Easy Guide: How To Flush A Pressurised Central Heating System

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At Grand Union, we specialise in helping our customers with central heating repair in Milton Keynes and the surrounding areas. We are specialists in radiator replacement in Newport Pagnell and we are recognised heating specialists in Buckinghamshire and surrounding counties. 

We know though, that there are a lot of you out there who will have a go yourselves and for you we have put together this handy step-by-step guide to help you carry out a problem free pressurised central heating system flush.  If you need us, however, we are the go to heating specialists in Northamptonshire and all surrounding areas and we will always be happy to carry out the work for you. 

Before you start, identify which system you have.  A sealed system will feature a pressure gauge, situated on the boiler.

You will need to understand how the boiler and your heating system work. You will also need to be able to connect and disconnect a radiator from your system. Once those boxes are ticked this is our 16-step guide.

First you will have to take a machine rental and purchase two chemicals: (One central heating inhibitor and one sludge remover)

  1. Drain a radiator: One in the bathroom is good as you will have water there and can drain the dirty water into the toilet. You may also have a bath to put the radiator in when it’s disconnected.
  1. Close the right and left radiator taps with some towels at the ready. Loosen but don’t disconnect one of the nuts that connects the radiator tap and the radiator. Have a flatish container ready for the water as it drains from the radiator (2-15min), the water may look very dark if the system is dirty.
  1. When all the water has been drained then you should disconnect the radiator taps, (right and left) from the radiator, carefully lifting it out. Connect the hoses from the machine directly to the 2 radiator taps (with supplied adaptors). Next open both valves on the radiator. Now you are in the system. The level of water may rise and might even overflow. Be ready. Now put the dump hose into the toilet, and tape it down.
  1. To fill the rental machine: put it in the shower tray or the bath, connecting the flow and put the dump hoses back according to instructions. Use the shower feed pipe, without the head, to fill the machine. Be careful not to overfill.
  1. Open the radiators right and left valves to the max, turned fully to the left.
  1. Start up the machine.
  1. Now start up the boiler – set the temperature at thirty degrees and programme on ‘constant’. The system should then heat up and the machine water will also heat up. (Some boilers won’t work when there is low pressure, so you may not be able to run your boiler while flushing.)
  1. Next add the chemical “sludge remover” while the machine runs according to the instruction – usually this will take around 2 hours. Do not leave the machine unattended.
  1. Isolate all but one radiator in the house. On all radiators, except the one to be cleaned first, turn off one valve – flow will be through this radiator. You can hammer the radiator gently with a rubber hammer to help sludge dislodge and then wait for five minutes before reversing the flow.
  1. Draining and filling: Ensure the dump hose is securely anchored in the toilet. Next you can discharge the dirty water from the radiator through the dump valve so that the dirty water flows into the toilet. The machine’s water level will drop so refill it every time water is dumped. Repeat these steps until the water is clear.
  1. Go to the next radiator and open the tap that is closed then close the one on the radiator that has just been flushed.
  1. To clean the radiator magnets, after each of the radiators has been cleaned, stop the machine then remove magnet or magnets and then wipe them clean. 
  1. Move through the system one after another reversing the flow and then dumping the water till it’s clean, cleaning the magnets as you go.
  1. Next the “inhibitor” chemical should be added to the machine and allowed to run for ten minutes.
  1. Now you can turn the machine off and close radiator taps in the bathroom disconnecting both the hoses and the machine.
  1. Wash the inside of the bathroom radiator (in the bath or shower cubicle) by filling it and then draining using the flexible shower hose utilised when you filled the machine. Once it is clean reconnect the radiator, open both of the taps and then bleed the air out.

At Grand Union Plumbing & Heating we are very proud of the high quality plumbing, heating and gas services that we offer. We are based in Milton Keynes, with many clients in the surrounding areas. Regardless of how complex or simple the job you need doing, you can rely on us, because we pride our selves on our customer service.  We are affordable and dependable, and we are ready to work with you on any project, large or small.

Are you a landlord looking for a plumber?

If you’re a landlord and you need experienced and swift action taken to replace an old cranky boiler, please don’t hesitate to give us a ring on 01908 034194. We’ll work quickly to get the problem resolved and keep your tenants happy.

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Areas we cover

  • Aylesbury
  • Bedford
  • Leighton Buzzard
  • Northampton
  • Olney
  • Towcester
  • Woburn Sands
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I’ve used Grand Union 3 times and every time the service is excellent and the price very reasonable. Engineers are very good at letting you know what they are doing and there are no hidden charges. They even managed to do a job at short notice. They work tidily and even followed up with a message to check my heating was still ok the following day. Highly recommend

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Excellent service from Mitch!
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Work swiftly done too, with clear explanations throughout.
Thanks again, Mitch.

