Cleaning out central heating system

Our hot water pump went recently and the bloke who replaced it said we had a lot of gunk in the system. The radiators still get hot all over but there is a lot of black sludge about. He quoted 450 to clean the system out with some sort of acid which I understand from a plumber friend is a reasonable price for this labour intensive job. However a few years ago when we had the boiler replaced the bloke who did it said there was a problem with our radiators and he couldn't degunk???
I fancy having a go at cleaning the system out myself. I want to know how to go about it. I understand you can buy stuff at the local DIY store for this job but how do I go about things.
Where do I put the stuff in? Expansion tank??
What do I need to turn off to drain the system? Expansion tank fill valve how about the cold water tank fill valve?
I will be draining the system from a valve on a radiator just by the front door, do I need to drain the hot water system aswell.
I presume I need to turn the boiler off throughout the drain?
Should I expect leaks from previously gunked up joins?
Genral advice aprreciated. Thanks in advance.
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Simple DIY guide to cleansing a CH system follows.
First, drain the system. You could refill and drain again immediately, which will clear some of the gunk.
Now take each radiator off in turn - close down the valves at each end of the radiator first. And be aware that you WILL get some black gunk coming out of the radiator as you attempt to move it - so pull back any carpets or whatever BEFORE you undo the valves. I can't stress this enough - if allowed to contact carpet material that gunk is there forever.
Plug the ends of the radiator, turn radiator upside down (this keeps the gunk in the radiator whilst you navigate your way outside) and take into back garden. Bolt garden hose on one end and let it have a good dose of mains water pressure. You will get a shedload of gunk out so don't do this on your prize patio or SWMBO will withdraw your marital rites for an indefinite period of time ;)
You can then put the hose on the other end to squirt in the opposite direction.
Once you've done the rads (and opened the valves again), refill the system (no inhibitor) and pump it around a bit (boiler off, thermostat high). Try closing off all but one radiator in turn, so that that particular radiator circuit gets a good flow. Then drain system again.
Last step can be repeated a couple of times for good measure. There will be some gunk in the pipework which will benefit from this.
Then refill, including inhibitor.
By this time and at this time of year you and your loved ones will be freezing.....best to leave this job until spring if you can..... :)
PoP
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wrote:

Cheers for your reply - very helpful.
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With a system as gunked as yours sounds to be, I would certainly go for the clean the radiators individually method that PoP describes.
I'd add acouple of things to the procedure though.
While each radiator is off the wall open each valve and allow some water to flush out. I generally use cat litter trays from the supermarket under each valve - right height and will take a lot of water. Be a little careful to avoid staining anything. This approach flushes most of the crud out of the pipework.
Once you have reassembled the system, If you are willing to do a little plumbing you can put a strainer on the return pipe to the boiler. Fit a full bore isolating valve either side of it such as a lever ball valve. As the system runs you then trap any circulating debris and can empty it once in a while without draining.
Another good idea, once you have the system flushed is to put in a flushing agent and run the system hot for a few hours. Then drain once more, flush (no need to remove radiators) and refill, adding inhibitor.
.andy
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PoP wrote:

radiators. Drain the system at the lowest point - there may also be a drain valve on the boiler. Switch the boiler off and leave the water supply on to the header tank, then with all radiator valves open, let it run until clear (or as clear as possible). Then turn off the mains supply to the header tank, either by the stop cock or tying up the valve. Let the system drain fully. Introduce some suitable cleaner (eg Fernox) into the header tank, close the drain valves, turn on the water supply and allow the system to fill. Run for a few days, or according to the instructions. Then flush and drain the system again, then fill with inhibitor (Fernox again possibly). It shouldn't be necessary to drain the secondary (hot water).
The whole procedure will take you several hours (you will need to bleed all radiators several times for instance), spread over a few days, but you shouldn't be deprived of heating for more than a hour or two. My system is almost 32 years old and still economically provides heating and hot water. It's even cheaper since I ditched the useless British Gas so-called 'service contract', whose solution to my last problem (knocking) was to offer me a new boiler for 2000. Solved the problem myself for less than 30.
Terry D.
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On Sun, 21 Dec 2003 19:15:02 -0000, "Terry D"

one that is, it won't - I have the tee-shirt from attempting it with a friend's system which had not been properly looked after.
Despite very thorough flushing and circulating chemical twice, the radiators were not cleared. The problem is that the path from the header tank to the drain cock did not go via any radiators so there was nothing to fluch them through - no exit path either.
In the end, the only way was to take all the radiators outside. They were loaded with sludge. After that was flushed out, a third dose of chemical cleaner removed what was left.
.andy
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450 seems a lot to flush a system, if that is what he was quoting on. That's 3 days labour! He may have been quoting to remove each radiator and flush them outside using a hose.
You do not need to worry about your hot water system, draining it etc, provided it is indirect, which i would think it is.
Turn off the expansion type fill valve and switch the boiler off. Drain it down through the drain cock, opening radiator bleed screws to let the air get in.
Once it is drained down, close off the bleed valves and fill the system up again. Repeat this draining and filling a few times until the water you are daining off seems to run pretty clear.
Then, add a bottle or two of system cleanser to the header tank and fill back up again. Run the system as normal for a couple of weeks then go through all the draining and filling again, but this time add an inhibitor/protector for the final fill.
You might get the odd leak but if it's gonna leak, it's gonna leak.
Hope that helps Rob
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It has a coil in a hot water cylinder.

Very helpful thanks very much.
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Hello Dave

Yep.

I use a stick across the header tank, with some string holding the ballvalve up to that. Most systems have a stop cock nearby for this purpose though.

Not in a normal system. The exit valve may block if it's particularly gunky - if it does, just attach the other end of your drain hosepipe to an outside tap and backflush briefly.

The whole CH electrical system - this includes the Pump and 3-way valve, obviously.

Fairly unlikely, but been known to happen. More usual is airlocks after refilling. Not a big problem.
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