I have moved into a house with a dog's breakfast of a CH system. The CH
system looks like it's been around a while, but fairly recently the hot
water tank has been replaced with a combi boiler. (Installed arse about
face as it happens - the TRVs on the (15) rads are all the older type that
are mono-directional - the combi had been connected into the existing
system the wrong way round.)
The towel rail in the bathroom has stopped heating up, but the other rads
are ok. There is no air in any of the rads. I think the system needs a
good clean and then some re-filled with some inhibitor.
The question: Is it a DIY job? None of my DIY books cover it and my
googling of the NG archives hasn't come up with anything definitive in the
Janet and John guide to flushing your system vein.
Yes it is.
You can do a Google Groups search for a method that I have used to
clean and flush successfully, by removing each radiator and taking it
outside for flushing with a mains hose or pressure washer.
This method works especially well if you have a sealed system with a
filling loop connected to the mains water supply. You can flush the
crud out at each radiator valve and clear the pipes very effectively.
Do watch out for sludge and other iron compounds from the radiators.
They are an indellible dye on carpets and furnishings. I use a
plastic bag on each valve tail held on with a rubber band.
I use a pair of new cat litter trays to collect water etc. from each
For the TRVs, if they are old enough to be unidirectional, then they
are probably beyond their useful life as well. While you are
cleaning the radiators, I would be inclined to replace them and also
Once the system is flushed clean, it is a good idea to put in a
chemical flushing agent and running the system hot for a few days.
Then rinse and flush thoroughly.
Finally refill, adding inhibitor. You can add a filling point to do
this by creating a vertical short stub of pipe on the system, closed
off with a cap and then using a funnel to add liquid inhibitor.
Alternatively, you can use a gel inhibitor which comes in mastic gun
cartridges. It is a lot easier and cleaner if you put this stuff
in through a vent before finally filling the system, rather than doing
so against the pressure afterwards.
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With all due respect I dont think that removing the rads and carrying
them out is the easiest way to flush a sealed system. If the system
was properly designed, (yes I know the OP called it a dogs breakfast)
the fill point and drain point should be on opposite sides. (i.e. one
on the hot flow from boiler, one on cold return). Then you can
switch off, release pressure at the release point.
connect fill and drain hoses, open drain valve, close pressure
Turn on supply. Then by opening and shutting valves round the house,
you can send a fast stream of water through each rad in turn. This
should remove a lot of the sludge except if a rad is solidly blocked;
if so, then you will have to remove that rad and flush it.
Refill as normal adding Fernox or similar sludge-remover. Repressurise,
run hot for a while as per instructions.
drain, flush, refill adding anti-rust.
If the drain point is on the wrong side of the system, you will need
to cut pipe and insert an extra drain point. You may prefer to remove
If you do decide to remove and flush out rads, note:
1. When you undo the joint, water will ooze out. When you definitely
separate the pipes, it will gush.
2. However carefully you tilt a rad to drain it, there is always a
little bit of mucky water lurking inside, waiting for a chance to run
out & ruin your carpet. Carry rads upside down, with ends plugged.
Making joints. I know that other posters use PTFE tape, I have had
better results with Fernox leak sealer (I have no connection with
Fernox). It comes in a toothpaste tube, you smear it onto the male
part of the joint, dont get it on your hands, bad for skin.
Well..... yes and no. I hadn't posted the full version of the
procedure this time because it is already in the Google archives, and
not all steps may be necessary.
I follow what you are saying and I did actually do this when
refurbishing my system with a new boiler. In principle it would work
well. However, there are a couple of points:
- If you have the filling point on one side of the boiler and the
drain close by on the other, then there is an easier flow path through
the boiler and you won't get that much water being forced through even
a single selected radiator. I fitted lever ball valves on each
side of the boiler - making sure a pressure relief valve was between
them. In any case this is a good plan because it avoids pushing crud
through the boiler. By adding another drain point I was able to
flush the boiler through independently as well. Then, as you say,
water can be forced through a single radiator to good effect. This
was my starting point, but I have to say that not a lot of material
came out or even much rusty water.
- I needed to replace the lockshield valves and TRVs on some of the
radiators (the lockshields were seeping and the TRVs had stopped
woring properly), and when I removed them I found quite a bit of old
copper swarf and a little sludge in the bottom of each radiator that I
investigated. It was really that which led me to systematically
take each radiator off and outside for a good flush through with a
pressure washer. This proved very effective at removing all of the
I replaced the lockshield valves with Pegler Terrier types, which are
pretty good quality and there is a version with built in drain point
in the tail. Thus it's possible to drain a radiator before or
without undoing the unions.
