DIY Central Heating Flush – simple & something you can & should do!

If you live in some sort of solid building in the UK, the chances are that you probably have a central heating system – and if you’re on this page, you’re probably wondering why central heating systems need to be flushed and whether you can flush your own central heating system with a DIY central heating flush?

If you have a gravity fed, open-vented central heating system with a feed & expansion header tank (possibly located in your loft), doing a DIY flush of your central heating system is a simple but effective way to potentially increase its efficiency, maybe lower your running costs & improve the system’s lifetime. It’s also useful to flush your central heating system if you’re looking at replacing radiators or making major pipework changes – not much point in replacing dirty radiators with new if you don’t clean the system.

Before I did this for the first time, I like many others (according to Google at any rate!) asked the question “Can you flush your own central heating system?” – and swiftly concluded that it’s definitely a task that you can do yourself with a few basic DIY skills.  A DIY central heating flush can be a little messy to do, but isn’t particularly hard – yet can make quite the difference.

Pressurised systems require extra steps and can be more difficult – do not use the instructions below as for a start, you won’t have a F&E tank to work with.

Over time & with normal usage, all heating systems build up a degree of “sludge” (iron oxide & dirt from water), which can restrict water circulation and cause your system to become more inefficient over time. Symptoms of a sludged system can include being slow to heat up, uneven heating of radiators, finding a radiator cold at the bottom, seeing black or very dirty water when bleeding radiators along with many more issues. It can also accelerate failure of key system components such as pumps; simply because they have to work harder than they should to move the water around the system’s pipework.

The most heavily affected systems might require more serious intervention/treatment in the form of a “power flush” (a process which essentially force-circulates water & cleaning chemicals around the system with more force than is possible via other means) but for many other systems, a DIY central heating flush may be sufficient to keep everything running in top condition.

There are many guides already available on the web that cover heating flushes in great detail along with discussing power flushes, power flush machines and other ways covering how to flush a gravity fed heating system, so I’m not going to go into much depth in this post. I have however summarised a few key steps below covering how to drain and complete a DIY central heating flush on your own home’s system – so that there’s hopefully enough detail for you to work out if this is something you’re confident to undertake and get you started.

Before you start, check that you have:

  • Access to the central heating system’s F&E header tank, and the ability to control water flow into the tank.
  • Access to a foul water drain (or toilet) within hosepipe reach of the F&E tank and a convenient drain point.
  • Access to a drain point/valve on your heating system
  • Sufficient length of garden hosepipe or similar to reach from the F&E tank to the nearby drain/toilet, and from the system drain point to a drain/toilet. These won’t be needed at the same time, so the same length of hose might be usable for both.

You will also need to buy:

Cleaning the system is a three step process (unless you’re powerflushing), as you need to flush & fill the system with the cleaning product; leave everything to circulate for a while and then flush/refill. It can take some time to carry out each flush/fill cycle depending on the size of your system, so ensure you have plenty of time and won’t need to abandon the job part-way through.
The ideal moment to start this process would be probably be if you needed to take a radiator off for any reason, replace valves or otherwise needed to drain the system down – as this would effectively empty out whatever’s currently in the system leaving it clear for you to introduce the cleaning products.

Assuming you’re not doing any other work on the system and that it’s currently full of water / working normally, we will need to drain/flush it through with clean water to clean out as much of the current sludge / dirty water as we can to start with. There’s not much point adding cleaning product to system full of dirty water as this will limit the product’s potential and will just mean that the process is not as efficient or as useful as it could otherwise be.

First things first – get a clean start

Before doing anything else however, go and have a look in the system’s Feed & Expansion (F&E) tank. If you’ve never properly flushed the system or cleaned the tank, I’d imagine you’ll find that it’s full of sludge. This usually takes the form of brown gunk on the tank’s walls, and can look like a thick layer of sand all over the tank’s base.

As a quick example of what you may be looking at, I don’t think our system’s F&E tank has never been properly cleaned out – despite all the work and changes that have been made to our system over the years since buying the property. At a rough guess, this is what the tank can look like after 20+ years of inattention…

It’s important if this is the case for your system to take the time now to clean out the F&E header tank, removing the sludge BEFORE starting to drain down or flush the central heating system. Otherwise, all you’ll do is disturb the gunk in your F&E tank by the act of it automatically refilling as its water level reduces; caused of course by the now dirty water being drawn into the system while you’re trying to flush it. This will just result in even more sludge entering the system, which will add to whatever’s in there already and limit the benefit of any further work.

