Installing, testing and maintaining vented hot water

This note follows a vented hot water job in the order you meet it.

Siting and supporting the cylinder

Each litre of stored water adds about a kilogram, so stand the cylinder upright on a level base that can permanently carry its full weight. A slatted linen shelf will not do, and tilting the cylinder does not help it drain. Keep hot pipe runs short and leave access to the immersion heater, thermostats and valves.

Connections, isolation and drain-off points

• Isolation: fit a servicing valve on the cold feed where it leaves the cistern. Closing it lets you work on the cylinder without draining the cistern; the main stopvalve and the float valve's servicing valve only stop the cistern refilling.
• Draining: fit the drain valve at the lowest point of the cold feed, close to the cylinder, so the cylinder and any sediment in its base can be drained. The primary circuit needs its own drain at its low points and air release at its high points.
• Open vent: it must stay open to the atmosphere, so never valve it off, and keep motorised valves out of the vent and the feed and expansion pipe.
• Overheat protection: in England a vented cylinder also needs either a non-self-resetting energy cut-out on each direct heat source, such as an immersion heater, plus an overheat cut-out on any indirect one, or a temperature relief valve. Replace a failed immersion heater thermostat with the same combined type, whose cut-out is reset by hand.

Filling and venting

1. Close the drain valve, open the cold feed valve and fill with a hot tap open.
2. Run each hot tap in turn until the air is purged and the water runs clear.
3. On an indirect cylinder with a two-port valve, set its lever to manual while the primary circuit fills, so the coil fills and vents, then return it to automatic.
4. Check for leaks. Never switch on the immersion heater or boiler until the cylinder is completely full.

Soundness testing

Every new system must be tested, flushed and, where necessary, disinfected before it is first used. The test pressure is 1½ times the maximum pressure the installation is designed to operate at. A system with no plastic pipe passes if, an hour after pumping to the test pressure, the pressure has held and there has been no visible leakage. A falling gauge fails even when no drip can be seen. Systems with plastic pipe use different tests.

Insulation

On a new system in England, insulate all primary circulation pipes for hot water, any secondary circulation pipework and every pipe connected to the cylinder for at least 1 m from the connection. When a cylinder is replaced in an existing home, insulate any accessible pipes in the dwelling. Pipes in unheated spaces also need protecting from frost.

Flushing and commissioning

• Flush thoroughly as close as possible to occupation. If a new system will stand for months first, flush it weekly rather than draining it, because damp pockets let biofilm grow.
• For Legionella control, store hot water at 60 °C or more and aim for at least 50 °C at each outlet within one minute. A longer wait points to a branch that is too long or poorly insulated.
• Prove the boiler interlock: with the heating off, turn the cylinder thermostat down and check that the valve closes and the boiler switches off.

Handover

Leave the owner the operating and maintenance instructions and the completed, signed commissioning checklist, and show them how the controls work.

Draining down

1. Switch off the immersion heater and the boiler.
2. Close the cold feed servicing valve.
3. Open a hot tap to let air in.
4. Run a hose from the drain valve to a point below the cylinder and open the valve.

Taking a system out of use

• Temporarily: a system left full but unused for months is recommissioned as if new: thoroughly flushed, cleaned and disinfected.
• Permanently: cut redundant pipework back to the pipe it branches from and cap it there, or better, take out the tee. A capped branch left full is a dead end where water stagnates.

Finding faults

• No water at hot or cistern-fed cold taps, mains tap fine: the cistern has run dry, for example because the float valve has stuck shut.
• Every hot tap weak, cistern-fed cold taps normal: a restriction on the cold feed, such as a servicing valve left partly closed.
• Cylinder cold after a refill, coil pipes gurgling: air trapped in the coil.
• Little hot water from an off-peak cylinder until the boost is used: the lower element, its thermostat or its time switch has failed.
• Water spurting from the open vent whenever the pump runs, even from cold: pumping over. Correct the vent and cold feed arrangement; never cap the vent.
• Kettle-like whistling and banging from the boiler: kettling, which is water boiling in the heat exchanger, for example where scale slows the flow.

Hard water and scale

Scale is deposited faster as water gets hotter, especially above 60 °C, so in very hard water keep stored water at 60 °C rather than hotter. In England, where hardness is above 200 ppm, treat the feed water to water heaters to reduce limescale. Heating inhibitor goes only in the primary circuit, never in water drawn off at the taps. Some boiler makers forbid filling a heating system with salt-softened water, so check first.