See my previous thread; a couple of you mentioned "pumping over". What are the most likely causes of this?
I've posted of pics of the heat exchanger 24 hours after the leak at
There are 2 things relevant to this;
1) Pressure is proportional/equivalent to the head of water. Old pressure gauges used to be called altitude gauges and were calibrated in both psi & feet head of water.In metric units Pressure = density x g x head.
If you attached transparent pipes to a water system, the vertical level of water would be proportional to the static pressure at that point.
2) When you have water flow through a system, there is a loss of pressure/head caused by the frictional resistance of the pipes and fittings. If you attached transparent pipes to a water pipe, the vertical level would be proportional to the static head at that point, as before, and the vertical level would drop as you progress along the pipes in the direction of flow. Pumps add head/pressure to a system.With an open system, there are usually 2 vertical pipes attached, namely the open vent & the cold feed. If there is a lot of pipe and fittings between the two connections there will be a large differential pressure/head between the two connection points. If this head difference exceeds the height of the open vent above the F&E tank water surface, then water will flow up the open vent and down the cold feed. This will churn up the water, dissolve oxygen from the atmosphere and cause rapid corrosion.
The usual situation is that the cold feed is on the boiler return, upstream of the pump, and the open vent is on the boiler flow, downstream of the pump. The pump creates a large head difference between the two connections and pumping over is more likely.
Usually the pump being at too high a speed. Set it on minimum and check the rads furthest away still get hot. If not, go up a notch. If it still pumps over on minimum with all the TRVs closed, come back for more help.
Too high a pump speed, or incorrect connection of the fill and vent pipes. [These should be connected into the flow pipe *before* the pump - and not more than 6" from each other.]
From your photos, its looks to me as if the heat exchanger has rusted from the *outside* inwards - rather than from the inside outwards - so that the black water may not have been too much of an issue. I suppose that if it has been fired up a lot of times without really getting hot - maybe in response to a frost stat in an unoccupied house - condensation *could* be the cause of the corrosion.
Where is the corrosion with respect to the connections? The return connection would be coolest and this is where on ewould expect most condensation and corrosion to occur.
On Tue, 20 Sep 2005 14:47:39 +0100, a particular chimpanzee named "Set Square" randomly hit the keyboard and produced:
It was installed in July without a room 'stat, so it would be firing continuously from the start of the heating season until I fitted one in October; my mum went into hospital in January for 6 months, so we set the 'stat to a low (16°C) setting until she came out. It would have been firing up one a day for the hot water cylinder (which we kept on). Is this long enough to corrode a cast iron heat exchanger?
Dunno! There's clearly something dire been going on. I wouldn't have believed that a new boiler could get in that state in 14 months whatever you did to it.
I suppose you are sure that it *was* new when it was installed - and that the plumber wasn't passing off a used one as new?
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