Question 1

Why is my T&P valve dripping or discharging?

Accepted Answer

A discharging T&P valve is almost always a symptom, not a fault in the valve itself. In London the most common cause is a waterlogged expansion vessel — when the air pre-charge collapses, expanding hot water has nowhere to go and the T&P lifts as the designed safety route. Less common causes are a failed PRV letting mains pressure spike inlet pressure past 6 bar, a failed working thermostat allowing stored water past 90°C, or a scaled T&P seat that has stopped reseating after a legitimate lift. The 15-minute diagnostic order is always vessel pre-charge first, then inlet pressure, then thermostat function, then seat inspection. Swapping the valve without finding the upstream fault wastes a part and leaves the safety route blowing again within weeks.

Question 2

What pressure should a T&P valve be rated to?

Accepted Answer

Every UK unvented hot water cylinder uses a combined temperature and pressure relief valve rated to lift at 7 bar (700 kPa) on pressure and 90°C on temperature. The 7 bar pressure setting sits deliberately above the 6 bar maximum that the inlet pressure-reducing valve permits, but below the 10 bar test pressure of a duplex stainless tank. Fitting a valve with a different pressure rating — a 6 bar valve, a 10 bar valve, or a sealed-system heating PRV — is a direct breach of BS EN 12897 and Approved Document G3. The 7 bar / 90°C dual-trigger is non-negotiable on every UK unvented cylinder.

Question 3

How long does a T&P valve replacement take?

Accepted Answer

A like-for-like swap on an accessible cylinder takes 60–90 minutes from arrival to handover. Drain-down, removal, refit and witnessed lift test sit inside that window. Where the cylinder boss thread is heavily scaled and needs cleaning, or where the discharge pipework needs re-routing to fix a G3 non-compliance, the visit can stretch to 2–3 hours. Combined T&P + expansion vessel work — which is what the diagnostic typically calls for — runs to two hours. Out-of-hours emergency visits cover the same scope at a fixed first-hour rate.

Question 4

Can I replace a T&P valve myself?

Accepted Answer

No. Approved Document G3 of the Building Regulations makes any modification to the sealed safety system of an unvented hot water cylinder a controlled service. A T&P valve replacement is a notifiable modification — the engineer must hold a current G3 ticket and the work must be notified to building control directly or through a competent person scheme. A DIY swap voids the manufacturer warranty (typically a 25-year tank cover), invalidates the home insurance, and exposes the owner to enforcement under the Building Act 1984. The valve itself is cheap; the legal and safety stack around it is not.

Question 5

Do I need an OEM brand-correct T&P valve, or will a generic one work?

Accepted Answer

Always brand-correct for the warranty path. The Megaflo Heatrae Sadia 95612893, the Gledhill XG090/XG107, the Kingspan Albion AT5/AT7 and the OSO 800 0042 are all WRAS-approved OEM parts and they preserve the 25-year tank warranty. A WRAS-listed generic valve from Caleffi or Reliance is legal and safe, but on the major UK brands the manufacturer parts register will flag a non-OEM swap and the tank warranty becomes a fight. On Megaflo cylinders in particular the brand-correct seat geometry matters — non-OEM lookalikes have a documented 6–12 month failure rate on Megaflo bosses.

Question 6

What is the manual lift lever on the T&P valve for?

Accepted Answer

Every UK unvented T&P valve carries a manual test lever. Lifting it briefly forces a discharge through the tundish — it is the only legitimate way to verify the safety device on a working cylinder. The annual G3 service includes a witnessed lift test of the lever, with the discharge confirmed to flow cleanly through the tundish and down the D1 pipework to the external termination. A valve that won't reseat after a lift test almost always indicates limescale on the seat — the next step is replacement.

Question 7

What is a tundish and why does it sit under the T&P valve?

Accepted Answer

The tundish is the small clear plastic funnel directly below the T&P discharge outlet. It does two jobs: it provides a visible 300 mm air gap (required by G3 to break the connection between the pressurised cylinder and the open drain), and it lets the homeowner see when the T&P has discharged. A wet tundish is the single most common reason a homeowner calls a cylinder engineer. The air gap requirement is why you can't pipe the T&P directly into a hopper, soil pipe or void — the tundish must be visible and the discharge must terminate outside.

Question 8

Where must the T&P discharge pipework terminate?