Fortunately, cleaning the F&E tank is extremely simple to do and if you’ve ever cleaned out a fish tank or something similar using a hosepipe syphon as a vacuum cleaner… then the same process will work a treat here.

With the system switched off (mainly to avoid any unexpected discharge of hot water from the vent pipework!), the first step is to suitably arrange your hose pipe so that the outlet end is secured into whatever drainage point you identified. This is important – as once water’s flowing, you don’t want to be worrying about the hose coming loose… Water from the tank is going to be dirty and full of all the gunk that’s currently stuck to the tank – would be useful to dispose of it cleanly! We were using a nearby bathroom/toilet, so were able to secure the hose under the toilet seat.
The other end of the hose needs to be taken to whereever the F&E tank is located, and temporarily secured so that it’s in the tank and under the surface of the water.
Next, we need to get the water flowing. There are countless ways to achieve this from using some sort of specialised suction pump through to simply getting some water into the hose (hint – try using the mains water feed into your F&E tank) and letting gravity take over to applying a vacuum or suction to the outlet end of the pipe. Take your pick! Obviously, I cannot recommend the old-fashioned approach of sucking on the hose to generate suction; the water in the F&E tank is probably filthy and you really don’t want to be running even the slightest risk of ingesting any of it.

Once you’ve got suction going and are sure the hosepipe is secure in whatever you’re using for a drain, head back up to the F&E tank and temporarily stop the mains water feed to the tank while you use the hose to start emptying out the water currently in it. You can do this by either switching off a handy valve if you have one on the feed pipework or by simply holding the ballcock valve closed while water drains out.

Once you can actually see the bottom of the tank, use the hosepipe to start removing gunk from the tank. The easiest way is to simply use your hands (with some disposable gloves on if you prefer) to slosh the water around and agitate the gunk at the bottom of the tank – sucking it up with the hosepipe. Don’t let the tank empty completely as once your hose comes out of the water you’ll have to start again in terms of getting things flowing.

Keep going with this until you’ve got the gunk off the sides and bottom of the tank, and have flushed it all through with clean water a couple of times – ending up with a clean-ish tank full of clean water!

Once you’re happy with the tank (don’t worry about getting it perfectly shiny & new-looking… that’s not likely to happen unless you replace it), carefully take the hose out of the tank; allowing any remaining water to drain. Check that it’s automatically refilled with clean water, with the ball valve operating correctly to shut off the water feed once full.

Next, find a drain point & get connected

With the preparation work completed and a clean F&E tank ready to go, switch the central heating system on and turn up its thermostat so that the pump & boiler run and get some circulation going around the pipework.
Find a convenient drain point and connect up your hosepipe, again ensuring that the outlet end is sufficiently secured in whatever you’re using as a drain as you don’t want to end up with a system-full of dirty water all over the floor.

By this point, the system’s hopefully warmed up a little so switch it off for now and open up the drain point’s valve. If it’s not been used for a while, you’ll probably need to apply some force to open the valve; especially if it’s been painted over a few times since last used. The valve might also leak a little – we found a small paint tray to be about the ideal size to sit under the valve & catch any drips.

With the valve open, you should hopefully now have a steady flow of warm dirty water out of the system with corresponding flow of clean water into the system from the F&E tank. You shouldn’t need to open bleed valves to get water flowing (unlike if you’re draining the system down to empty) as your F&E tank should be able to continue replenishing water as fast as it drains out.
Assuming the water’s brown or black, leave it running until it starts to clear. Depending on the size of system (number of radiators / amount of pipework etc), this might take some time. Once the water clears, close the drain valve and fire up the system. Leave it to heat up and circulate the water for 15-20 minutes, before switching off and reopening the drain valve. With a little luck the water will still be running clear – if not, leave it to flush again until it does.

Once the water is running clear, head back up to the F&E header tank and close the ball valve or tie up the ballcock float so that the tank begins to empty. Once it’s down to about a third full, pour in your bottle of system cleaner (eg Fernox Central Heating Cleaner F3) and wait for the majority of remaining water to drain in to the system. Before the water level drops below the feed pipework to the system, re-open the ball valve and let the tank start to refill.
Quickly head back down to the drain point, and close the valve to end the flush process.

