If you’ve got an old central heating system or if you‘re intending to install a new boiler you should clean out the radiators and pipes now and again. Over time, a central heating system corrodes and forms a sludge of corroded metal and scale. This, then, leads to blockages and if you install a new boiler, you run the risk of damaging it.
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It’ll take a certified plumber about a day to do the work. And, the average cost of a power flush will be between £300 and £600 depending on the number of radiators you have.
You need a certified plumber to give your system a power flush. This involves flushing chemically treated water around the system at high pressure to clean away all the solids.
So, it’s essential to flush a combi boiler and its associated central heating circuit to make sure the system gives you heat efficiently.
How Much Does a Power Flush Cost?
The table below outlines aspects of power flush costs – you can compare plumbing prices & use these figures as an estimate for plumbing quotes you receive.
|Job Description||Average costs||Duration|
|Power flush only||6 to 10 radiators||£300 to £450||1 day|
|11 to 15 radiators||£400 to £600||2 days|
|Stand–alone chemical cleanse. Add powerful chemical to system & allow to stand for a few days. Drain & refill with clean water with inhibitor||6 to 10 radiators||£380||0.5 day|
|11 to 15 radiators||£500|
|Magnetic filter||Labour £70 per hour||1 to 2 hours|
|Cost of filter variable depending on model|
|Add inhibitor. This must be done after every drain down and when installing a new boiler||£70 per hour||1 hour|
All prices include VAT. Furthermore, prices might differ depending on whereabouts you live in the country. Usually, London and area cost about 20% higher than elsewhere. Remember, if you’re including other work such as installing a new boiler, some of these costs will be reduced because they’ll be included in the installation work anyway.
Note – See how much a central heating installation costs
What does a power flush involve?
1. Initial flush
Firstly, the plumber will check each radiator for any cold spots that indicate blockages.
Then, protect all furniture and carpets from splashes.
After this, test the pH of water with a testing kit.
Open all radiator valves and connect power flusher to pipework to flush through pipes and boiler. The initial flush uses chemicals, debris mobiliser, scale and corrosion inhibitor, leak sealer and neutralising crystals. Moreover, the flushing machine holds all these additives and pumps them around the system. Usually, the initial flush takes about 20 to 30 minutes.
Next, isolate each radiator in turn and flush using a rubber mallet to loosen any debris and sludge inside the radiator.
2. Second flush
Isolate and clean each radiator separately by pushing the liquid back and forth through each radiator until water appears clear.
Then, flush through the entire central heating system until water is clear.
Finally, add an inhibitor and circulate this through the circuit. Then, refit the radiator valves.
3. Final check
Check the temperatures of all radiators to confirm that the system readings are within 10%and 20% of each other. However, if they aren’t, then you still have debris in the system.
Next, bleed all the radiators.
It’s now completed and, the plumber will give you a certificate of completion.
The plumber might decide that you need an alternative method of cleaning for your situation. These include a chemical flush and a magnetic filter.
A magnetic filter is a small device containing a very powerful magnet. Usually, it’s permanently installed on the water return pipe just before the boiler. So, as dirty water passes through the filter, it collects magnetic and non-magnetic solids to prevent these damaging the boiler. Generally, you only need to have the filter cleaned out annually and the system will stay clean. However, have a power flush first, otherwise, the filter will clog up very quickly.
If you only have a mild sludge problem, the plumber will decide that a chemical cleanse is enough. First, the plumber adds a chemical to the system and allows it to stand for a specified time (hours or days) depending on the manufacturer’s instructions. Then, this is drained down followed by a refill with clean water and inhibitor.
Usually, these two alternatives work well for small amounts of sludge. But, only the power flush can dislodge heavy sludge solids.
What are the signs that I need a power flush?
A plumber can easily determine if you need a power flush or not. But, you can keep an eye out for certain signs to know when to call in the plumber.
