How to do a DIY power flush on your open vented central heating system

First you need to know which system you have; sealed systems have a pressure gauge on the boiler while open vented systems have a f&e water tank in the loft. If your system has a pressure gauge on the boiler and no water tank in the loft switch to the DIY guide for power flushing sealed central heating systems.

(At no point will we take any liability for your actions when using this guide or any of the information provided by us. If you don't know what you are doing - Get a professional in)

This one is harder than the sealed system. We are assuming that you have sufficient knowledge to understand the working of the central heating system and the boiler. Your plumbing skills should at the very least be up to connecting and disconnecting a radiator from the system. This is not the professional "how to" version, but it will work well for systems that are currently working fine with no problems or faults on them. If your system has a problem – use a professional like one of our local members it's more affordable than you think. You can get a free quote by contacting us. The biggest problem with doing a DIY power flush is that you don't have a "proof of power flush" when you are finished; it is a certificate issued by professional power flush companies showing that the power flush was completed and to a professional standard. Insurance companies tend to ask for these, before they are willing to carry out any further work on your system. If you don't have a proof of power flush, they are likely to refuse work and future cover.

NEVER USE ACID to do the power flush, leave that to the experts. A power flush machine has a flow rate of around 100liters per minute, so it can fill your bath in one minute - if you are unsure about what you are doing it might be better to let a professional handle it. Here we aim to explain how to use a power flush machine correctly.

Rent a machine and buy 2 chemicals (1* sludge remover and 1* central heating inhibitor). You will also need 1* 22mm push fit cap and a "bung"(it's a black rubber wedge shaped plug) used for closing off pipes temporarily.

1) Feed and expansion tank: You have to clean this tank, we do this by using a garden hose like a vacuum cleaner. You will need two people for this. Fill the hose with water, then hold bath ends shut. One goes into the loft where the tank is (normally the small one of the two) making sure he/she knows where to turn the water off to the tank, the other has the bottom end of the hose in the toilet. Still closing it off with his/her finger. The person in the loft has to disturb all of the sludge at the bottom of the tank by hand, the water will go very dark give the sides a wipe while you are at it. Now stick your end of the hose into the tank and tell the guy downstairs to take his finger of the end and aim into the toilet. The water will get drained down, you should move your end around the bottom of the tank to suck up all of the sludge. Continue doing this while letting the tank fill, until the water is really clean. Now turn the water to the tank off and continue draining it till completely empty.

2) Plugging the tank: (picture) There should be a 22mm copper pipe ending over the tank, stick the 22mm push fit cap onto it and make sure its secure. Now show the “pipe bung” into pipe that comes out the bottom of the tank, you do this from inside the empty tank. This stops the water from backfilling up into the tank while you do the power flush.

3) Drain radiator: Bathroom radiator is best because you have a water supply, toilet to drain dirty water down and a water proof floor. Close both taps on the radiator – left and right. Have some cloths / towels ready. Loosen (not disconnect) one of the nuts that connecting the radiator tap to the radiator. You will need to be patient and have a flat dish or try to catch the water in as the radiator drains down; this takes 2-15min depending on radiator size. The water is normally black or brown on dirty systems.

4) Connect machine: When you are sure that all the water has been drained out. Disconnect both left and right radiator taps from the radiator and lift the radiator out. Putting it in the bath (if you have one) is normally best. Now connect the two hoses that come from the power flush machine to the two radiator taps – you should be supplied with adaptors. Now open the two valves of the bathroom radiator – you are now connected into the system. The water level in your machine will rise even overflow as the pressurized system backflows into your machine. Terminate the dump hose in toilet, taping it down is a good idea.

5) Filling machine: put the power flush machine in the bath or shower tray and connect the flow, return and dump hoses to it (read machine instructions). Most bathrooms have shower head connected to a flexible silver hose, disconnect the showerhead and use hose to fill machine. Be careful not to overfill, if it’s not in a bath or shower tray.

6) Open radiators: Go to all the radiators and open both left and right valve to full – turn all the way left.

7) Starting up: Plug in and start the machine – note the direction of flow can be changed by flipping a switch or turning a valve (read instructions). Now one person goes and checks in the loft that the “bung” or 22mm cap has not come out – do this often while you are flushing.

8) Start the boiler by turning the temperature up to 30 degrees and putting programmer to constant. The whole system should heat up including the water in your machine. Note: some boilers will not work when the pressure is too low, boilers on these systems should run fine. There is a chance that you cannot get the boiler to run while flushing, just do it without it. If you are going to flush the system with the boiler
off you will need to manually open the 2-port or 3-port valve while flushing

9) Now add the “sludge remover” chemical and leave it in with the machine running for as long as it says on the back of the container – normally 2 hours. Supervise machine at all times; turn if off if you leave the room.

10) Isolate all radiators but 1: Turn off one valve on all of the radiators but 1 (the one you want to clean first) All of the flow is now concentrated through this one radiator. If you have a rubber hammer you can hammer the radiator to loosen sludge (bottom 20cm is best all the way along) Wait 5min then reverse the flow on the machine.

11) Fill and drain: Make sure the dump hose is terminated in the toilet. You now have to discharge the dirty water by opening the “dump” valve. Open it, you will see the dirty water coming out in the toilet. As you dump the water level in the machine will drop, you will need to fill it back up every time you dump. Try dumping a lot of water then filling with a lot. Do this until the water is as clean as tap water, then move on to the next radiator by opening the one closed tap on the next radiator before closing the one on the radiator you just flushed.

12) Cleaning the magnets: Stop the machine after every radiator is cleaned. Take the magnet/s out and wipe clean, they will have loads of black sludge if your system is dirty.

13) Move trough system one radiator at a time: reversing flow, dumping water till clean and cleaning magnets. Till they are all done, once the last radiator’s water is “tap water” clean.

14) You can turn the machine off, close the bathroom radiator taps and disconnect hoses and machine
15) Now undo the “bung” and cap on the tank in the loft and add the inhibitor straight into the tank. Open the water supply to the tank.

16) Wash bathroom radiator internally by filling and draining it using the flexible silver hose you used to fill the machine with (put radiator in bath). Once clean reconnect radiator, open both taps and bleed air out. Remember to set the 2-port or 3-port valve back to closed posittion.


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