As an alternative, if you have a Magnaclean or similar fitted you can of course use that to dose the system with cleaner chemicals if you prefer.  Don’t forget to empty & clean the Maganaclean after flushing the system however!

At this point, you should have a central heating system that’s full of relatively clean water & cleaning chemicals.
Disconnect your hosepipe and leave the system to run as normal for a week or so, circulating the cleaning chemicals around the pipework & all your radiators.


After a week of normal usage, repeat the process above for flushing the system – starting after the system’s been able to heat up to normal temperature.
This time, you’re flushing out the cleaning chemicals along with whatever sludge (suspended in the water) it’s been able to dislodge and again let the water run until clear.
Once you’re happy that you have clean water running out, close the F&E tank’s ball valve and pour in your bottle of inhibitor (eg: Fernox Central Heating Protector F1). If you have a Magnaclean fitted, empty & clean the unit once you’re seeing clean water and perhaps use it to dose the system with inhibitor.

With the inhibitor in the system, leave the F&E tank to refill and close your drain point’s valve.

Fire everything up, and let the system heat up to temperature. Check all your radiators are hot all over, and are not cold at the top. Its worth then working your way around and quickly bleeding them after a flush just to ensure no air has been caught somewhere.

Hopefully you’ll now have a heating system that’s working more efficiently, heats up quicker and might even last a little longer – along with saving yourself some cash by doing the work yourself.

Obviously, if you’re not confident in your ability to follow the steps above & deal with any problems should they occur, do not attempt to carry out a DIY central heating system flush.  Book a heating engineer or plumber to do it for you. I do not take any responsibility for anything that goes wrong, or right, should you choose to follow the notes above.

You do so ENTIRELY at your own risk.

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    1. No problem – great to hear it’s useful. Can be a bit daunting if you’ve never done it before, but is really simple to do.
      Good luck with it!

  1. great article!! really glad I found it – one question though, in my case the same system is my heating AND hot water, so surely if I leave the cleaning chemicals in for a few days I can’t use my hot water, as surely it comes from same tank? so my bath water etc would then contain the chemicals?

    1. Thanks Paul.
      As mentioned via email, Darren, you’ll be fine as the water flowing around your radiators doesn’t ever mix with the stored hot water in the tank.

  2. Thank you both for your replies – my tank in loft has the expansion? bit on top of main tank i.e. it is one tank split into two I assume – so I assume this is same principle as you explain and as per diagram?

  3. reason I ask is that if I run the hot tap or my bath – then the expansion tank at top starts to fill up again i.e. so assumed that it was for hot water not heating

  4. That’s the hot water cylinder fill tank, that IS for your hot water, DO NOT put the treatment in that tank. The F&E tank is sepetate and smaller.

    1. Okay, now I am confused then – as original reply mentioned a tank heating the water. But the only tank I have is this one I described. There must be some other tank or something I have not noticed – will have another look in the loft, is it a tank I am looking for, or just a plastic container with a lid? – Thanks for replying

    2. Just been in loft and think I have been missing the obvious!! have got a photo, so perhaps I could get it to you so someone could just confirm what is what for me?

      The main tank is a fortic combination unit type 3 and there is another small tank hanging from loft roof surrounded by white insulation panels I assume – this must be the F&E tank?

      If I can send photo to you then that will confirm it all

      1. The Fortic tank is a combined hot storage & cold water cistern from looking at details online. The smaller tank on the top fills the lower larger tank, which will be connected to your heating system along with probably having an electric immersion heater fitted?

        It’s not overly clear what pipework runs to the hanging tank from your pics but that is probably the heating feed/expansion tank. Does it have an outlet pipe connected into your heating pipework somewhere, a mains cold feed supply with a ballcock and an open (vent) pipe from your heating system, perhaps hooked over the top so it vents into the tank?

        Could be awkward to inspect or do anything with it though – ideally need to be able to see into it to establish the state it’s in / clean etc!

