When it comes to unvented water cylinders like a megaflo, many homeowners enjoy the benefits of full mains pressure at every tap, improved flow in showers, increased efficiency, and overall cost-effectiveness. However, some may experience an issue with water dripping through the tundish on their unvented water cylinder. In this article, we will explore the reasons behind this issue and provide potential solutions.

What is a Tundish and its Purpose?

A tundish is a simple, yet essential safety device found in unvented water cylinder systems. Its primary purpose is to act as an overflow for the pressure relief valves (PRVs), ensuring that any excess water or steam is safely directed to a drain. The secondary purpose of the tundish is to serve as a visual indicator of any issues within the system, as water dripping from the tundish can alert homeowners to potential problems.

Common Reasons for Drips through the Tundish

1. Expansion and Contraction

During normal operation, an unvented water cylinder undergoes expansion and contraction as the water is heated and cooled. This can cause a small amount of water to be released through the pressure relief valve and into the tundish. In such cases, occasional dripping from the tundish is expected and should not be a cause for concern.

2. Excessive Pressure

If you notice a constant drip or a significant amount of water coming from the tundish, this may indicate excessive pressure within the system. Excessive pressure can be caused by a faulty expansion vessel, incorrect system pressure settings, or a malfunctioning pressure reducing valve.

3. Faulty Temperature and Pressure Relief Valve (TPRV)

Another potential cause for water dripping through the tundish is a faulty temperature and pressure relief valve (TPRV). The TPRV is designed to release water from the system if the temperature or pressure exceeds safe limits. If this valve is malfunctioning or has become worn, it can cause water to be released through the tundish more frequently than necessary.

4. High Mains Water Pressure

In some cases, high mains water pressure can cause water to flow through the pressure relief valve and into the tundish. This can be resolved by installing a pressure reducing valve on the incoming mains water supply to regulate the pressure within the system.

How to Resolve Drips through the Tundish

1. Check the Expansion Vessel

An expansion vessel is designed to accommodate the expansion of water as it heats up within the unvented cylinder. If the vessel is faulty or has lost its air charge, it can result in excessive pressure within the system. Checking and recharging the expansion vessel can help resolve the issue of water dripping through the tundish.

2. Inspect the Pressure Reducing Valve

The pressure reducing valve regulates the incoming mains water pressure to ensure it remains within the safe limits for the unvented water cylinder. If this valve is malfunctioning, it can cause excessive pressure in the system, leading to water dripping through the tundish. In this case, it is necessary to inspect and potentially replace the pressure reducing valve.

3. Replace the TPRV

If the temperature and pressure relief valve (TPRV) is faulty, it may need to be replaced to resolve the issue of water dripping through the tundish. A qualified professional should carry out this replacement to ensure the correct operation of the valve and the safety of the system.

4. Install a Pressure Reducing Valve

If high mains water is causing the issue you might need to install a new pressure-reducing valve.

How to Prevent and Resolve Tundish-Dripping Issues

Regular Maintenance and Inspections

To prevent issues with dripping through the tundish, it’s essential to conduct regular maintenance and inspections on your unvented water cylinder. This will help identify any potential problems before they escalate, and ensure that your system is operating at its optimal efficiency. Schedule annual check-ups with a qualified engineer who specializes in unvented water cylinders to keep your system in top shape.

Pressure Relief Valve Replacement

If you’re experiencing a drip through the tundish due to a faulty pressure relief valve, it’s crucial to replace the valve as soon as possible. A qualified engineer can perform this task, ensuring that the new valve is correctly installed and functioning as intended. Timely replacement of the pressure relief valve can prevent further damage to your unvented water cylinder and protect your property from potential water damage.

Expansion Vessel Servicing

As mentioned earlier, issues with the expansion vessel can lead to dripping through the tundish. To prevent this, it’s essential to have your expansion vessel serviced regularly. A qualified engineer can check the air pressure, membrane integrity, and overall condition of the expansion vessel. If any issues are detected, they can be addressed promptly, preventing more severe problems down the line.

Proper Installation and Commissioning

When installing a new unvented water cylinder, it’s vital to ensure that it is properly installed and commissioned by a qualified engineer. This will help prevent any issues with dripping through the tundish from the start. The engineer will verify that all safety devices, including the pressure relief valve and expansion vessel, are correctly installed and functioning. Proper installation and commissioning will provide you with peace of mind, knowing that your unvented water cylinder is operating safely and efficiently.


In conclusion, a drip through the tundish on your unvented water cylinder is a sign of an issue within your system, often related to pressure relief valve malfunction, temperature and pressure relief valve failure, or expansion vessel problems. Regular maintenance, inspections, and servicing by qualified engineers can help prevent and resolve these issues, ensuring that your unvented water cylinder operates safely and efficiently. By addressing these concerns promptly, you can protect your property from water damage and enjoy the many benefits of your unvented hot water system.


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