaning at all, we will tell you that, as we do on when you do not need a power flush.
Related guides
Power flush or new boiler, answered
Will a power flush make my old boiler last longer?
An engineer told me I need a new boiler. Should I get a second opinion?
Can a power flush fix a boiler that keeps losing pressure?
How old does a boiler need to be before replacing beats cleaning?
Is it worth flushing if I plan to replace the boiler in a year or two?
Do you fit boilers if that turns out to be what I need?
Not sure which one you need?
Tell us what your heating is doing and we will tell you honestly whether it is a cleaning job or a boiler job. We do not sell boilers, so there is no reason for us to push you toward one.