        1. Hi Tim, yes that hanging tank has a ballcock connected to cold feed, a vent pipe hooked over top and another pipe from bottom going into mass of pipes for central heating – so this must be the right tanks

          Location is not good as nearly impossible to get to easily – have managed to get a photo though which I have emailed you, looks dirty, but cleaning it out could be difficult due to location

  5. I have been through this process about 4 weeks ago. In addition I flushed every radiator by disconnecting each from system and cleaning it under mains pressure by moving it to rear garden. Unfortunately my two biggest double radiators in conservatory are ice cold. I think my problem is sludge in the pipes under the floor boards on the ground floor, since my heating water drain cock is higher then the pipes under the floor boards. I have a very expensive very nicely fitted carpet throughout the house. At my age and small pension, I can not afford to disturb the carpet. Question any advice about cleaning the under floor 22mm pipes please. Look forward towards your kind response.

    1. Hi there, I don’t profess to be a plumber but it does sound like your pipes are blocked in some way or potentially the valves could be stuck closed? Simple way to test that theory would probably be to take the problem radiator off / disconnect it, and then open each valve in turn to see if you can get any water flow through it. TRV valves can get stuck closed too – try taking the control head off the valve and ensure the pin operates properly. Hope this is of some use!

  6. Hi Tim, I recently had to go through this whole process. (Clearing sludge) my system was renewed 6 years ago, but they did not clean out the F & E tank ?? so, not realising this I drained system then put the Fernox into tank, unaware of all the crap inside it. Needless to say all that crap then went into the system ??? So now starting at the begining again…..flushed etc, i then put my 2nd fernox via radiator (after cleaning out the tank of course) Let the whole thing run a few hours checked the tank What was clean water was now brown water. presumably from the safety vent pipe from boiler?? is that normal? was it drawing from around the boiler ? So frustrating ..but getting there slowly.
    interested that you first off didnt find it necessary to open any rad bleed point to flush out system ??

  7. Sorry Max – just seen this! I’d imagine all the stuff in the tank would have indeed flushed through into the system if it wasn’t cleared first. More flushing would probably work to help clear that one! No idea unfortunately why you’d end up with brown water in the tank unless perhaps the system is overpumping or regularly venting into it? Might be worth watching the vent while the system’s running to see if it’s dumping water in there.

    I don’t recall needing to open a bleed point however – possibly because I didn’t try and drain it; just flush through with a constant supply of water flowing in from the f/e tank and effectively out of a drain point.

  8. Hi I recently cleaned the system out but only did it once didn’t reflush after chemicals 1st question is that ok? Also I have an open vented system and to get more pressure to some still cold radiators have put flexi pipe to top up pressure from the mains to boiler is that ok?

    1. Hi Wendy, what chemicals did you leave in? If it was the cleaner, you might be better off to check with the manufacturer’s instructions as to how long it can be left in a system. Otherwise all the gunk it’s managed to dissolve/release will still be in the system. In terms of the open system & more pressure, good question. I’m not a plumber but if it’s open vented, that is probably not going to solve your problem as it’s not a pressurised system.

      If you have TRV valves on the problem radiators, it’s worth removing the thermostatic heads and checking whether the valves are functioning correctly; there should be a pin visible underneath the head that moves freely up and down.. if it’s stuck and can’t be freed with a few gentle taps then you may need to replace the valve. If the pin moves OK, then try running the system without the thermostatic heads and see if the radiators warm up.

      If they still remain cold, probably best to then check whether you’ve got decent water flow through the pipework to them. Can verify this by turning off both the valves, placing a bowl underneath one of them to catch the water and then loosening the connection from one valve to the radiator. Drain out the water from the radiator and then gently open the valve.. all being well you should be getting a decent flow through the valve. Close it & repeat with the other. If the flow’s OK, then maybe flush the radiator through outside to remove any persistent sludge and re-fit. If the flow’s not OK, then either the valves are not opening properly or there’s a problem / blockage / airlock in the pipework feeding that radiator somewhere. Might be easiest to get someone in to powerflush the system at that point unless you fancy tackling it yourself..

  9. Hi we have flushed the radiators out and just letting it get pumped thro system but noticed constant drip coming from header tank when put heating on, is that the ball cock?

    1. Hi Genny, from the sound of things either the ballcock isn’t fully closing the water feed or perhaps your system’s overpumping. If it happens when your system is off, then it’s probably the ballcock; if it only happens when the system’s running, then may be worth checking for air in radiators at the highest point of your system and potentially turning your pump’s speed down a notch.

