Capped Relief Valve and a Tripping Water Heater
A 15-litre, 2 kW storage electric water heater was installed in a first-floor bathroom of a three-storey house in Ludhiana. The supply is intermittent, so a 1 HP pump fills the overhead tank at night, and static pressure at the heater inlet has been measured at 5.5 bar at 2 a.m. The heater inlet has a stop valve and a non-return valve, and the factory-fitted T&P relief valve (marked 0.8 MPa / 90 °C) discharges through 15 mm PVC pipe to a point 100 mm above the floor, where the plumber has fitted a threaded cap "because it kept dripping and wetting the floor". The thermostat is set at 70 °C. The customer reports that the casing feels warm, the 6 A MCB has tripped twice in a fortnight, and there is a faint hot-plastic smell. There is no RCCB on the circuit and the earth wire from the heater is clamped to the cold-water GI pipe. The heater sits in a corner with 40 mm clearance to the wall.
1. Why does the relief valve keep dripping, and what is wrong with the two "fixes" that were applied to stop it?
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The drip is a safety device doing its job on an installation designed to make it operate.
Why it lifts. The heater is fed through a non-return valve and a stop valve, so heated water has nowhere to expand. Water expands about 1.5–2 per cent by volume from cold to 70 °C - roughly 300 mL in a 15-litre tank. In a closed system that volume is incompressible and pressure climbs quickly. The night-time static inlet pressure is already 5.5 bar; a modest further rise crosses the 0.8 MPa (8 bar) set point and the valve opens. It reseats after a tap is used or the water cools, so the drip is cyclic, mostly at night.
Fix 1 - the cap. Capping or plugging a T&P valve removes the last pressure protection. If the thermostat sticks closed, the element keeps heating; the tank can split or the heater be ejected, and a cap that blows off is a projectile. This is a defect to refuse to leave live.
Fix 2 - 15 mm PVC discharge. PVC softens around 60–70 °C while the discharge is near boiling; it can sag, kink or burst exactly when needed. The discharge must be full-bore metal or CPVC, gravity-run, with no trap, no valve and no reduction, terminating visibly about 150 mm above the floor or outside.
Correct arrangement: a pressure-reducing valve set to 3.5 bar upstream of the heater, an expansion vessel sized roughly at 10–12 per cent of tank volume, no valve between the heater and its relief valve, thermostat turned down to 60 °C.
2. The MCB tripping, warm casing and hot-plastic smell - what do you check, and what do you do about it?
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Together these point to an electrical fault inside the heater or its supply, and the plumbing is secondary.
Isolate first. Switch off the MCB and the local isolator, lock and tag, prove dead. Do not reset the breaker a third time "to see if it holds".
Checks in order:
- Element insulation - disconnect the element tails and megger each terminal to earth at 500 V. Below about 1 MΩ means the element has absorbed water or cracked and must be replaced.
- Thermostat and thermal cut-out - check continuity and for a stuck contact. A thermostat that never opens lets the element run continuously, which explains both the smell and the excessive relief-valve operation.
- Supply and terminations - look for burnt or loose terminals, moisture at the terminal-cover gasket, and cable sizing. 2 kW at 230 V draws about 8.7 A, so a 6 A MCB is undersized and is itself a reason for tripping on cold start; a 16 A device is needed.
- Earthing - a clamp on a GI pipe is not an earth. A plastic section, jointing compound or a removed pipe can leave the casing live. Test earth continuity, run a proper earth from the distribution board, and fit a 30 mA RCCB on the circuit.
Action: replace a failed element, fit the correct MCB and RCCB, renew the terminal-cover seal, re-megger after the repair, and only then recommission.
3. What goes on the handover and service report, and what will you refuse to do?
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Refuse to leave the heater in service on a capped relief valve. Isolate it, write on the job card "T&P valve capped by others; heater left isolated pending correction", and get the customer to sign.
Report entries: insulation resistance and earth-continuity readings; RCCB trip test result; relief valve rating and the new full-bore discharge route with its termination point; PRV setting of 3.5 bar; expansion vessel size and pre-charge; thermostat setting of 60 °C; MCB and RCCB ratings; and the clearance from combustible surfaces per the maker's instructions (the 40 mm gap here is likely below the specified minimum).
Customer advice, in plain words: never plug, cap or block the relief valve - a drip is a message, not a fault; if it drips often, run a hot tap; have the unit serviced annually (element descaling, anode check, relief valve lift test); do not box the heater in or cover it; switch off the circuit before any maintenance. Warn about scald risk at 70 °C - 60 °C storage is safer and still delivers about 50 °C at the outlet, and it cuts scaling. Leave a copy of the report and the manufacturer's manual with the owner.