With the radiator removed, the valves either side of the boiler can be
opened and mains pressure applied to the system. Each radiator
valve can be opened in turn at the location where the radiator is
removed and the pipework leading to it is flushed out rather than
anything being recirculated at all. I found that I got quite a bit
more copper swarf out this way and a small amount of iron crud as
It's also worth mentioning that for a more complete job, fitting an
inline strainer on the boiler return is a good plan.
I found that this was worth doing as well.
Very important and there is no harm in overdosing to some extent.
That's true if they were full. Generally it is possible to drain
upstairs radiators through the pipework, although downstairs ones at
the unions. Hence the suggestion of using cat litter trays as they
will take the water from most sizes of radiator.
Yep. I used "condoms" made from small sandwich bags held on the
tails with rubber bands.
You can get PTFE liquid thread sealer which is also effective as a
substitute for the tape - probably similar stuff to the leak sealer.
Radiator tails often do seem to have looser threads than other
threaded joints. I've also found the thicker PTFE tape used for gas
(on yellow reels) to be good for this.
To email, substitute .nospam with .gl
On Tue, 21 Oct 2003 18:49:30 +0100, M. Damerell wrote:
Could you clarify exactly what you mean by 'fill point' and 'drain point'
- because I suspect I have neither. Oh, hang on - I'm wrong. Re-reading
the rather good http://www.makewrite.demon.co.uk/SealedCH.html I certainly
have a Fill point, but I can't see any sign of a drain point. Neither
could a plumber who replaced a radiator for me a couple of months ago. He
drained (more or less) the system by shoving a bit of hosepipe into the
rad in the kitchen. The fill hose is connected and has been much used
until the plumber replaced the faulty rad which was leaking and
de-pressurising the system.
I understand the fundamentals behind what you're saying but I'm struggling
with some terminology. 'pressure release valve' - is that the bypass valve
that connected CH flow and return just below the boiler?
Will I definitely have a drain point and I just haven't found it yet? Or
could it be that the installers were too busy eating digestives to bother?
Thanks for all the info - food for thought.
On Tue, 21 Oct 2003 22:09:02 +0100, Dougie Nisbet wrote:
[ ... ]
Nah - I'll answer my own question. I presume it's the 'pressure relief
system' mentioned in the manual for my boiler (a RAVENHEAT RSF 100ET). I
can't get access to it without taking the cover of the boiler.
Look for something like
fitted at a low point normally near the boiler.
There should really be one at every low point.
Otherwise, after you have drained via a low radiator, you could fit
one using a push fit tee.
No. Take a look at www.bes.ltd.uk and the section Plumbing->Central
Heating->Central Heating Sealed System Equipment.
You should find a valve somewhere (typically near the expansion
vessel) with a knurled knob. It could also be near the filling point.
It may well have a label with 3 bar on it and there should be an exit
pipe to a drain. However, since there are other questionnable
aspects of the system then it may not have that. There *must* be
one on the system somewhere though.
It is possible that it is inside the boiler, especially if the
expansion vessel is in there as well.
If you don't have a pressure relief valve, then it is probably time to
seek professional help because the system would be dangerous without
one. There is no reason why yoiu couldn't DIY the addition of such a
valve, but absence would be an indicator that there could be other
Assuming you can find the valve, it is not a good idea to use it to
drain or even depressurise the system. Pieces of crud or scale
could easily become caught in it and then the valve won't seal again
and a new one will be required.
Based on some of the oddities you've mentioned, this is entirely
possible. It's laziness at best and incompetence at worst.
If you have any doubts about the system, it's also possible that the
gas installation has not been done properly either.
I don't want to frighten you, but if you are not 100% certain, then a
prudent move would be to get a CORGI registered heating engineer to
look at it (not BG since they are generally expensive).
To email, substitute .nospam with .gl
On Tue, 21 Oct 2003 23:14:31 +0100, Andy Hall wrote:
[ .. ]
I suspect I won't find one. The reason I say that is that the house has a
raised floor and a lot of the CH pipework (22mm plastic) runs underneath
the floor. Now that I know what I'm looking for I'll go underground and
have another look - but presumably if there is one below the floor it'd
have limited effectiveness as the water would have to go uphill to get to
a drain. It was a factor in my thinking about trying to go down the route
of forcing water around the entire CH system as it's possible that a lot
of the 22 mm pipework under the floor will be acting as a sump and
This is true. Do take care that you don't start pushing gunk into
the boiler as you do this.
