Vented vs Unvented Hot Water Cylinders in Irish Homes - BER Assessments Dublin Ireland
- Mihai
- 2 days ago
- 3 min read
Choosing the right hot water cylinder is an important decision for homeowners in Ireland. The two most common options are vented and unvented hot water cylinders. While both store and supply hot water, they are designed for very different operating pressures and use different safety methods.
Understanding these differences is essential for safety, performance, and compliance.
🔹 Vented Hot Water Cylinders
What Is a Vented Cylinder?
A vented hot water cylinder is the traditional type, commonly found in older Irish homes. It is supplied from a cold water storage tank, usually located in the attic.
Key Identifying Features
Made of copper
Designed to withstand normal (low) pressure only
Has an open vent pipe rising from the top of the cylinder
➡️ The copper construction is a defining feature, as vented cylinders are not designed for high pressure.
How Safety Is Achieved
Water expansion is released through the open vent
Excess pressure safely returns to the attic tank
System pressure depends on the height of the tank, not the mains
Advantages
Simple, proven technology
Lower installation cost
No specialist certification required
Suitable for gravity-fed systems
Limitations
Lower water pressure at taps and showers
Requires attic storage tanks
Less suitable for modern bathrooms
🔹 Unvented Hot Water Cylinders
What Is an Unvented Cylinder?
An unvented cylinder connects directly to the mains water supply, delivering hot water at mains pressure throughout the house.
Key Identifying Features
Made of steel
Designed to withstand much higher pressure
No open vent
Fitted with multiple safety devices
➡️ Steel construction is essential to safely resist high internal pressure.
Safety Components (Critical)
Because the system is sealed, unvented cylinders rely on:
Pressure relief valve
Temperature relief valve
Expansion vessel or internal expansion device
Tundish to show visible discharge
These components prevent dangerous pressure or temperature build-up.
🔴 Does an Unvented Cylinder Need an Expansion Vessel?
Short Answer: Yes — in most cases.
When water is heated, it expands. In an unvented (sealed) system, this expansion must be absorbed safely.
How Expansion Is Controlled
Most systems use a red expansion vessel on the cold water inlet
Some cylinders have internal expansion control, removing the need for an external vessel
➡️ Expansion control is always required, whether internal or external.
🔵⚪ Expansion Vessel Colours Explained
Colour | Typical Use | Suitable for Unvented DHW? |
🔴 Red | Domestic hot water | ✅ Yes |
🔵 Blue | Cold water only | ❌ No |
⚪ White / Grey | Heating systems | ❌ No |
Using the wrong vessel type can lead to:
Premature failure
Safety valve discharge
Non-compliance with regulations
🧠 Interesting Things Homeowners Often Don’t Know
A copper vented cylinder cannot be converted to unvented use
Unvented cylinders work particularly well with heat pumps
Poor mains pressure can still limit unvented performance
Regular inspection of safety valves is essential
🇮🇪 Irish Installation Considerations
Unvented cylinders must be installed by a competent person
Discharge pipework and tundish installation are mandatory
Manufacturer instructions and Building Regulations must be followed
Annual safety checks are strongly recommended
✅ Final Thoughts
Vented cylinders rely on copper construction and open venting to safely manage low pressure
Unvented cylinders rely on steel construction and controlled expansion using safety valves and expansion vessels
Both systems are safe when correctly designed and installed, but they are not interchangeable.
For independent advice on domestic hot water systems, energy efficiency, and compliance in Ireland, MTS DNC Energy Consultants Limited are here to help.
📢 Need Help with Your BER Assessment?
At MTS DNC Energy Consultants Limited, we provide accurate and up-to-date BER assessments based on the latest SEAI standards. Whether you're renovating, building, or just curious about how your current building ranks, we’re here to help.
📞 Contact us today for expert advice and a tailored assessment.
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