Installing a mains pressure water heater can provide efficient, high-pressure hot water for your home or business. Whether you're upgrading your system or fitting a new unit, this comprehensive guide will walk you through the process. At Quality Heating, we specialise in premium water heaters and accessories to ensure safe, compliant installations. Remember, for unvented systems over 15 litres, professional installation by a qualified engineer is required to meet UK Building Regulations (G3 compliance).
Important Safety Note: Always turn off the water and electricity supply before starting. If you're not confident with plumbing or electrical work, consult a professional. Improper installation can lead to leaks, pressure issues, or safety hazards.
Why Choose Mains Pressure Water Heaters?
Mains pressure water heaters deliver hot water at full mains pressure, ideal for powerful showers and multiple outlets. They are energy-efficient and compatible with modern UK homes. For more details on selecting the right model, check our Water Heaters Buying Guide.
What You'll Need for Installation
- Your chosen mains pressure water heater (browse our collection here).
- Potable expansion vessel to handle thermal expansion – essential for system safety. Explore our range at Potable Expansion Vessels.
- Water heater connection set, including safety valves and fittings. We recommend our Water Heaters Connection Set also Fitting kit (6 Bar) for a complete, compliant kit.
- Basic tools: adjustable spanner, pipe cutter, PTFE tape, screwdriver, and spirit level.
- Flexible hoses, compression fittings, and isolation valves.
Our Water Heaters Connection Set also Fitting Kit(6 Bar) includes everything from pressure relief valves to flexible hoses, making installation straightforward and ensuring G3 compliance. Buy now to simplify your setup.
Electrical Installation Requirements
For 3kW Storage Water Heaters
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Circuit Protection: Install a 16A Residual Current Breaker with Overcurrent protection (RCBO) to safeguard against electrical faults and ensure compliance with IET Wiring Regulations (BS 7671).
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Isolation Switch: Use a 20A double pole switch for safe disconnection of the heater from the mains supply. This must be easily accessible and located near the heater.
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Wiring: Use 2.5mm² heat-resistant cable rated for at least 85°C, connected to a dedicated circuit with a minimum 20A capacity. An electrician should verify the earthing and bonding.
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Power Supply: Ensure a 230V AC single-phase supply with a maximum load of 3kW.
For 2kW or Less Storage Water Heaters
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Circuit Protection: A 13A fused spur with a 3A or 13A fuse (depending on manufacturer specifications) is sufficient for lower power models.
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Isolation Switch: A 13A double pole switch can be used for isolation, ensuring it is accessible.
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Wiring: Use 1.5mm² heat-resistant cable rated for at least 85°C, connected to a standard 13A socket or fused spur.
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Power Supply: Ensure a 230V AC single-phase supply with a maximum load of 2kW.
Always hire a qualified electrician to perform electrical connections and certify the installation to meet UK standards.
Step-by-Step Installation Guide
Step 1: Prepare the Installation Site
Choose a location close to your water supply and electrical outlet, such as under a sink or in a utility room. Ensure there's space for the heater, expansion vessel, and drainage. Mount the unit vertically or horizontally as per the manufacturer's instructions.
Image: Boiler Safety Unit DN15 15mm Compression – Key components include shut-off valve, check valve, relief valve, and rotatable funnel.
Step 2: Install the Cold Water Inlet and Safety Unit
Connect the cold water supply to the inlet of the boiler safety unit. This unit must be fitted to the cold water inlet of your water heater. Use compression fittings for a secure seal.
The boiler safety unit (often called a pressure relief valve assembly) plays a crucial role:
- Supply of Water: Allows cold water to enter the heater.
- Shut-Off Capability: Enables isolation of the supply line for maintenance.
- Overpressure Protection: The relief valve opens if pressure exceeds safe levels (e.g., 6-10 bar), preventing damage or bursts.
- Backflow Prevention: A check valve stops warm water from flowing back into the cold supply, reducing contamination risks.
Refer to the schematic below for connection points:
Under-sink installation showing the Boiler Safety Unit (highlighted) connected to the cold inlet.
Step 3: Fit the Expansion Vessel
An expansion vessel is vital for mains pressure systems to absorb water expansion during heating, preventing pressure spikes. Size it based on your heater's capacity (e.g., 6-8L vessel for a 200L tank). Connect it to the cold water line before the heater.
For reliable options, visit our Potable Expansion Vessels collection. Incorrect sizing can lead to system failure, so consult our buying guide for sizing tips.
Step 4: Connect Hot Water Outlet and Drainage
Attach the hot water outlet to your taps or showers using flexible hoses. Install a tundish and drain pipe for the relief valve to safely discharge excess water. Ensure the drain leads to a safe location.
Overview of a complete under-sink installation with boiler and connections.
Step 5: Electrical Connection and Testing
Wire the heater to a dedicated circuit with appropriate amperage (consult an electrician). Fill the system with water, check for leaks, and test the pressure relief valve. Set the thermostat to 60°C for safety and efficiency.
Step 6: Final Checks and Commissioning
Verify all connections, insulate pipes to reduce heat loss, and run a full test cycle. For larger systems, have a qualified engineer certify the installation.
Components of our Water Heaters Connection Set (6 Bar), including valves, hoses, and fittings.
The Role of the Boiler Safety Unit (Pressure Relief Valve)
The boiler safety unit, compliant with EN 1488 standards, is a multifunctional device installed on the cold water inlet. It includes a shut-off valve, check valve, test plug, relief valve, rotatable funnel, and adapter for flexible hoses. Operating at up to 65°C continuously (90°C peak for 1 hour) and 2.200 L/h at 1 bar, it protects against overpressure (available in 6, 7, 8, 9, 10 bar options). This prevents system damage and ensures safe operation by venting excess pressure and stopping backflow.
Why Use a Potable Expansion Vessel?
Expansion vessels are crucial for unvented mains pressure systems to manage thermal expansion. Without one, pressure could build dangerously. Our collection offers sizes from small 2L units for compact heaters to larger vessels for big systems. Shop now at Potable Expansion Vessels for certified, durable options.
Frequently Asked Questions
Have more questions? Visit our Water Heaters Frequently Asked Questions page for in-depth answers on types, capacities, and maintenance.
Ready to Install? Shop with Quality Heating
Get everything you need from our store, including the Water Heaters Fitting Kit (6 Bar) and expansion vessels. For expert advice or custom quotes, contact us today.
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