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Water dripping from a pipe outside? Check your unvented cylinder
If you’ve got a mains pressure hot water cylinder and water is dripping from a pipe outside, or into the funnel next to the cylinder, a safety valve is opening. It’s telling you something needs looking at.
- 1tundish to check, next to the cylinder
- 12months between cylinder services
- £120repair visit, including VAT
Unvented cylinder
An unvented cylinder stores your hot water at mains pressure. That’s what gives you strong showers, and it’s why the cylinder has safety valves.
If the pressure or temperature inside goes too high, a valve opens and lets water out. That water drops through a funnel next to the cylinder, called a tundish, and down a pipe to the outside where it can be seen.
So a drip outside isn’t the problem itself. It’s the cylinder telling you something else is wrong.
Why it’s dripping
The usual causes, most common first.
The expansion vessel has lost its charge
The small tank next to the cylinder has an air cushion inside that soaks up the extra volume as water heats. When the air leaks away, the pressure climbs every time the water heats up, and the relief valve lets some out. It’s a common cause, and it’s checked and topped up at a service.
The incoming mains pressure is too high
The cylinder has a valve that lowers the mains pressure on the way in. If it fails, too much pressure reaches the cylinder.
A worn safety valve
Valves wear, or a bit of debris stops one seating properly, and it lets water by.
Overheating
If a thermostat or control fails and the water gets too hot, the temperature and pressure relief valve opens. This one comes out hot.
Cylinder cupboard
What to do now
Check the tundish first. It tells you how urgent it is.
Look at the tundish
It’s the funnel with a gap in it, next to the cylinder. An occasional drop can happen. A regular drip or a steady flow isn’t normal.
Never block it
Don’t cap, tape or bung the tundish or the pipe outside. It’s a safety device, and blocking it can be dangerous.
If it’s hot or pouring
Keep clear, it can scald. Switch off the boiler and the immersion heater, turn off the mains stop tap, and call us.
If it’s a slow cold drip
It’s not an emergency, but book a visit soon. It won’t fix itself, and it’s mains water going down the drain.
Leave the valves to us
Only an engineer with the unvented qualification should work on the cylinder’s safety valves or expansion vessel. Ours hold it, as well as Gas Safe registration.
Why the cylinder needs a yearly service
Most drips are caught before they start.
An unvented cylinder needs its own service every year, separate from the boiler. Manufacturers’ guarantees usually depend on it.
- Expansion vessel checked and recharged
- Safety valves tested
- Pressure reducing valve checked
- Discharge pipe and tundish checked
Our engineers can service the cylinder and the boiler that heats it in the same visit.
One visit for both
Which one sounds like you?
- Hot water or steam from the pipeSwitch off and call
Boiler and immersion off, stop tap off.
- Slow, cold dripBook a repair visit
£120 including VAT, before any parts.
- Not serviced in over a yearBook a cylinder service
Cylinder and boiler in one visit.
- Not sure what you’ve gotAsk us
A photo of the cupboard helps.
Common questions
Is a dripping unvented cylinder dangerous?
A slow, cold drip isn’t an emergency, but it means something isn’t right and shouldn’t be ignored. Hot water or steam coming out is more serious. Keep clear, switch off the boiler and immersion, and call an engineer.
What is a tundish?
A funnel with an air gap, fitted on the discharge pipe close to the cylinder’s safety valves. The gap lets you see when a valve is letting water out, and stops waste water being drawn back into the system.
Why does it only drip when the water is heating up?
Water expands as it heats. If the expansion vessel has lost its air charge, there’s nowhere for that extra water to go, so the pressure rises and the relief valve lets some out.
Can I recharge the expansion vessel myself?
It’s best left to an engineer with the unvented qualification. The vessel has to be set to the right pressure for your cylinder, and the safety valves checked at the same time.
How often should an unvented cylinder be serviced?
Every year. Most manufacturers’ guarantees depend on it, and it’s when the expansion vessel and safety valves are checked.
I have a combi boiler. Could this be my boiler?
Combis have a pressure relief valve too, with a pipe that usually goes outside. If you’ve no cylinder and water is coming from that pipe, the boiler’s pressure is too high. Our boiler pressure guide explains what to do.
Related pages
Sources: Viessmann, what is a tundish?; Nu-Heat, hot water cylinder troubleshooting. Checked 1 November 2026.
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