  10. Great article – thank you. will know (or let my husband to read this) what to do next time.
    We have a new boiler installed two months ago, a chemical flush of the system was included in the quote. Unfortunately, my dear husband did drill through one of the radiator pipes recently and, on top of ruining our carpet, we have noticed that the water was very very dirty. Does it mean that the engineer didnt do his job correctly, as i would assume the water to be clear-ish, or the colour of the water has nothing to do with the supposed flush? Do I complain to the installers? we have spend a small fortune on the new boiler and if the job hasnt been done properly, i would not be impressed…
    Many thanks in advance!!!

    1. It’s really hard to comment further on that one to be honest. Chemical cleans are not really any different to the diy approach I’ve described – in that you introduce a cleaner chemical to the system, run for a while and then flush everything out. The results depend entirely on the state of the system and how much effort / time you put into flushing. No heating system will be pristinely clean or have clear water circulating around it though. If you’re concerned suggest speaking to whoever installed the boiler, attempting a couple of DIY cleans or maybe getting it powerflushed. If everything works and the radiators heat up quickly however, would suggest not worrying about it.

  11. Excellent article Tim, very well written and precise.

    I have done this a few times in the past and had forgotten some of the tricks.

    I was thinking that one trick for filling a hose with water prior so as to act as a syphon would be simply to put the hose in the bath to fill the hose, then put something heavyish on the hose to keep it in place in the bath and clamp one end with something and then take it to the expansion tank to do the syphoning( can one use one’s thumb? Might be tricky as one negotiates getting into the loft)

  12. Hi Tim. I’ve got plumbers doing a chemical flush for me and they’re advising I should have the heating on for a week to ensure the chemicals work properly. Issue they are trying to resolve is a blockage causing boiler to overheat so it cuts out after about 10 minutes. Do you think having heating on that much is necessary? Thanks

  13. Could possibly use a cork or wine bottle stopper on the hose before entering the loft. Would obviously need to use the wine if you opened it!

  14. Very simple and easy to follow guide, I had a blockage in the system, located the block with a magnet, dislodged it but used your guide to clean out the system and was amazed to see how much residue and muck was in the system. Central heating is working great now. Very useful advice for a novice.

  15. So pleased I found this guide thank you for putting it out there! As a single woman and full time carer for my elderly mother money is tight so when I was told by a “Professional” Plumber my heating issue was not air in the system as I had suggested it was sludge in the system and needed a £700 power flush naturally I was horrified especially as the house is on the market!
    After googling power flushes and various other topics I came across your blog and thought reckon I can do this and it is certainly worth a try before getting in another “professional”. So after a £30 spend on Fernox Cleaner x 2, Inhibitor and a length of clear hose (figured that would be better to see the colour of the water) I was ready to go. I was a little concerned bout getting the syphon action going but after wedging the house under the toilet seat and getting the other end into the roof I used the fill action in the tank to get the hose full of water then dunked it into the tank and away it went! Way easier than I was anticipating. The F & E tank was a bit grotty but didn’t take too long to clean which was pleasing. So then off to the radiator in the utility hose attached fed through cat flap and out to the drain, armed with my new multi tool (£1.99 Screwfix) I opened the radiator and expected to see the same grimy water as in the tank. To my amazement it was totally clear! I ran this process for almost 4hrs as I couldn’t find anything on the internet that said how long it would take to flush a 14 radiator system but during that time the water was always clear just lots of bubbles. I then went back to the roof emptied the cleaner into tank sprinted back to radiator and shut it down. I ran the system for a week with the cleaner in it and during that time the system worked perfectly.
    My original issue was that when the hot water had been on for 30 minutes there would be a horrendous noise from the airing cupboard it would sound like Vesuvius was erupting in the tank the pump would start clicking and banging then it would all shut down locking out the boiler so no heating for a couple of hours. But now it is working perfectly.
    To be honest I was reluctant to repeat this the following week to put the inhibitor in as it was all working fine but knew I had to and was interested to see if the cleaner had produced anything. Well it hadn’t when I repeated the drain process again the water was clear. I didn’t let it run so long this time before putting the inhibitor in maybe a couple of hours. It’s been 2 weeks now and touch wood it is all working fine in fact better than it has ever worked since the new boiler was installed in 2012.
    So massive thanks again for your easy to follow guide which gave me the confidence to have a go and saved me £670.
    Best regards,