Used grand Union plumbing to service my boiler and couldn’t of asked for a better service. They were on time and actioned the job in a clean and professional manner. Very nice to see they were operating in a covid secure manner without being promoted too

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Highly recommend Grand Union Plumbing & Heating. We called Mitch after getting nowhere with the first 2 heating engineers we contacted. In contrast to them, Mitch quickly diagnosed the fault, ordered the part and replaced. Totally professional and good value for money.
Mike Kennedy
I found Grand Union on local heroes. Id given up on British gas and NPower/homeserve.
Mitch was honest from the get go. Came round and gave me quote got the worl done within a couple of days once parts came.
Very highly professional. Very friendly. Perfect on time. Clean. Safe. What else can I say. I will be using only Grand Union here on and cannot recommend enough.
Amazing service Mitch Andy and Cory all amazing guys came out Xmas Eve went above and beyond to get me sorted and were amazing with my children letting them watch and so polite couldn’t recommend highly enough 10/10
Steaves Jen
Highly recommend the team to fix any plumbing or heating issues. They arrived as scheduled and it was great to have such a high standard of service. Will definitely be my contact for anymore plumbing issues and for boiler services.
Owen Thomas
Contacted them after having three boiler services being cancelled by a Big Five company only resulting to a wait till May for the 4th appointment. From the first day I contacted them Andy came around in a week’s time and provided excellent customer service and serviced our boiler very efficiently and quickly. The appointment ran a bit late due to another emergency call but both Mitch and Andy were apologetic and polite. Appreciate there excellent customer service on top of a job done well. Will be calling on them in future again for sure. Thank you.
Kaanita Khan
Would highly recommend Grand Union plumbing, really knowledgeable team. They were able to identify the problem with our heating pressure gauge, and release valve really quickly and stop our leak straight away. Thanks
Emma Thompson
Love these guys, have been using them for years and will keep using them for years in the future. They’re helpful, do amazing work each and every time, and are respectful of your home, both Andy and Cory took their shoes off when they came into the house recently, Cory put down dust sheets when he was fitting my new radiator in the bathroom (tiled floor wipes clean, but still put it down).
100% recommend them for any and all plumbing works and looking forward to having them do more work for me later in the year.
Ian Kockelbergh

It’s not very often that you find a quality tradesperson who is reliable, honest, and competitive on his pricing. Mitch has done a number of jobs in our house, including installing a new boiler, water tank and radiators. All to a brilliant standard. I will happily recommend Mitch and Grand Union Plumbing!


I have called Mitch out twice since moving into our new house a month ago. Both times he has managed to fit in an appointment fairly quickly to resolve our issues. He has been professional and friendly, offering advice on other issues that I have seen. I would definitely recommend his services.

Simon Edwards

We have used Grand Union Plumbing several times as we had a really tricky fault. We found Mitch to be really excellent. Very happy to recommend, 10/10

Annette McDade

Called Mitch out on very short notice, extremely reliable and great rates, would highly recommend Grand Union Plumbing and Heating. Also provided advice with other queries aside from the job at hand- with no reluctance. Great service, thank you for your hard work and honesty Mitch.

Dushan Gnanakumar
Great service, polite, attention to detail. Would highly recommend.
Ajvinder Dhatt
Had a leak that needed fixing asap and Mitch gave me advice over the phone to deal with it until he could get to me, then came out within the hour on a weekend! Would highly recommend
Hannah Manners
Great company would highly recommend for any heating & plumbing issues
Terry Adolpho

Definitely recommend these guys. Had a job done by them and they wasted no time to get it done and made sure I was completely happy before any payments. Nowadays companies just want to rush and take your money. Big thumps up

Tevaugh Grant
We always use Grand Union for serving and any other unforseen problems. The service and professionalism is 1st class. Would definitely recommend!!
Stephanie Horne
A great company to do business with… I used them recently for an emergency with a domestic heating system which was rectified very efficiently. A second problem (some months later) and there was no decision to make, called them and they sorted it in no time with minimum fuss, mess and very reasonably priced.
Trevor Gordon

Have used Grand Union twice now. They are very quick to respond to enquiries, reasonably priced, tidy workers and very friendly. I can’t fault them. Definitely my go to guys now for any plumbing needs.
Highly recommended

Emma Verrall
Great service from Mitch. Have used a few times for boiler service, repairs and outdoor tap installation. Always very prompt service, turns up on time and reasonable prices. Would definitely recommend!
Jon Hood

I found Grand Union Plumbing through the Gas Safe Register. I had a boiler service and gas safety certificate. Mitchell was very professional and the prices were very reasonable. I would definitely recommend and use again.

Rebecca Hughes
Mitch was very helpful when fitting my new boiler – he was very fair about price and I couldn’t recommend Mitch enough. Thanks Mitch!
Reece Hussain


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