Why is my boiler losing pressure and dripping outside?
If your boiler is losing pressure combined with a copper pipe dripping externally, the TPRV to you boiler is faulty
What is a TPRV and How Does it Work?
A TPRV (Temperature and Pressure Relief Valve) is a critical safety device found on all combi boilers, system boilers, sealed system kits, and unvented cylinders. Its purpose is to protect the appliance and system from dangerous levels of pressure or temperature.
- On Boilers (Combi/System/Sealed Systems):
- The TPRV is designed to open automatically if the pressure exceeds 3 bar.
- It also provides protection against overheating – typically set to open at 90–95°C, depending on the manufacturer.
- When activated, the valve discharges excess pressure or heat safely to the outside of the property.
- These are rated to withstand higher pressures.
- A typical unvented cylinder TPRV will open at 6 bar (sometimes up to 10 bar, depending on brand).
- The temperature setting is again usually around 95°C.
Why Do TPRVs Start Dripping or Fail?
A TPRV is a moving mechanical part, and like any valve, it can wear out or fail. Common reasons include:
- Sludge or debris preventing it from resealing after activation.
- Constant exposure to high pressure – for example, if the expansion vessel fails, system pressure will rise too frequently and strain the TPRV until it weakens.
- Repeated overheating – frequent boiler or cylinder temperatures above 90–95°C force the valve to activate, which can shorten its lifespan.
Example of a TPRV Becoming Damaged
One example is when a system pump seizes, causing the boiler to overheat above 90°C. The TPRV will then open to relieve excess heat. On an older, dirty system, sludge can stop it sealing properly, leaving it dripping and needing replacement.
Another common scenario is when a faulty expansion vessel allows system pressure to climb too high. The TPRV then has to discharge frequently, which eventually causes the valve itself to weaken and fail completely.
◐ Summary: A TPRV is a safety device designed to release excess pressure and temperature. However, if the system suffers from issues such as a failed expansion vessel, seized pump, or repeated overheating, the valve may become damaged or fail. In such cases, the TPRV must be replaced alongside addressing the underlying fault.
What is a TPRV and Why Do They Fail?
A TPRV (Temperature and Pressure Relief Valve) is a vital safety device found on combi boilers, system boilers, sealed system kits, and unvented hot water cylinders. It protects the appliance and wider heating system from dangerous levels of pressure or temperature. When operating correctly, a TPRV discharges excess pressure or heat through a safe outlet pipe, preventing serious damage or safety risks.
How a TPRV Works
- On Boilers (Combi/System/Sealed Systems):
- The TPRV is set to open automatically at 3 bar pressure.
- It also activates if the boiler temperature rises above 90–95°C (depending on brand).
- Once triggered, it discharges hot water and pressure to outside.
- Cylinders are rated for higher pressures than boilers.
- Their TPRVs usually open at 6 bar, though some models operate at 10 bar.
- The temperature limit is typically 95°C.
- Discharge pipework must always comply with G3 Building Regulations, ensuring safe routing and visible tundish installation.
Why TPRVs Start Dripping or Fail
A TPRV is a mechanical part, and like any valve, it can wear out over time. Failure is relatively common and can be caused by:
- Sludge or Debris
- When a TPRV opens, system dirt, sludge, or scale can lodge in the valve seat.
- This prevents the valve from sealing fully when it closes, leaving a continuous drip from the discharge pipe.
- Constant High Pressure
- If the expansion vessel fails, system pressure can rise excessively.
- The TPRV is forced to open repeatedly, which weakens the spring mechanism and shortens its life.
- Repeated Overheating
- If the boiler or cylinder frequently exceeds 90–95°C due to circulation issues (e.g. seized pump, faulty thermostat, blocked heat exchanger), the valve is forced to activate.
- This puts excessive strain on the TPRV until it eventually fails.
Example of TPRV Damage in Real Systems
- A seized circulating pump can cause boiler temperatures to exceed 90°C. The TPRV will then open, but on an older system with dirty water, sludge prevents it resealing. The valve continues dripping and requires replacement.
- A faulty expansion vessel can allow pressure to climb past 3 bar on a boiler. This causes the TPRV to open frequently. Constant strain weakens the valve until it eventually fails completely, even if the original expansion vessel fault is later repaired.
What to Do if Your TPRV is Dripping
- Do not ignore it – a dripping discharge pipe means the TPRV has either activated or failed.
- Check the system pressure – if pressure regularly exceeds 3 bar, the expansion vessel may also need replacing.
- Flush and clean the system – sludge and debris are common culprits for TPRV resealing issues.
- Replace the TPRV – once compromised, the valve should always be replaced.
Conclusion
A Temperature and Pressure Relief Valve is a small but essential component that ensures your boiler, sealed system, or unvented cylinder operates safely. However, TPRVs do fail – often due to faulty expansion vessels, constant high pressure, overheating, or sludge in the system. If your TPRV is dripping or leaking, it must be replaced by a Gas Safe or G3 qualified engineer, and the underlying system fault should also be addressed.