    1. Hi Alison, thanks for taking the time to comment. Lovely to hear it’s gone well and been useful! Sounds like the problem may have been air in the system or something similar – so by flushing it all out you’ve hopefully dislodged the air and solved the problem. Would imagine it was cleaned during the boiler install so hopefully didn’t have years of dirt to clean out. No bad thing 🙂
      Best wishes, Tim

  16. Hi Tim, followed your instructions to the letter and it all worked beautifully no issues, I even fitted a new pump whilst the system was drained. It’s people like you that make the internet worthwhile.
    Thanks for the great advice.

  17. Great article with sound advice. Only one warning about header tanks and that is based upon my unexpected experience: Stopcock to F&E tank for the heating system itself (small tank) was shut off but outside overflow kept running from time to time. How could water from central heating be escaping if the supply was turned off? – Answer:- there was a pinhole leak in the spiral inside the hot water tank, the source of heating to it from the boiler, and so water feeding the hot taps from the main body of the hot tank, was migrating into the central heating water and so constantly topping it up.
    New hot water tank required and solved problem.

  18. Hello, and thanks for the useful information.
    When you say : >>The other end of the hose needs to be taken to whereever the F&E tank is located, and temporarily secured so that it’s in the tank and under the surface of the water.<<
    I thought I could do this in 2 different actions and 2 different and unconnected smaller hoses. No?
    1- 2m hose to drain radiators…Hose on the last radiator with draining valve ( on ground floor near front door)
    2- Another 2m . hose just to empty overflow tank ( in loft over 2 floors above ground floor). into a large bucket.
    Is this ok or not?

    1. Hello, probably depends on the state of your overflow/header tank & how much you feel like carrying buckets up and down ladders. Personally, using a long hose seemed the quickest/easiest option to avoid extra mess while emptying the tank. If a bucket works for you, perfect!

      Also can’t see why doing it in two actions wouldn’t be fine. Idea of emptying the header tank is to try and reduce the amount of additional sludge/dirt getting into your system while flushing it through so providing that happens before you start draining water out of the last radiator, great 🙂

  19. I’m going to try this on my radiator system,I don’t have a tank it’s a condensing boiler,do I have to take each radiator off as I don’t have the drain off point to attach the water pipe,cheers.

  20. Great blog.

    I made the mistake of ‘draining’ down my system after studying too many youtube videos. On refilling and adding system cleaner, I ended up with an airlock somewhere and couldn’t prime the water pump located in airing cupboard on first floor (Conventional boiler is in ground floor). Called out a heating engineer who sorted it out the airlock. He also advised flushing rather than draining to avoid a repeat of problem and having to call him out again!

    After a week, the F/E tank was dirty again and required cleaning out a second time before commencing the flushing process.

    One thing I have observed with this flushing-only method is cold water entering the system does not really flush through the upstairs radiators. The upstairs rads don’t ever start to turn ‘stone cold’ to suggest it is being infused with fresh water, so there is diluted system cleaner still lurking in them.

    I’ve so far flushed the system 3 times and running up heating for an hour in between flushes to try and reduce the overall concentration of system cleaner still left in the system before finally adding inhibitor.

    The 18 yr old boiler sounds a bit better after each flush.

  21. Hello, after a lot of searching, I found your article, which is exactly what I needed. Have performed all the steps several years ago, but without the cleaner chemical stage. I plan to follow your guide exactly April 2020. Been getting H2S smell from rads, well only one actually, need bleeding every 2-3 days, only a little gas, but think the system needs a clean out…..some rads 17 years old, some replaced recently, condensing boiler is 10 years old…..so bit of a mix…..but need to protect what I have, thanks again great article.

  22. Hi Tim thanks for your guide. Many years ago I flushed the whole system so its due another one. Just thinking with our ‘traditional 2 pipe system / header tank ….. if I shut off ALL the radiators should the flushing from the header tank automatically pass through the boiler ? Our drain point is at the lowest point within the system. Hope you can advise .