Accepted Answer

The discharge pipework from the tundish must be at least one pipe size larger than the inlet to the T&P valve (typically 22 mm copper from a 15 mm valve), with a continuous fall, no more than three bends, and a maximum equivalent length per the G3 Approved Document Table — D1 from tundish, then D2 to outside. It must terminate to a visible, safe external location — usually a gully, an external wall low-level discharge, or in some installations a Hotun air-break direct to a soil pipe. Internal discharge into hoppers, soil pipes or voids is a Building Regulations breach and the single most common fail we find on inherited installs.

Question 9

Will a new T&P valve fix my dripping cylinder permanently?

Accepted Answer

Only if the upstream fault has been corrected first. If the T&P was lifting because the expansion vessel was waterlogged, fitting a new valve without recharging or replacing the vessel buys you 2–6 weeks before the new valve is doing the same job and dripping again. If the T&P was lifting because the PRV had failed and mains pressure was spiking past 6 bar, the new valve is back to dripping within days. A correctly diagnosed T&P replacement — vessel checked, PRV checked, thermostat checked first — lasts 8–12 years before the next preventive swap, the same as the valve manufacturer's published seat life.

Question 10

Does a T&P valve replacement need to be notified to Building Control?

Accepted Answer

Yes. Approved Document G3 of the Building Regulations treats any modification to the sealed safety system of an unvented hot water cylinder as a controlled service. The notification is filed by the G3 engineer through a competent person scheme (we use WaterSafe / BESCA) — the homeowner does not have to call the local authority. The Building Regulations Compliance Certificate is posted to the property within 30 days. That paperwork is what the home insurance, the cylinder warranty registry and any future sale survey will expect to see.

Question 11

How long should a T&P valve last before it needs replacing?

Accepted Answer

A correctly fitted WRAS-approved T&P valve on a London cylinder typically lasts 8–12 years before preventive replacement. In a soft-water postcode (most of NW London on the lower Thames Lee Valley supply) a valve can run 12–15 years without intervention. In hard-water postcodes (E, EC, N, SW, W on Thames Water mains-fed supply) the seat scales faster and replacement often falls due at 6–9 years. Annual G3 servicing extends valve life by catching limescale binding early through the lift-lever test.

Question 12

Is the T&P valve the same as the PRV (pressure reducing valve)?

Accepted Answer

No — they are two different safety devices on different parts of the cylinder. The PRV (pressure reducing valve) sits on the cold inlet group, before the cylinder, and limits mains pressure to a safe working level of 3.0–3.5 bar (capped at 6 bar maximum). The T&P (temperature and pressure relief valve) sits on the top of the cylinder body itself and discharges if stored water reaches 90°C or internal pressure reaches 7 bar. Both are tested annually but the diagnostics and the replacement parts are different. Confusing the two is the single most common mistake on DIY forum posts.

Question 13

What's the difference between a T&P valve and an expansion relief valve?

Accepted Answer

On some smaller or older UK unvented cylinders the expansion relief and the temperature/pressure relief are two separate valves — the expansion valve lifts on pressure rise (usually around 6 bar) due to thermal expansion, while the T&P lifts on the dual 7 bar / 90°C trigger as the layered safety. On modern combined-spec cylinders (every recent Megaflo, Gledhill, Kingspan Albion, OSO model) the two functions are integrated into a single combined T&P valve. Knowing which design you have matters when ordering parts and when reading discharge symptoms — call us with the cylinder data plate photo and we'll tell you which it is.

Question 14

Can a T&P valve cause a cylinder to explode?

Accepted Answer

The opposite — a working T&P valve is the device that prevents catastrophic failure. The well-publicised UK unvented cylinder incidents (Cheshire 2016, Norfolk 2018) involved installations where the T&P had been deliberately plugged off, bypassed or removed — every layered safety device had been defeated. A correctly fitted and annually tested T&P valve is the engineered reason a sealed pressure vessel storing 60°C water cannot fail catastrophically. Removing or capping a T&P is one of the few mechanical decisions in domestic plumbing that puts lives at direct risk.

Question 15

Will replacing the T&P valve affect my cylinder warranty?

Accepted Answer

Only positively, if the brand-correct OEM part is used and the work is notified through G3. A documented G3-engineer T&P replacement, with a brand-correct WRAS-approved OEM part and a signed benchmark logbook entry, preserves the full 25-year tank warranty on Megaflo, Gledhill, Kingspan Albion, OSO and Range cylinders. A DIY swap, or a non-OEM lookalike valve, voids the warranty the moment the manufacturer's parts register flags the change. We update the parts register on every brand-name install we touch.

