In article ,
Pete Verdon d writes:
Part of the Great Bathroom Rebuild involves fitting a big new towel
rail, and this naturally involved breaking into the (sealed, combi)
central heating circuit. At the moment I just have two capped-off stubs
sticking out of the wall, ready to plumb in the rail/radiator this
weekend. For the couple of weeks since I fitted those stubs, I just
topped the system back up (didn't have to drain it fully down) via the
filling loop, but once I fit the radiator I'll want to properly
"recommission" the system with an appropriate additive.

I have no idea what was in there previously. I also think I might have a
small leak somewhere, as the system needs a little more topping up than

If you have a leak, the inhibitor gets used up quite quickly,
both due to having to process the air (oxygen) which leaks in,
and due to being diluted by leaking/topping up.

it probably should (not loads though). Note that this predates my work
on it! Can you get leak-sealing additives that might sort this? It's
possible (not having found dampness elsewhere) that the leak is in the
boiler, which is moderately decrepit and will probably get replaced in a
year or so, which is why I haven't worried about it too much.

Try to find the leak. I did a leak check on my system when
replacing the inhibitor this year. System needs to be off
and cold. Pressurise it by topping up, but up to the level
it normally runs at when hot (but not into the red).
Leave it a few hours, and then go around checking all the
joints, valve stems, bleeding valves and radiator blanking
plugs for moisture. With the system being cold, the leaked
moisture will hang around, whereas it might normally dry
up before being noticed otherwise. Then drain down and
fix/replace all the leaking parts you found. In my case,
it was a couple of valve stems and a 1/4 turn full bore
isolation valve shaft. Although these showed up in my test,
I only topped the system up about 3 times in 7 years, so
they weren't particularly significant. (You won't normally
get 7 years life out of inhibitor in most sealed systems,
because they aren't well enough sealed, but this one was.)
In your case, the leak should be more obvious and easy to
find.

Other than that, I just need a decent inhibitor, or whatever one
normally adds (not had to before). Any particular one recommended?

Finally, how do you actually get the stuff into the system? I've only
ever known my Dad's approach, of tipping it into the expansion tank of a
conventional system, but mine is sealed. I have a normal filling loop
under the boiler, and (luckily - I hear some people don't) an outside
drain cock. Is there perhaps some cunning fitting that you put inline
with the filling loop, to push a bottleful of gunk in ahead of the water?

Do I need to completely empty the system first or anything?

It's a good idea to do this occasionally anyway, particularly
if the water is taking on a dark tinge due to inhibitor having
run out. What colour was the water you drained out?

I flushed mine twice with water (partly because of draining
down again to replace the leaking valves after the test,
and then repeating the test), before refilling for real.
It didn't really need flushing as the 7 year old inhibitor
was still working and the 7 year old water still clear.
If a system is sluged up, then you should consider adding a
flushing cleaner too, rather than just water.

Any advice on getting the air out again afterwards? This towel rail will
be the highest point on the system, though I'm not sure everything will
necessarily run nicely upwards towards it.

Refill/repressurise (having remembered to close off the bleed
keys;-). I start at the bottom and work up, bleeding all the
radiators. If the boiler has a bleeding point, bleed that too,
and likewise any bleeding points you have on raised pipework.
You will probably have to return to the filling loop to
repressurise whilst doing this.

What you want to do now is to run the pump without the boiler
firing up. On both the systems I do regularly, that's not easy,
but I can acheive it by turning off the gas supply to the
boiler, and then generating a call for heat. In both cases,
I only get a few minutes before the boiler gives up trying to
light the gas, locks out, and cuts off the pump, but this can
be repeated. This will move more air trapped in the pipes into
the radiators, and you bleed them again. You must be sure water
is pumping though the boiler, and there isn't an air-lock in
there, as that could wreck it when the gas eventually fires up.
You can probably hear bubbles going through the system if all is
OK. You'll hear bubbles in the system for a day or two until all
the air is transfered into the radiators and finally blead out.

--
Andrew Gabriel
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