    1. Hi Mark, sorry for not coming back to you faster – completely missed your comment. Good question to be honest, but am kind of assuming you’ve found the answer to this by now! Would depend on your pipework at a rough guess and whether shutting off the radiators still allows circulation to continue. Would hope it would do, but can’t really help with that!

  23. Iam pleased that I have now found a web site that goes into the full details of chemically flushing a central heating system as Iam thinking of getting a local heating company to carry this out and I wanted to know exactly how this should be done so that I can see if this is what they will do. My heating system was installed in 1981 using microbore pipes and another heating company was supposed to have flushed the system out a few years ago but having read this article I don’t think that they did it properly as the radiators take a long time to heat up and they didn’t first drain down the system and then refill it and allow the chemical to flow around the system for a few week before finally flushing it.

  24. Hi Tim. Great blog. I have a drain point on all my downstairs radiators so wondering if I need to drain from all of them or if just one is enough? My thoughts are that the other downstairs radiators will not benefit from this and any sludge will remain unless I drain from each point. All advice appreciated.

    1. Hi Ron, Absolutely not an expert on this (!) but in theory if you drain from one leaving the water running for long enough to start getting cleaner water from the drain point it should hopefully be sufficient to dislodge the sludge in other radiators. That said if your radiators are all slow to heat or cold at the bottom you might need to take them off and use a hose to flush each through individually. Can probably do that in situ but outdoors tends to be less messy.
      Adding a strong dose of a chemical cleaner for a week or two along with running the system to warm it all up before you start flushing can help break it all down too.

  25. Currently having a rough time with central heating to cut a long story short the system had to be drained via the boiler itself to replace a faulty 3 way valve at its last service September 2021. Unfortunately all the build up of dirt was dragged into the system and needs a similar treatment to Alison to clean it only thing is I do not have a drain point is their an another option or do I need to get one fit.

    1. Sorry it’s taken a while to reply – hoping it’s all sorted by now? If not, simplest answer might be to take off a radiator at the lowest point of the system and drain via the radiator valves. Close them both, crack the connection to one end of the radiator and be prepared to let it drain into drip trays / bucket etc. Once the water stops flowing, open the bleed valve to allow air into the radiator and drain the rest of the water. Once that’s stopped, loosen the radiator valve / tail connection and pull them apart… ideally connect a hose to the valve to make this easier, ensure the end’s in a bucket or otherwise safe and then open the valve to drain the rest of the system. Recommend then fitting a drain valve – ideally more than one if you have multiple low points in pipework as that will make things easier to flush in the future.

  26. Hi Tim. Brilliant article. Only question I have is do you run the pump when you are draining the system or leave it switched off. Peter Sadler

    1. I’d run the system for a while first to let things warm up and get the water circulating before draining.. then switch off & drain. Wouldn’t run the pump while draining to avoid risking running it dry.

  27. Hi
    Moved into old house 3 months ago, developed terrible cough and breathlessness, 5 courses of antibiotics and 10 weeks later slowly recovering, they now think may be legionaries disease I also bought hot tub When moved in, now reading up house was empty for year, I should treat cold water tank as suspect, so put a cup chlorine granules in that today and let run real hot before running though hot taps hour later, just in case…
    Noticed the little tank next to it was empty bone dry looks been dry long time with brown powder on bottom, also the big cold water tank is half full of white sand looking, I’m reading that maybe anti hard water bag basically rotten and dumped… I’m looking to replace both tanks this week as now paranoid then will try this flush…. any tips on replacing tanks???
    Regards Justin.

    1. Sorry to hear you’ve not been well, sounds like one heck of a start to the new house.

      Replacing the tanks sounds like a great move from what you’ve described. It’s pretty straightforward as long as you’re comfortable working in the loft and able to make up the right connections.
      Biggest tip would be to measure the loft hatch dimensions (assuming that’s where your tanks are) and make sure that whatever tanks you order will fit through the hatch.

      For replacing them, turn off any water supplies to your existing tanks, drain them either by running taps / draining heating or using a hose or two. Then, disconnect from supply / overflow and feed to the house; move them out of the way and install the new ones re-making the connections and fitting valves etc. Switch on the water and carefully start to slowly fill, checking they’re settled into place and water tight before you let them completely fill.
      Good opportunity to flush the heating at the same time if you’re replacing that header tank!

      Hardest bit is often then getting the old tanks out of the loft !!

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