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. A vertical cylinder needs a level base rated for its full operating weight; other approved orientations need the manufacturer’s specified supports. 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: place a valve as the cold feed exits the storage cistern towards the cylinder. Close it to repair the cylinder while leaving the cistern full; the main stopvalve or float-valve isolator only stops that cistern refilling.
• Draining: put the drain cock low on the cylinder cold feed, near the vessel. That position lets water and sediment leave the cylinder. Drain the primary circuit separately at its low points and provide air release at its high points.
• Open vent: leave its route to atmosphere unobstructed, with no valve. On an open-vented primary heating circuit, the feed and expansion pipe also needs an unobstructed connection. This is separate from the domestic cylinder cold-feed servicing valve described above.
• 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.

Plastic hot pipework

Plastic pipe lengthens noticeably as it warms. Wavin's installer guide says an exposed Hep2O run carrying hot water will snake slightly between its clips, especially where it has been threaded through joists. That movement does not cause airlocks or affect how the system works, and in floors and roof spaces it is taken up along the run. Clip plastic pipe at the spacing its manufacturer gives, which is closer than for copper.

Immersion heaters and cylinder thermostats

Check an immersion heater's length against the cylinder before fitting it. Kingspan's Albion Vented installation guide says a top-mounted element must be at least 200 mm shorter than the cylinder's overall height, and a side-mounted one at least 100 mm shorter than its diameter. A strap-on cylinder thermostat goes in direct contact with the metal body: remove a section of foam no larger than the thermostat's footprint, rather than strapping it over the insulation or cutting away a wide band. Heatrae Sadia's immersion heaters screw into a 2¼-inch BSP boss. Its 11-inch and 14-inch elements are fitted horizontally and longer ones vertically, always leaving at least 15 mm between the end of the element and the inside of the cylinder.

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. Replacement electrical work also needs its required safety checks before energising.

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 if you can see no drip. Check temperature stability, the test equipment and possible leaks before deciding the cause. 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, the guidance covers accessible heating and hot-water pipes throughout 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 the conventional Legionella temperature regime, store hot water at 60 °C or more and check for at least 50 °C within one minute on the unblended hot supply (55 °C in healthcare premises). A TMV deliberately delivers cooler water, so check upstream of it. If the temperature rises slowly, investigate the branch, the flow and the source temperature: it could be any of them.
• Prove the boiler interlock: with the heating off and the cylinder calling for heat, satisfy the cylinder thermostat and check that the valve closes and the boiler stops, allowing for the model’s pump overrun and protective functions.

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. Safely isolate the heat sources and prevent them restarting. Before electrical work, secure the isolation and prove dead. Water may still be hot; allow for safe handling and discharge.
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: inspect the cistern level and feed. A failed refill valve is one possible cause of an empty cistern.
• Every hot tap weak, cistern-fed cold taps normal: investigate the shared hot-water path, including a restricted cold feed or partly closed servicing valve.
• Cylinder cold after a refill, coil pipes gurgling: check for trapped air and inadequate primary circulation; confirm the cause before venting or further work.
• Little hot water from an off-peak cylinder until the boost is used: check the off-peak supply, the time settings, the lower element and its controls before deciding which part has failed.
• Water spurting from the open vent whenever the pump runs, even from cold: pumping over. Check pump duty, restrictions, system head and the feed/vent layout before choosing the correction; never cap the vent.
• Kettle-like whistling and banging from the boiler: investigate local overheating, restricted circulation and deposits against the boiler’s instructions. The noise alone does not mean scale is the cause.

Hard water and scale

Hotter water generally forms more scale. Use the settings the equipment allows while still meeting the hygiene regime: the PremierPlus direct-cylinder instructions, for example, give 60–65 °C in very hard water. The dial setting does not tell you the temperature of the whole store. In England, where hardness is above 200 ppm as calcium carbonate, 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.

Shower pumps on a vented system

A pump boosts a gravity-fed shower. Stuart Turner's standard pumps, which other makers call positive head pumps, switch on when their flow sensor detects water already flowing by gravity, so the shower must give at least 1 litre a minute with the pump off: measure what it gives in one minute. Below that, fit a universal (negative head) pump, which senses the fall in pressure when the shower opens and needs no gravity flow.

Air in the hot supply makes a pump noisy, swings the temperature and wears the pump out. Salamander advises an anti-aeration flange, which draws water from deeper in the cylinder, below the air that collects at the top, with a dedicated feed from the flange to the pump's hot inlet. Top-entry flanges, such as the Surrey type, screw onto the dome's hot draw-off; the No Stop Essex flange goes into a hole drilled in the cylinder's side, and suits a pump that must sit above the cylinder. Stuart Turner's conditions for a pump under a bath include dedicated 22 mm hot and cold feeds, free of air (15 mm only at the pump), within 4 m of the cylinder.

• Too fierce a spray: close the outlet isolating valves by no more than 30%; never restrict the supply side.
• Pulses while the shower runs: a blockage, such as a scaled-up head or a collapsed hose, is building back-pressure.
• A standard pump runs on, but stops when its outlet isolators close: it is pumping to an outlet left open.
• A universal pump cycles with everything closed: look for leaks; if it still cycles with its outlet isolators shut, its own non-return valves are letting by.