We've been having trouble with pumping over. On investigation the overflow only goes about a foot above water level at its highest, so it wouldn't take much to make water come out of it.
But what is the overflow for?
Is it just for filling the system?
What would happen if I blocked it?
Andy
This depends on the positioning of the feed and expansion pipe connections to the system, relative to the pump position and other flow restrictions.
It can happen if a system is retrofitted with TRV's, increasing the pressure difference when valves shut off, if the system wasn't designed for that originally.
Having the feed and expansion pipes connected nearby to each other avoids this. They can be one common pipe, but it has to be a thicker pipe, and it can make filling and bleeding the system more difficult.
It's not an overflow, it's to allow air to escape, and if a boiler stat fails on, it allows steam to escape.
Not at all.
You might end up with air trapped in parts of the system, and if the boiler went wrong, you might end up with an explosion of steam and super-heated water from somewhere, as the fill pipe is usally thinner and more difficult to eject the water from.
Another thing that causes pumping over is if a new boiler is fitted. The waterways are smaller and longer so creating a large pressure difference. You can especially get a problem if the new boiler has the circulating pump is inside the boiler cabinet. New heating systems are piped up differently to what they used to be.
When a new boiler is fitted the original pipework often needs to be altered.
This needs fixing toot sweet. You lose a lot of heat when it's pumping over plus all the water vapour in the roof space can cause condensation and rot. The water gets oxygenated leading to corrosion in your heating system.
Sometimes they get round it by converting the system to a pressurised one. Avoid this if you can, there is a lot more stuff in a pressurised system that can go wrong.
On the other hand, there's only a finite quantity of water to escape when it does go wrong.
Owain
Yes if you want to see what happens if it overheats, there were some admittedly over the top demos on Myth Busters, in one case it went through a ceiling and roof. Not a good thing. Brian
Blocking it is not a good idea. Things are usually there for a reason.
Two simple solutions:
(1) Turn down the pump
(2) Graft in a small length of pipe to take the pipe higher.
As far as I know the overflow is there to allow for the difference between the volume of water when hot and cold. As the water heats up it should move up the pipe, and as it cools it should move down. If the system is properly set up the water should never come out, but the pipe is also a safety measure should anything more serious cause the system to build up pressure. It also copes with the difference in pressure when the pump is running.
If you keep pumping over there will be a constant flow of aerated water into your system; as the water heats up it spills hot water over into the header tank and as it cools down this extra water is drawn back into the system from the header tank. This can lead to corrosion in the system.
So no, please do not block it. Like screwing down a safety valve because it keeps letting dangerous pressure out. Addressing the symptom not the cause.
Cheers
Dave R
Do you mean really mean overflow or the vent pipe coming from the systems primary?
An overflow should always be above the water line other wise will run down it and out of the warning pipe outside. Normally caused by the ball valve jamming open but badly setup expansion tanks can also do it when the primary water expands on heating and raises the level in the expansion tank.
An expansion pipe is there to vent the primary circuit or the domestic hotwater (the primary over the expansion tank, the DHW over the cold water storage tank). Niether should be blocked. Doing so runs the risk of an explosion should the boiler fail into "on" mode.
Assuming your "overflow" is really the primary's expansion pipe then the bend being only a foot above the water level is decidedly low.
Is the pump over a constant drip/trickle when the system is running or only a little spurt when the pump starts and/or stops?
Has the system ever been balanced?
If TRV's are fitted is there a suitable (possibly automatic) bypass?
not at all, and a single header tank pipe is allowed. Hazard only occurs if both pipes are blocked AND both boiler stats AND HW/CH stats all fail.
Low head is fine is the 2 pipes connect close together on the primary circuit. OTOH if they're on oposite sides of the boiler you'd need more than a foot of head.
NT
Since this post is last -
I stand corrected. The vent pipe.
There are two pipes into the loft - one from the bottom of the header tank, and one into the air above water level.
Expansion should occur up the other pipe if I block off the one in the air.
One of the rads has both the lockshield and TRV set wide open.
I'll look into tracing where they go. This may be difficult - the boiler is in a cupboard in the ground floor bathroom, and about 10 pipes go upwards from there. Failing that, splicing in a higher loop should do.
Thanks Andy
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