Brand	OEM part	Fits cylinders	Notes
Heatrae Sadia (Megaflo Eco / CL)	95612893 — combined T&P, 7 bar / 90°C	All Megaflo Eco, Megaflo Eco Plus, Megaflo CL Indirect, PremierPlus	Stocked on every van. Brand-correct seat geometry — non-OEM lookalikes can fail in 6–12 months on Megaflo bosses.
Gledhill	XG090 / XG107 — combined T&P, 7 bar / 90°C	StainlessLite Plus, EnviroFoam, BoilerMate	OEM Gledhill part required to preserve the 25-year tank warranty.
Kingspan Albion	AT5 / AT7 — combined T&P, 7 bar / 90°C	Ultrasteel, Ultrasteel Plus, Aerocyl	Direct manufacturer part. WRAS-approved replacement.
OSO Hotwater	OSO 800 0042 — combined T&P, 7 bar / 90°C	Super S, Super SX, Delta Coil	Marine-grade duplex bodies; brand-correct part essential to preserve the 25-year stainless warranty.
Range / Tribune	RG09 — combined T&P, 7 bar / 90°C	Tribune HE, Tribune Xpress	Common in 2000s London social housing stock. Parts stocked.
Telford	TFT-090 — combined T&P, 7 bar / 90°C	Tempest, Hurricane, Maxistore	Country-stock cylinders — used on London period flats with rear extensions.

Job	What's included	Typical cost
Diagnostic call-out (T&P discharging)	On-site G3 diagnostic: vessel pre-charge test, inlet pressure check, thermostat function, T&P seat inspection. Written quote on the spot.	£95–£140
T&P valve replacement — like-for-like (15 mm, 7 bar / 90°C)	Replacement combined temperature & pressure relief valve, drain-down, isolation, refit, witnessed lift test, discharge route inspection, recommissioning.	£195–£260
T&P valve + discharge pipework re-route	Valve swap plus 22 mm copper discharge re-run to compliant external termination (D1/D2 lengths to G3 Table), tundish gap reset, certificate updated.	£295–£480
T&P + expansion vessel replacement (combined visit)	Often the real fix when a T&P is dripping. Replace both, recharge vessel to 3.0 bar, witnessed T&P lift, full recommissioning.	£365–£520
T&P + PRV (inlet control group) replacement	Replace failed pressure-reducing valve and T&P together, balance inlet to 3.0–3.5 bar working pressure, strainer clean, leak test.	£345–£495
Emergency / out-of-hours T&P replacement	Evenings, weekends, bank holidays. First-hour fixed + parts. 24/7 cover across all 32 London boroughs.	£245–£340
T&P valve replacement on Megaflo / Megaflo Eco	OEM Heatrae Sadia / Megaflo T&P assembly, brand-correct seat and discharge fitting, manufacturer warranty preserved.	£215–£290
T&P valve replacement on Gledhill / Kingspan Albion	OEM Gledhill or Kingspan T&P assembly, brand-correct factory part, 25-year tank warranty preserved.	£215–£285

T&P Valve Replacement London

What is a T&P relief valve?

T&P valve specification — what every London cylinder needs

Pressure setting — 7 bar

Temperature setting — 90°C

Connection — 15 mm BSP

WRAS approval

BS EN 1490 compliance

Manual lift lever

Brand-correct parts — by cylinder manufacturer

T&P valve replacement cost London — 2026 prices

The 5-step replacement process

Isolate & depressurise

Remove the failed valve

Fit the new T&P (7 bar / 90°C)

Reconnect discharge & tundish

Pressurise & witnessed lift test

G3 compliance & Benchmark commissioning

Approved Document G3 — Hot water safety

Combined temperature and pressure relief valves

Indirectly heated unvented closed storage water heaters

Water Supply (Water Fittings) Regulations 1999

HHIC Benchmark commissioning scheme

Common T&P fault scenarios — worked examples

Camden mansion block, 4th floor — Megaflo Eco 170L T&P dripping intermittently

Islington garden flat — Gledhill StainlessLite Plus 210L blowing T&P on heat-up

Hackney terraced house — Kingspan Albion Ultrasteel 250L T&P seat scored by limescale

Wandsworth new-build, 2-bed flat — generic valve fitted by previous trader, failed in 11 months

Westminster top-floor mansion flat — discharge pipe non-compliant, T&P valve fine

T&P dripping into a saucer right now?

Areas covered across London

Frequently asked questions

Service Area — All London Boroughs

T&P valve dripping, blowing or seized?

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