urgent - heating pumping over problem

Hi All
Bizarre circumstances with one of my customers.
I got a phone call at beginning of week - no hot water. Said I couldn't go until next day - they got someone else.
Customer phoned yesterday to say they now had no central heating or hot water. I went and had a look. Seemed system air locked, so sent some mains water down the vent pipe...
All seemed ok, rads heating up, hot water circulating through cylinder - so left.
Customer called - no hot water... me thinks - this is the original problem, other plumber managed to air-lock system and not fix hot water problem...
Took a while to figure out - very hot water at outlet of cylinder, cold water at tap - how come?! Went up to attic and found the clip holding the cylinder vent pipe had broken, the vent pipe now below the surface of the tank - so syponing to the taps - bizarre... and other plumber had obviously not sorted.
Now customer has hot water & central heating. However I noticed that the vent pipe for the central heating is constantly running - pumping over, the water in the header tank roasting. This had been going on all day, and condensation now dripping off roof in attic. What could cause this to happen to a previously working system?
The vent (22mm) and feed (15mm) pipes connect together within 4" of each other at the inlet to the pump. Although the vent is pumping over, the tank is not filling, so the water going in to the tank through the vent must be leaving through the feed pipe. It's as if the connection between the two pipes at the pump inlet is blocked... only thing I can think of, but doesn't seem likely.
Please help!!
Cheers Colin
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In an earlier contribution to this discussion,

Did you, or the other plumber, turn the pump speed up to try to get the system going when it was airlocked? If so, turn it down again!
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Is not the feed to the tank direct from the mains, to the ballcock and to the tank? If so, the vent doen't come into the equation.
Make sure the vent rises above the level of which the ballcock closes off. Otherwise it wont work. Air will also get into the system if the filling pipe from the header tank is blocked. Water will not get into the system and eventually, as steam vents, the system will be replaced with air.
I hope i'm on the right track here
DS
wrote:

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Turn the pump speed down and see what happens. It's probably been turned up by other plumber to make a fast buck.
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How do you know it wasn't pumping over before?
You can always make the vent loop higher, which allows it to resist more pressure differential, if you aren't able to reduce the pump speed in order to reduce the pressure differential.
Of course, the best solution is to rip header tanks and replace with a proper sealed system, if the boiler supports it.
Another thing that can affect it is the setting of any bypass valve fitted, particularly on a S plan system, or one containing TRVs on most/all of the radiators.
Christian.
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There is a good chance that the cold feed pipe is blocked up. Check the f&e tank to see if there is sludge on the bottom, if so, there is a good chance it will be blocked further down too. I remember finding a similar problem to this, we replaced the said pipe and hey presto !! bob's me grannie !!!!:o)
mike
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That wouldn't cause the reported problem though: there'd be nowhere for the water being pumped over to go.
I encountered this problem on a system which (a) had the pump set at max - turning it down stopped it pumping over (b) the opening on the vent pipe was just an inch or so above the level of water in the header tank, which meant that as soon as pumping over started it would only require a pressure difference of 1" WG or so to maintain the flow. This was sorted by shortening the downturned part of the vent pipe and lowering the level of water in the tank.
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On Wed, 14 Jan 2004 23:09:22 +0000, Colin freeserve wrote:

Since the pipework is essentially correct then there must be some sort of blockage as you describe. This may have been going on for some while with lots of bad news for later on.
Here are some things you could try. 1) Chemical cleanser (i.e X300 say) 2) Dismantling and manual cleaning. 3) More pump to try to shift the blockage.
Simply raising the height of the vent "over-loop" won't really fix it but might make more pressure on the (partial) blockage.
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