The main things to look out for are signs that the radiators are clogged with solid sludge. So, look out for problems with heat transfer from the radiators. Usually, some areas will be warm and others will be cold while the pipes stay hot. Sometimes, the radiators need bleeding and the water is discoloured. Often, the entire system, boiler, pipes and radiators make a knocking sound. The boiler might even overheat because it’s working too hard to heat your house. Or, the thermostat continually cuts in and out and the boiler whistles. Finally, if you have a combi boiler, look out for warm water coming from the hot tap, because the heat exchanger is blocked.
DIY vs Hiring a Plumber
You might think that power flushing costs a lot of money. And, you would be correct. However, don’t be tempted to try the job yourself, to save some money. It’s not really a DIY job as only someone who knows what they’re doing can do the job safely. Not only that, but you need a good quality power flushing machine, other special equipment, the correct tools, and you’re dealing with high pressures and corrosive chemicals. If it’s not done properly, you will damage the heating system and especially the boiler. In fact, most boiler manufacturers will extend their warranty if you have power flushing done professionally.
Hire a plumber or heating engineer with the correct qualifications, to be sure they know what they’re doing. And, don’t forget the correct insurances so they’re covered in case things go wrong.
First, you need a qualified heating engineer who can work on the type of boiler you have. These days, just about all heating engineers can work on gas, oil and electricity fuelled boilers so there shouldn’t be any problem there.
Then, you need someone qualified to operate a power flushing machine. Usually, the flushing machine manufacturers run training courses for heating engineers who buy their pumps. So, the engineer will have a certificate stating he is qualified to operate it.
Note: Read our full plumber cost guide.
Should my plumber be a part of any trade body, organisation or hold certifications?
To work with a domestic heating system, you need to have specific qualifications relevant to the trade. Usually, the most common are:
- City & Guilds Plumbing and Domestic Heating NVQ2.
- City & Guilds Plumbing and Domestic Heating NVQ3. This achievement also awards the plumber the ‘Gas Safe licence to practice’.
- Register with the Gas Safe Register. This is a legal requirement for anyone working on a heating system.
- Be suitably qualified to operate a specific power flushing machine.
Power Flush Cost FAQ
How long does a power flush take?
This depends on the size of your system and how long it’s been since you had it cleaned. Usually, a power flush can take about one day. However, on particularly complicated heating systems, a power flush can take up to two days.
How often is a power flush needed?
Usually, you only need to have your system cleaned out by a power flush every five or six years. However, if you use good quality materials in your system you might be able to get away with once every ten years.
What’s the difference between a power flush and a chemical flush?
You can imagine the chemical flush to be a basic version of the power flush. The difference is that the power flush uses an electric pump to pump water back and forth. Whereas, the chemical flush relies on gravity.
Can Power Flushing cause leaks?
It’s not common for a power flush to cause leaks in radiators. However, sometimes the high fluid flow might dislodge a rust scab covering a hole. In this case, you’ll create a leak.
How do I clean the sludge out of my central heating system?
There are two ways to remove sludge from your heating system. First, use a power flushing machine to remove the sludge. But remember, you should hire a professional to do this. Secondly, you can flush the radiator yourself using a garden hose. But, first, disconnect the radiator from the pipework and take it outside to do this. It’s messy! Then, let all the sludge run directly into a domestic drain. Never empty the sludge into an open watercourse, pond or stream. Otherwise, you will poison any animals that use them.
How long does it take to drain the central heating system?
Usually, an average central heating system will take about 20 minutes to drain. However, if you have a more complicated system, it might take longer. Sometimes, as much as 2 hours from start to finish!
What causes black sludge in the radiator?
It’s formed by the corrosion of the inner lining of the radiators and other metal parts of the system. Because rust and metal create the sludge, it’s heavier than water and settles out in radiators, pipes and the heat exchanger. Then, the sludge builds up over time, restricts water flow and makes the pump work harder. Eventually, the pump stops working and you’ll need extensive repairs or a new one.
Get Power Flush Quotes
It makes sense to keep the inside of your central heating system clean so that everything operates at optimum efficiency. Complete the form on this page to request 3-4 quotes from certified plumbers and heating engineers, this will give you a better idea of how much a power flush costs in your area.