Safety Valves and Pressure Relief Valves for Combi Boilers and Unvented Cylinders: Selection, Testing, and Discharge

Pressure safety valves are a critical safety device in domestic and commercial heating and hot water systems. They are the last line of defence against dangerous over-pressurisation — a condition that can lead to catastrophic vessel failure. This guide covers the types of safety valve used in UK heating systems, selection criteria, discharge pipework requirements, testing procedures, and the regulations governing their installation and replacement.

Why Safety Valves Are Critical

Modern sealed heating systems, combi boilers, and unvented hot water cylinders all operate under pressure. Without functional pressure relief, a failure of the expansion vessel, a faulty filling loop left open, or a stuck thermostatic control could cause pressure to rise to dangerous levels. Safety valves are designed to open at a defined set pressure and discharge water or steam, reducing system pressure before catastrophic failure occurs. A correctly installed and functioning safety valve can prevent boiler explosions, cylinder ruptures, and the extreme scalding risk associated with superheated water suddenly released at atmospheric pressure.

Types of Safety Valve Used in UK Heating

Pressure Relief Valves (PRVs) — Sealed Heating Systems

The most common safety device in domestic sealed central heating systems is the pressure relief valve set at 3 bar. This valve is fitted at or adjacent to the boiler and protects the sealed system circuit. It opens when circuit pressure exceeds 3 bar and closes (ideally) when pressure returns below the set point.

PRVs for heating systems are manufactured to BS EN ISO 4126-1 and must be marked with their set pressure (usually 3 bar) and discharge capacity. They are typically fitted in the flow pipe close to the boiler heat exchanger — the highest-temperature point — and must not be isolated from the system by any valve.

Standard specifications for domestic sealed system PRVs:

Set pressure: 3 bar (consistent with standard boiler maximum working pressure)
Connection size: ½" BSP male (most common) or ¾" BSP for higher-output boilers
Discharge: tundish to indirect drain or outside
Standard: BS EN ISO 4126-1

Temperature and Pressure Relief (TPR) Valves — Unvented Cylinders

Unvented hot water cylinders require a combination temperature and pressure relief valve (TPR valve or T&P valve). This is a more complex device with two independent triggers: a pressure element and a thermal element. It opens if either:

System pressure exceeds the set pressure (typically 7 bar for unvented cylinders, consistent with a maximum working pressure of 6 bar), OR
Water temperature exceeds the thermal set point (typically 90-95°C, protecting against thermostatic control failure)

TPR valves for unvented cylinders must comply with BS EN 1490 and be approved as part of the cylinder manufacturer's package. They are not interchangeable between makes and models without checking the manufacturer's specification — the discharge capacity must match the cylinder's maximum heat input. Unvented cylinder work is restricted to G3-qualified engineers (Part G of the Building Regulations).

Expansion Relief Valves — Cold Water Supply

Where a check valve (non-return valve) or pressure reducing valve is fitted in the cold water supply to a hot water storage vessel, thermal expansion of the stored water on heating has nowhere to go — it cannot push back through the check valve. An expansion relief valve (typically set at 6 bar for mains-fed systems, or matched to the vessel working pressure) is fitted downstream of the check valve to relieve this expansion pressure.

On properly designed unvented systems, the expansion vessel accommodates normal thermal expansion; the expansion relief valve is a secondary safety device for abnormal expansion. The expansion vessel must be correctly sized and pre-charged for this to work — see Article #29 (Expansion Vessels) for sizing detail.

Boiler Relief Valves — High-Pressure (Combination) Systems

Some high-efficiency combi boilers include an integral pressure relief valve factory-set and not field-adjustable. In these cases, the boiler's built-in PRV satisfies the protection requirement, but a discharge pipework connection must still be made to a suitable tundish and drain point. Refer to the boiler installation manual for the required PRV discharge pipework size and routing.

Selection and Sizing

Selecting the correct safety valve involves matching three parameters:

Set Pressure

The set pressure must be below the maximum allowable working pressure (MAWP) of any component in the system. For sealed heating systems, the maximum working pressure is typically 3 bar (matching the PRV set pressure). The static fill pressure at the lowest point (usually the PRV location) must be at least 0.5 bar below the PRV set point to prevent nuisance operation on thermal expansion. Normal fill pressure is typically 1.0-1.5 bar cold; maximum operating pressure (when hot) should not exceed 2.5 bar to leave 0.5 bar margin below the 3 bar set point.

Discharge Capacity

The valve's certified discharge capacity must exceed the maximum heat input of the system. For domestic combi boilers (15-40kW), a standard ½" PRV rated at 3 bar with a discharge capacity of 3-4 kW is typically adequate. For unvented cylinders, the TPR valve must be capable of discharging all heat input from the immersion heater or primary coil — cylinder manufacturers specify the required valve capacity in their installation documentation.

Connection Size

Most domestic PRVs are ½" or ¾" BSP male. The discharge pipework must be the same size as or larger than the valve outlet — never reduce the discharge pipework downstream of the valve. Where a tundish is used, the pipe from the valve to the tundish must be the same bore as the valve outlet; the pipe from the tundish to drain may need to be upsized to ensure adequate discharge flow (D2 calculation per BS 6700 or manufacturer's guidance).

Discharge Pipework Requirements

Safety valve discharge must be routed safely — superheated water or steam discharging during an over-pressure event can cause severe scalding. Requirements:

Tundish

A tundish (open funnel) must be fitted in the discharge pipework where it can be observed, typically close to the valve itself. The tundish provides:

Visual indication that the valve has opened (water visible in the tundish)
An air break to prevent back-siphonage from the drain into the hot water system
A point where discharge can be checked before reaching the termination point

For unvented cylinders, the tundish must be within 600mm of the TPR valve and in the same enclosure or in a visible location. The discharge pipework from the tundish to the termination point must fall continuously (no upward runs) and be as short and direct as possible.

Pipe Material and Size

Discharge pipework must be capable of withstanding the maximum temperature of discharge — this is often superheated water above 100°C. Suitable materials: copper (most common), stainless steel, or CPVC. Plastic waste pipe is not suitable for TPR valve discharge from unvented cylinders due to the temperature risk, though it may be acceptable for low-set-pressure PRV discharge on sealed heating systems where temperatures are lower — check with the manufacturer.

Minimum pipe sizes: same bore as valve outlet for the connection to the tundish; typically one pipe size larger for the run from tundish to termination to account for additional friction losses.

Termination Point

The discharge must terminate in a safe, observable location where it will not cause harm or damage. Suitable termination points:

External wall termination discharging over a drain or gully at low level (150mm above gully level)
Internal floor drain or trapped gully within a plant room
NOT into a soil stack, sealed drain, or any location where back-pressure could develop
NOT above electrical equipment, gas meters, or in a location where hot discharge would be a scalding hazard to building occupants

Discharge pipework must be secured against movement during discharge — a safety valve discharging significant flow generates thrust that can dislodge unsecured pipework.

Common Causes of Safety Valve Operation

Understanding why a safety valve has lifted is essential — replacing a leaking valve without addressing the root cause will result in the new valve failing in the same way. Common causes:

Sealed Heating System PRV Weeping or Lifting

Oversized or failed expansion vessel: the most common cause. If the vessel membrane has failed or the pre-charge pressure is incorrect, the system cannot accommodate thermal expansion and pressure rises to the PRV set point on every heating cycle. Test and replace or re-charge the expansion vessel first — see Article #29.
Filling loop left open or leaking: mains water pressure (typically 3+ bar) forces pressure above the PRV set point. Check and close/isolate the filling loop. A dripping filling loop needle valve will slowly raise system pressure over days.
PRV seat fouled or damaged: calcium scale or debris on the valve seat causes weeping. A new valve may seal correctly initially but lift again if the root cause (scale, overpressure events) is not resolved.
Incorrect PRV set pressure: if a replacement PRV was installed at a higher set pressure than required, the system may routinely operate above the original set pressure.

Unvented Cylinder TPR Valve Operating

Thermostat failure: the cylinder thermostat failed to cut off the heat source (immersion heater or primary coil). The thermal element of the TPR valve opens. This is a serious fault — the cylinder's thermostatic and high-limit thermostat controls should both be checked and replaced.
Expansion vessel failure: loss of pre-charge in the cylinder's dedicated expansion vessel causes pressure to rise rapidly on heating. Restore vessel pre-charge or replace the vessel.
Cold water supply pressure exceeding valve set pressure: mains pressure exceeding 6 bar triggers the pressure element. A pressure reducing valve (PRV) is required in the cold water supply, set to maintain inlet pressure below 3.5 bar for typical 6 bar working pressure cylinders. See Article #16 (Pressure Reducing Valves).

Testing Safety Valves

Safety valves should be tested at least annually as part of system service. For unvented cylinders, testing the TPR valve is an annual service requirement under G3 regulations.

Test Procedure for Sealed System PRVs

Ensure discharge pipework is clear and will discharge to drain safely
Locate the test lever on the valve body
Briefly lift the test lever (one to two seconds) — water should flow freely from the discharge
Release the lever — flow should stop completely
If the valve continues to weep after testing, the seat may be fouled; a further brief lift may clear debris from the seat. If weeping continues, replace the valve.
Check that system pressure (as shown on the gauge) returns to normal fill pressure within a few minutes

Do not leave a test lever in the lifted position — the system will depressurise fully and require refilling. Do not test a PRV if you cannot safely observe the discharge point.

Test Procedure for TPR Valves

Unvented cylinder TPR valve testing follows the same lever-lift procedure, but the temperature element cannot be tested without heating the water to near the trigger temperature (90-95°C) — this is not practical during a routine service call. Annual testing confirms the pressure element is operational; thermal element operation is confirmed by checking the element rating stamp and valve condition.

Replacement Considerations

When replacing a safety valve:

Match the set pressure exactly to the original — do not upgrade to a higher set pressure without recalculating system MAWP
For unvented cylinder TPR valves, use only the manufacturer's approved replacement valve for that cylinder model — cylinder G3 compliance depends on the entire assembly including the approved valve
Inspect and clean the valve seat area in the system before fitting a new valve
Check and replace the sealing washer/gasket on the new valve installation
Pressure-test the system after installation to confirm the new valve seals at normal operating pressure
Issue a Minor Works Certificate (or update the cylinder's unvented commissioning document) after TPR valve replacement

Regulatory Framework

Key regulations governing safety valve installation and unvented cylinder work:

Building Regulations Part G3: unvented hot water storage systems — requires installation by G3-qualified engineers and specific safety devices including the TPR valve, pressure reducing valve, expansion vessel, and discharge pipework
Building Regulations Part L (conservation of fuel and energy): requires efficient hot water system design
BS EN ISO 4126-1: safety valves for steam and gas installations (referenced for PRVs)
BS EN 1490: building valves — combined temperature and pressure relief valves (TPR valves for unvented cylinders)
WRAS approval: valves in contact with potable water must be WRAS-approved. Unvented cylinder TPR valves are typically WRAS-approved as part of the cylinder package.
CIPHE/Institute of Plumbing guidance: discharge pipework routing and tundish requirements are specified in the technical appendices to Building Regulations Part G

All work on unvented hot water systems in dwellings is notifiable under Part G3. The installing engineer must be qualified (BPEC or City & Guilds G3 unvented hot water systems certificate or equivalent) and must issue appropriate commissioning documentation.

Products from APM Plumbing & Electrical

APM Plumbing & Electrical does not currently stock 3 bar PRVs or tundishes as standalone products. The following related items support pressure testing, cold supply check valve requirements, and service isolation work covered in this guide:

KeyPlumb RWC Pressure Gauge 63mm 0–10 Bar ¼" MBSP

Expansion Vessel Check Valve 2Ltr (Cold Supply Non-Return)

JG Speedfit Double Check Valve 15mm

Embrass Peerless Isolation Valve Angle Chrome 15mm

Visit us at APM Electricals, 31 Inglis Road, London, W5 3RL. Call 020 8819 5852.

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Safety Valves and Pressure Relief Valves for Combi Boilers and Unvented Cylinders