If your unvented cylinder is leaking into the tundish, it means one of three safety devices — the expansion vessel, temperature and pressure relief valve (TPRV), or pressure relief valve (PRV) — has activated or failed, and the system should be inspected by a qualified engineer without delay.
A tundish is a small, black funnel-shaped component installed on the discharge pipework of an unvented hot water cylinder. Its purpose is to visibly indicate when water is being discharged from the system — a warning sign that pressure or temperature has exceeded safe levels.
Important: Under normal conditions, no water should flow or drip into the tundish. Any discharge typically signals a fault with one of the cylinder’s safety devices.
If you do, it’s an indication that pressure or temperature in the system has reached a level that has triggered a safety response — or that one of the safety devices is faulty.
Unvented cylinders are pressurised systems and must include the following safety components to prevent overpressure or overheating:
This device absorbs the pressure caused by thermal expansion as water heats up. If the air charge inside is lost, or the vessel fails, the excess pressure has nowhere to go and will force discharge through the tundish.
Symptom: Water discharges during cylinder heating cycles.
The TPRV is mounted directly on the cylinder and will release hot water if either temperature or pressure exceeds its safety limits (usually above 90°C or 6 bar). This is a last-line safety measure.
Symptom: Hot water leaks into the tundish, sometimes continuously.
Usually located on the cold inlet combination valve, the PRV opens if mains pressure becomes too high. It protects the cylinder from pressure surges or failed regulation.
Symptom: Cold water dripping into the tundish even when the cylinder is idle.
Yes — even a small or occasional drip is a warning that the system is not operating correctly. The safety devices are either being triggered due to excess pressure/temperature or are leaking due to failure.
Leaving it unresolved may lead to:
Contact a qualified G3-certified heating engineer as soon as possible. They will:
A leak into the tundish is not normal — it’s your unvented cylinder telling you that pressure or temperature control has been compromised. Whether it’s the expansion vessel, the TPRV, or the PRV, these issues require expert inspection and should not be ignored.
For unvented cylinder servicing, repairs, or diagnostic checks, always use a certified engineer with G3 qualification.
20th August, 2026
18th March, 2026
