Overflowing expansion pipe


Postby panda » Tue Sep 22, 2009 1:15 pm

I can usually fix any small to medium plumbing jobs around the house but this one is beyond my DIY knowlege base.
Due to house alterations, I have renewed and moved the feeder tank for central heating, leaving it at approx the same level it was before, but now it is next to the HW feeder tank, approx 30 feet away from directly above the boiler, as it was before. All 4 pipes are reconnected - water in, overflow, water out to radiators with pressure expansion pipe now exiting through wall to outside instead of into header tank (for practical reasons), but a foot or so [u]higher[/u] than it was previously.
The de-aerator looked decidedly dicy, so I replaced it, repositioning some pipework to accommodate the differing positions of the in and out connectors.
I did not interfere with any other pipes, electrics or stop-taps.
The trouble now is that the radiators work fine all the time; the hot water heats up and all the taps have hot water in them; BUT when the hot water heating systen is turned on, hot water gushes out of the expansion pipe outlet to outside of house.
But the water is coming out 2 or 3 feet HIGHER than the water level in either of the feeder tanks which are approx 20 feet above the height of the pump.
Boiler etc is traditional type and fairly new, with a Potterton Kingfisher Mf boiler and a Grundfos Super Selectric pump. Seems to me that I have caused a balancing problem with the pump, but how do I solve it?
The pump has a 3-position click-switch and slot which twists round using a edge of a coin, but I have no idea what either of them does.
panda
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Postby plumbbob » Tue Sep 22, 2009 10:33 pm

The easy bit first.

The three settings on the pump are simply speeds settings with one being the slowest. Slowing the speed down may reduce the problem, but it may cause other problems and anyway, it is not the answer.

The expansion pipe you have fed to outside must be moved back into its proper position ie, back in to the header tank. Apart from the possibility of boiling water venting from the pipe, which would do more than startle anyone close by outside, you have broken the closed circuit of the heating system. It is not unusual for a small spurt of water to come out of the expansion each time the pump starts up which will now need to be replaced with fresh water. Not good!

As for the over pumping. Is the expansion pipe highest point still higher than the water level in the header tank? Have you altered the point where the tank feed pipe joins the heating circuit or where the expansion pipe leaves the circuit? Over pumping is caused by these two pipes being connected in the wrong place.

I should examine the de-aerator arrangement too as incorrect plumbing may cause the fault you describe.
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Postby panda » Wed Sep 23, 2009 1:29 pm

Looks as though I need to move the feeder tank back to its original position, on bedroom wall and reinstate the pipework between it and the boiler.
Pity, 'cos the bedroom looks a lot prettier now without the tank on the wall.
I had failed to appreciate the consequencies of breaking of the closed system and fitting the cold feed to radiators elsewhere.
Everything had seemed to work fine until I went outside whilst the hot water heating for taps was turned on.
Now seen elsewhere that correct relative positioning of pipes is:
"open vent, cold feed connection then pump."
Can somebody please confirm this is correct for me, so I can get busy correcting my mistakes.
Thanks to all.
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Postby plumbbob » Wed Sep 23, 2009 9:35 pm

Why can't you extend the expansion pipe so it ends up back over the header tank in it's current position rather than moving the whole lot back?

Apart from allowing trapped air to escape, the expansion must cater for expanding water and steam should the boiler thermostat fail. As long as the pipe leads from the top of the boiler to the tank without any valves etc, you can route it anywhere. (Use common sense though!)

I don't subscribe to the rule you mention, because I don't think it is necessary. What is important is as I said, the venting of boiling water. Wherever the expansion is, place the cold feed within 6" to prevent a pressure difference between the two.
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Postby panda » Fri Sep 25, 2009 8:13 pm

Thanks for reply, PB.
To connect the cold feed back to the position it occupied previously without moving the header tank entails complications that make it a non-starter - instead of being on the 1st floor bedroom wall, it would have to go on the 2nd floor bedroom wall (no radiators on that floor), with holes through Victorian plaster architrave etc.
Best to put it back where it was and make a neater job of it than whoever did it before.
Many thanks and I am now somewhat wiser than I was before.
Now, where did I put that blowlamp and solder?
panda
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