Overflow pipe and water rushing and banging

Blowheads · 27-01-2026 12:39PM #1

Open vented system (small tank in attic)

Wet stove in kitchen

Oil boiler

2 zone heat + HW

Not the first time this happened, hence the concern.

Most rads have thermostatic valves but main rad in landing plus bathroom rads normal valve fully open.

Stove is ticking away last night 9pm, circulating pump running. Want to heat upstairs before night so turn on upstairs zone (Drayton Wiser App).

Shortly after the overflow pipe starts pouring water out the back. The this almighty banging and water rushing upstairs starts, by the time I'm there I can't tell what's gone on. The pipe from the top of the cylinder to the attic is gone cold, then shortly after it's back hot again.

It's not the ball cock in the main tank, that was replaced recently and this only happened after this scenario, so it's not an irregular issue from the ball cock.

It's not the first time this happened, few times also in the past.

Anyone any ideas where to start looking?

winklepicker2025 · 27-01-2026 08:19PM

a picture of the hot press/attic tank set up would be good

Lenar3556 · 27-01-2026 08:34PM

Likely a circulation issue of some description which is preventing heat from leaving the stove.

Blowheads · 28-01-2026 09:15AM

Hopefully ye might see something

Jonnyc135 · 30-01-2026 07:34PM

https://www.boards.ie/discussion/comment/124186486#Comment_124186486

is the pipe going from the stove rising all the way to the hot press and T into the top coil of the cylinder, then does the other side of the T go directly upwards to the expansion tank in the attic. I take this is a twin coil cyclinder

Jonnyc135 · 30-01-2026 07:45PM

where is the pipe stat on the pipe coming from the stove and what temp is it set at

Jonnyc135 · 30-01-2026 07:55PM

if I was to hazard a guess, I would say the thermosyphon pull on the gravity circuit is slow. If this is the case and the stat is set too high or far away from the stove the stove may well start to over boil quickly, often see it if it was windy and there was a good pull on the chimney. If it start to boil quicker that the gravity circuit can bring it away to the where the stat is on the pipe for the circulating pump to kick in, it will pitch into expansion tank and bang and rattle like mad.

Blowheads · 01-02-2026 12:38AM

https://www.boards.ie/discussion/comment/124193396#Comment_124193396

Stove pipe stat is direct in the cloak room under the cylinder..stat is set at 55-60.

The horizonal run from stove to stat is about 3 or 4 m.

Blowheads · 01-02-2026 12:39AM

Twin coil cylinder

Blowheads · 01-02-2026 12:42AM

I can say the stove will boil if not managed, usually only at the start or the lighting process..

We might leave the bottom door open to light the fire and if not managed it could boil the stove. But we watch and manage this by turning down the stat to kick in the circulating pump. Once it kicks in we set the stove stat back to 55/60.

After that and usually we don't need to touch the stat.

The nights in question for the overflow, the stove pump runs fine

Jonnyc135 · 01-02-2026 07:26PM

https://www.boards.ie/discussion/comment/124196776#Comment_124196776

I always find the pipe stat takes along time to kick in the first time as they are usually induction and its operation is like a bimetallic strip type operation. Is the cloak room on the same ground floor as the stove?

Blowheads · 01-02-2026 08:25PM

Yes, the cloak room is 3m run horizontal to the stove, then turns vertical to into the hot press.

I'd say only 5% of the time do we need to manually trigger the stat. But this seems not to be the issue here as both stove and oil can work in harmony most of the time. The plumber has several rads open permanently for circulation

Jonnyc135 · 01-02-2026 09:04PM

that horizontal run kills the thermosyphon effect that creates the natural circulation flow from hot water rising due to the boiler in the stove prior to the pump kicking in. This pipe should always rise from the stove even a gradual slope upwards would work.

Common problem in solid fuel installations, the flow is to slow and cannot get away from the stove through natural convection when pump not engaged, it takes along time for it to reach the temp stat as it is in the hot press 4m horizontally away, this is causing the boil over as the pipe is slow to heat due to poor circulation up and trip the pipe stat telling the pump to come on.
in this case I would look at getting the pipe stat as close as possible to the stove to stop the boiling over and reduce the stat temp down to 45-50 degrees.

Poor thermosyphon flow here is the issue, if your stove is even lit and the electricity goes - immediately turn down all dampers and close up the stove.

Jonnyc135 · 01-02-2026 09:05PM

I take it the oil burner has its own pipe stat and circulating pump?

Blowheads · 02-02-2026 08:07PM

https://www.boards.ie/discussion/comment/124198811#Comment_124198811

Yes, own stat and kill switch, and pump.

Blowheads · 02-02-2026 08:09PM

Thanks for the info, but this I already know and understand. Very rarely do we need to trigger the stove stat manually and once it's running it runs fine all day.

This does not cause the issue above as both the stove and oil run hand in hand except when ? causes the overflow issue?

Overflow pipe and water rushing and banging

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