CH system won't refill

2 Oct 2005
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United Kingdom

I've just replaced 5 rads and thought I'd use the opportunity to flush the system. I drained the system at the drain cock by the boiler and the other at the lowest rad.

Yesterday I added some Sentinel X400 sludge remover, opened all the rad valves and turned the water back on at the header tank. System filled with lots of gurgling, ran it for 24 hrs. (I didn't bother bleeding any of the rads As I figured any sludge would only be in the lower parts of the rads anyway). Drained it this evening from both cocks with a reassuring medium brown stuff running out.

I've now reopened the valve to the header tank to be greeted with a slow trickle from the header tank (it fills OK and the ball cock cuts off when it should), not the whole system gurgling and filling like yesterday. Opening the odd bleed vent on an upstairs or downstairs rad makes no difference.

I feel there must be an airlock somewhere but I'm not sure where, or how to clear it.

Ideas gratefully accepted.
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You may have got a sludge blockage. It very often occurs where the feed pipe (usually 15mm) comes down and joins the 22mm or 28mm circulating pipes.

If you have compression joints here, undo them and prod up and down the pipes.

If you have soldered joints you would have to cut it, and later rejoin with compression.

Remember that when you cut the pipe open all the water in the F&E will gush out and squirt up your arm before soaking the carpet and the ceiling below.

Some people say you can knock the blockage out of the way, by putting either an Aquavac or a garden hose on the vent pipe (22mm) which curves over the F&E
Thanks for that. I'll give it a go tomorrow.

Does sludge have to be bad to get to that stage? The boiler was replaced a couple of years ago and we flushed and refilled the system with inhibitor then. The initial drain down at the weekend only showed a bit of terminal black stuff, most of it was a very pale brown. All the rads have always been hot with no cold spots. I suppose the sludge remover stirs it all up? It did fill and drain quite readily yesterday.

I'm not keen to cut the pipes as the small loft over the boiler is very cramped with two HW cyliders and lots of fibreglass. I think I'll try the idea of power flushing. I shall put the hose back on the drain cock and attach a garden hose to the vent pipe. Is that what you are suggesting?

When I replaced the F&E earlier in the year I put a drain cock on that so's I could drain the tank if needed. Could I attach the garden hose to that and blow mains water down the system?
The vent pipe is open. If you blow down or suck up it, the water will try to come up or go down the f&e pipe. If you put a hosepipe on the drain cock it will come out of the open vent.

I have not used a hosepipe in this way and would be anxious that the mains pressure (if the blockage does not clear) might do some damage.

But someone will be along in a minute who has done it.

BTW the pipework cutting is not usually done on the loft, it is more likely in the airing cupboard, where the Tee is.
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I think that you have said that you left X400 in your system for about one day ???

The instructions say 2-3 weeks and all professionals know that is a minimum and 3-5 weeks is better.

There are better chemicals to use!

The X400 instructions said a minimum of 2 hours or until satisfactory perrformance was restored. Given that I already had a satisfactory performance (until now) I thought 24 hours would be OK. What should I have used instead?
Sorted. It was an air lock caused by the added on way that the system has built up over the years.

I put the garden hose on the drain cock and positioned the wife in the loft to shout when the F&E level looked like approaching the overflow.

There were a series of gurgling and farting sounds (pipework, not wife) and the system filled and is now working. Many thanks to all for the advice.

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