Try to get the Pegler Terrier 367 lockshields with drainoff.
Unfortunately Screwfix don't seem to have them, but Plumb Center do as
part number 250291
These have the drain (more neatly) inboard of the valve. The effect
is that you can turn off both valves at the radiator and drain it.
Or by judicious opening of the valves use it as a flush out point for
the pipework or pipework and radiator.
I was thinking more of their lockshields but these should be good as
well. I have the Invensys ones which have also proved pretty
On 29 Oct 2003 18:17:22 GMT, email@example.com wrote:
Well, that dates us doesn't it? :-)
I spent many a happy hour with that game and an ASCII version of Star
Trek running on an Intel development system.
Now look at what it takes to do games........
To email, substitute .nospam with .gl
Dammit, meant to cancel that rather than send it off half-cock. But now
I'm committed (Magnus Magnuson mode): so, if I navigate my way to nr11
Downing St, will I find a pirate's chest with all the little treasures
I've accumulated so far neatly stashed in there? (Hey, if I'm *really*
luck, I'll find a Red Button and a Green Button to push in some out-of-the-way
Alternative Seat Of Government too; then again, the (Big) Red Button in
an ASOG might do something a bit worse than open up the sluice gates of
a single dam ;-)
On Wed, 29 Oct 2003 17:27:39 +0000, Andy Hall wrote:
I'm trying not to. There are two taps directly underneath the boiler that
I can use to isolate before I start my adventures.
This approach appeals to me a lot. I think I mentioned somewhere else that
the plastic 'branch lines' for the rads was 15mm. I was wrong. I
discovered this when my box of 20 drain off valves arrived from Screwfix
this morning and I held one up to the radiator to compare. One or two
enthusiastically expressed expletives later and I thought it'd probably a
good idea to measure the diameter of the plastic pipe - which is 10mm. I'm
not quite sure how I stand with Screwfix with my box of probably useless
valves - I've dropped them an email to see if they'll take them back.
I'm looking at the Peglers you mentioned. Are they expensive or what? Are
you thinking along the lines of http://makeashorterlink.com/?Y3F842066 -
and is it really nearly 12 quid each? It raises another question which is
probably blindingly obvious to most people and I think I can work out the
answer to but would appreciate confirmation: the numbers in the
description - 10mmx1/2 - is the 10mm the diameter of the pipe, and 1/2 -
what's that. 1/2 inch. Is that the diameter of the radiator socket? I
don't want to go and order the wrong thing again.
If you bought as a consumer, and really you would have done, then they
have to within a reaonable time because of the Distance Selling
Regulations. In practice I think that you will find that they will
That's list. You should be able to negotiate to a much lower price
1/2" means 1/2" BSP thread for the tail.
On Mon, 20 Oct 2003 21:59:06 +0100, Andy Hall wrote:
Thanks - that sounds rather encouraging. The towel rail in the bathroom
has capped tops (what are they called) which I need to investigate but
look very promising in the longer term for adding inhibiter. The bathroom
is on the ground floor so I guess I may have to more or less completely
drain the system when I decide to do this after cleaning.
I'm not sure I'll bother replacing the LSVs and TRVs at the time of
flushing. Partly because of cost although I've not investigated this) but
mostly because I'm not a wild fan of TRVs and recently wired in a Danfoss
TP5 room thermostat
http://www.screwfix.com/app/sfd/cat/pro.jsp?id9276&tsh941 into the
combi which I find much easier and useful. Almost all the rads have the
TRVs fully open and I control temperature and timer with the Danfoss.
I've already read the other posts in this thread and tracked down some of
the archived material and it's raised more questions. I'm not wild about
the idea of removing and flushing 16 rads, but certainly think that doing
the bathroom rail and using it as the access point for flushing the system
is the way for me to go, and I'll see how it goes from there. One problem
I may have of trying to flush water around the CH circuit as my mains
water pressure is pretty poor anyway (lead piping) and I'm not convinced
that it will suffice to give my rads the colonic irrigation they so much
desire. That's another thought - if I shove a hose up the backside of rad
at poor mains pressure, am I not contravening some law about risking
backflow of dirty water into the mains?
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