Water flowing out of Central Heating Vent Pipe

[stonem64]

I have an open vent Y-plan system with Baxi Boiler (set at 3) and Grandfos 15-50 pump (set at 1). The vent pipe and expansion pipe leading to the header tank are straight after the boiler before the before the pump. All raidiators are working fine but the problem that I have is that whenever the pump runs water flows out of the vent pipe into the header tank.

I have spoken to the guy who services the boiler and he says it is sludge restricting flow and that I will need to get the sytem chemical cleaned with one of thoses machines that put inplace of the pump. I am not convinced on this as:

- the pump is effectively sucking water from the vent pipe so if there were a restriction I would have thought this slowed up the flow stopping the water going up the pipe, also

-All the raidiaters are working fine

Changing the speed does not seem to affect the flow however stopping the motor by puting the switch between speeds, immediatly stops the flow out of the vent.

Not realising the cause I beleive I have had this problem for a good few years as I remember see condensation on the roofing felt at least 3 years ago. I resently dosed the system with Sentinal X400 and have changed the pump and it during this time I notice the condensation and realised it was the steam from the header tank.

Any suggestions why I am getting water through the vent pipe before I hire the cleaning pump?

[Ed Sirett]

Drain, dismantle and clean out manually. Which may have to be done anyway. There is a serious partial blockage between the vent and feed connection in the primary circuit. The effect of this pumping over will have been to fill the entire system with rust. The radiators may well be close to leaking.

If the system works, put a lid on the tank to stop the steam doing damage. And resign yourself to a complete and thorough overhaul in the spring.

[stonem64]

Ed,

Thanks for the advice.

The vent pipe and the feed are 3" apart, so I guess I just need to dismantle (heat up the soldered joints in this area) after draining down the system and clean out? I have already sealed up the lid to stop the condensation. Recently when I drained down the system the water was clear (not rusty) so I am hoping I have not got a rust problem as I have always had an inhibitor in the system.

Mike

[John Stumbles]

What distance is there between the end of the vent pipe and the water level in the header tank? If it's only an inch or so a surge of pressure which pushes water over the vent pipe can start a syphonic action which keeps the flow going quite easily. You should have the vent pipe stopping about an inch below the rim of the tank, and the water level coming only halfway or so up the tank when hot, giving several inches clearance. The vent pipe should also be in a loop (inverted U shape) the top of which is say 4" or so above the rim.

[Ed Sirett]

If and only if every thing is working really well then putting a higher loop in the vent pipe may be sufficient to prevent the pumping over.

[stonem64]

Ed,

Just a quick note to let you know that your diagnosis was spot on, there was a blockage (very small hole through which water could flow) between the expansion pipe and the vent pipe. I replaced the effected areas and everything is running fine. My only worry was the colour of the water (orange/brown) that I drained off which I assumes indicates corrosion? The water was not this colour when I drained off the system a month ago, to change the pump.

I am guessing that I had next to no flow on the old pump (hence the lack of heating furthest from the boiler) and that water was not flowing out of the vent pipe. Changing the pump added flow but with the restriction forced the water up through the vent pipe and back down through expansion pipe.

I have refilled the system and have added X400 desludger and am planning to leave this in for a couple of weeks after which I will drain down again and then refill and dose up again with X100.

Many thanks for your advise

Best regards

Mike Ed Sirett wrote: