Summary

A fire valve (also called a fusible link valve or thermal trip valve) is a mandatory safety component on oil-fired heating installations. Its purpose is straightforward: if the area around the boiler or appliance reaches a temperature indicating fire, a fusible link in the sensor melts and spring-activates a valve in the oil supply line, cutting off the fuel. This prevents an uncontrolled feed of oil onto a fire.

The fire valve is a passive, fail-safe device — it requires no power, no electronics, and no user action to operate in an emergency. Its only weakness is that it must be accessible for manual reset by a trained engineer after operation (and after the root cause has been identified and resolved). Automatic reset fire valves are not acceptable in domestic installations.

OFTEC technical standard OFS T200 covers fire valve specification, siting, and installation. The Building Regulations Approved Document J references the OFTEC standards for compliance. Failure to install a correctly sited fire valve on an oil system is a notifiable breach of the building regulations.

Key Facts

  • Mandatory installation — a fire valve is required on all oil supply pipework to any oil-fired appliance in a domestic property; no exceptions
  • Valve types — fusible link type: a wax-filled capsule or alloy plug in the sensor that melts at 95°C±5°C; spring returns the valve to closed position when link melts
  • Operating temperature — 95°C standard domestic rating (suitable for all domestic boilers and range cookers); higher-rated valves (120°C, 150°C) exist for commercial applications — do not use higher-rated valves where 95°C is specified
  • Manual reset only — all domestic fire valves must require manual reset by removing and replacing the fusible link and physically resetting the valve mechanism; automatic reset is not permitted
  • Valve body location — must be installed in the oil supply line on the supply side of the appliance; typically in the meter/filter cupboard or at the tank, but must be remote from the fire hazard area
  • Remote sensor location — the sensor head is connected to the valve body by a capillary tube; sensor is sited at the point of highest fire risk: 100–500mm above the top of the appliance, or in the boiler flue compartment (where a flue fire would be detected)
  • Maximum capillary tube length — manufacturer-specific, typically 1,000–2,000mm; do not extend or modify the capillary tube
  • Two-valve installations — where the fire risk extends to both the boiler room and the fuel store (e.g. an oil tank room adjacent to the boiler), two fire valves may be required, each with a sensor in the appropriate zone
  • Testing — fire valves cannot be tested by applying heat (this activates the valve and requires a new fusible link); check valve is not corroded, capillary tube intact, and valve operation is documented at each annual service
  • Annual service requirement — OFTEC engineers must inspect the fire valve at every annual service and record its presence and condition on OFT105
  • Replacement after operation — after any fire valve trips, the fusible link must be replaced with a new OEM-specification link; a non-standard link could have the wrong melting point, which is dangerous
  • Pipe size compatibility — fire valves are available for 8mm, 10mm, and larger copper pipe; confirm the valve body size matches the oil supply pipe

Quick Reference Table

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Specification Requirement
Operating temperature 95°C ±5°C (domestic installations)
Sensor siting height above appliance 100–500mm above top of appliance
Reset type Manual only (no automatic reset)
Capillary tube length Per manufacturer spec (typically ≤2,000mm)
Valve body position In oil supply line, away from fire hazard area
Standard reference OFTEC OFS T200; BS 5410-1:2019
Mandatory? Yes — all domestic oil installations
Annual inspection required? Yes — OFTEC OFT105 records condition
Pipe sizes available 8mm, 10mm (domestic); larger for commercial

Detailed Guidance

How the Fire Valve Works

The fire valve consists of two parts connected by a capillary tube: the valve body (installed in the oil supply line) and the remote sensor head (positioned near the appliance).

The sensor head contains a hermetically sealed capsule filled with a wax compound. At room temperature, the wax is solid and holds the capillary tube pressurised, which in turn holds the valve body in the open position against a spring. If fire causes the sensor area to reach 95°C, the wax liquefies, pressure drops in the capillary tube, and the spring closes the valve, stopping the oil flow.

This operation is irreversible without a replacement fusible link — once the wax melts, the capsule cannot reseal. This is intentional and is a safety feature.

Sensor Positioning

Correct sensor positioning is critical to the fire valve functioning as intended. An incorrectly sited sensor may fail to detect a fire (if too far from the fire risk area) or may operate unnecessarily in normal service (if placed in a location that reaches 95°C during normal boiler operation — e.g., too close to the flue).

Standard domestic boiler:

  • Primary sensor position: 100–500mm directly above the top of the boiler cabinet, on the front face or within the boiler housing if the manufacturer provides a sensor pocket
  • Secondary consideration: if the boiler has a flue compartment where a flue fire could develop, place the sensor in that space

Range cooker (Aga, Rayburn, Esse):

  • Sensor should be positioned above the appliance in the hottest zone — typically above the combustion chamber access, not above the hot plates (which are separated by insulation)
  • Do not place the sensor directly on the flue (too hot in normal operation for some positions)

External boiler house or utility room:

  • If the boiler is installed in an outbuilding or separate room, the sensor goes in that room; the valve body may be in the oil supply at the tank or en route
  • Consider whether a secondary valve is needed at the tank if there is a significant oil supply run through the building before reaching the boiler room

Garage installations:

  • Sensor above the boiler; valve body typically within the supply pipe run in the garage

Part J Compliance and OFTEC OFS T200

Building Regulations Approved Document J requires that oil storage and supply systems protect against fire risk. The specific requirement for fire valves is referenced to the OFTEC standard OFS T200, which is the technical specification that defines:

  • Valve operating temperature requirements
  • Reset mechanism requirements (manual only)
  • Sensor siting rules
  • Capillary tube length and handling requirements
  • Test and inspection procedures

OFTEC-registered engineers self-certify compliance with Part J. The OFTEC commissioning certificate (OFT105) includes a specific field for confirming the fire valve has been installed and inspected. Installations without a properly sited fire valve cannot be certified as Part J compliant.

Manual Reset Procedure

After a fire valve trips — whether from an actual fire or from a false trip due to overheating near the sensor — the following procedure is required before the system can be returned to service:

  1. Identify and resolve the root cause — if an actual fire occurred, make safe, inspect for damage to pipework, tank, and appliance before resetting
  2. Source a replacement fusible link — obtain the correct OEM-specification link (correct operating temperature for the valve model)
  3. Replace the fusible link — follow the manufacturer's procedure; typically involves removing the sensor head, fitting the new capsule/link, and reassembling
  4. Manually reset the valve body — most designs require compressing the valve against the spring until it latches in the open position; this is held by the pressurised capillary
  5. Check for oil supply — confirm oil is flowing freely to the appliance before attempting to restart
  6. Re-commission if necessary — after any fuel supply interruption, re-prime the burner as required

Do not reuse a tripped fusible link — once activated, the link is destroyed and cannot be re-rated.

Common Installation Errors

Error 1: Valve body in the fire zone: Some engineers mistakenly install the valve body adjacent to the boiler. The valve body must be outside the fire hazard area — if the valve body itself gets hot, it can compromise the valve mechanism or the oil supply pipe connection. The valve body goes in the supply line at a safe distance.

Error 2: Sensor too high: Placing the sensor higher than 500mm above the appliance risks delayed response — smoke and hot gases rise, so a sensor too far above the appliance may not reach 95°C quickly enough in an early-stage fire. The 100–500mm range is the OFTEC-specified zone.

Error 3: Wrong temperature rating: Using a 120°C or 150°C fire valve (commercial grade) in a domestic installation because it was on the van. The higher trip temperature means the valve activates later in a fire, reducing its protective value.

Error 4: Automatic reset valve: Some older or commercial valves have automatic reset mechanisms. These are not permitted in domestic oil installations. After a fire, an automatic reset would restart the oil supply while the fire may still be present.

Error 5: Extended or modified capillary: Some installers extend the capillary tube to reach a more convenient sensor position. The capillary tube and sensor are a matched, calibrated system — modification changes the pressure-volume relationship and may prevent operation at the correct temperature.

Frequently Asked Questions

How do I test a fire valve without activating it?

You cannot test the actual operation of a fire valve without melting the fusible link (which means fitting a replacement). At the annual service, the engineer should:

  • Visually inspect the valve body for corrosion and physical damage
  • Check the capillary tube for kinks, cracks, or damage
  • Confirm the sensor head is secure in its correct position
  • Document the inspection on the service record
  • Advise replacement of the fire valve if it appears corroded or is over 10 years old (component reliability diminishes with age)

Does the fire valve protect against boiler malfunction as well as fire?

No. The fire valve only responds to heat at the sensor location reaching 95°C. This is a temperature reached only in fire conditions — normal boiler operation does not reach this temperature in the sensor zone. The boiler's own safety devices (high limit thermostat, pressure relief valve, flue overheat sensor) protect against internal boiler faults.

The fire valve tripped and there was no fire — why?

The sensor area may have been exposed to unusual heat: a blocked flue causing combustion gas backflow into the boiler room, an overheating appliance (overfired burner), or unusual ambient conditions (a room that gets excessively hot in summer near the appliance). Check the boiler operation, flue, and burner settings before resetting. If no clear cause is found, the fire valve sensor may be too close to a heat source in normal operation — reposition it within the 100–500mm guidance range.

My customer's boiler is 25 years old and never had a fire valve — is it legal?

Pre-existing installations that predate the regulatory requirement are not retrospectively required to be upgraded under building regulations. However, OFTEC guidance is clear that fire valves are required on all oil installations as a matter of safety. When servicing an existing system without a fire valve, the OFTEC engineer should advise the customer in writing that the installation does not comply with current standards and that a fire valve should be fitted. Document this advice on the service record. Fitting the fire valve at service is strongly recommended.

Regulations & Standards