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Installing a Hot-Water Cylinder

If you are constantly running out of hot water, a new hot water cylinder can provide a faster heat-up time and increased capacity.


Plumbing skills are required and electrical knowledge would be useful.


  • 1 in male iron to compression 28mm or 22mm copper (for bottom cold-feed connection)
  • 22mm copper tube (or 28mm if the hit water is on gravity circulation)
  • 1 in male iron thread to copper angled connector (for top of cylinder)
  • Cylinder
  • Cylinder unions or 1 in female iron thread to compression
  • Immersion heater or blanking plate


  • Adjustable plumber’s wrenches
  • Blow lamp (if you use solder fittings)
  • Immersion-heater spanner
  • Pipe cutters or junior hacksaw

A direct cylinder with and immersion heater has just two connections so it should be very easy to fit.

Installing an indirect cylinder, which has a coil inside for transferring heat from the boiler, is slightly more involved as the boiler and heating circuit will need to be drained down.

If your home has an older style gravity-fed primary hot water circuit. Installing a pumped system could be more efficient.


Turn off the hot water a few hours before draining down a system- this will prevent you being scalded if a hose slips off. Isolate electrics before disconnecting the immersion heater. A cylinder is heavy so get help to carry it, especially on stairs.

“A hot-water cylinder can be heated by a boiler or with an electric immersion heater. It is even possible to run both at once if you want a really quick heat-up time, but most people save the electric immersion heater for heating water in summer or for backup in case the boiler breaks down.”

There are also economy 7 cylinders for use with off peak electricity. These have two immersion heaters; one in the top and one in the bottom. The top heater is wired up to standard rate electricity to provide daytime top up. The lower heater is wired up to the white meter supply for night-time heating of the whole cylinder.

Preventing freezing of vent pipes

  • If feed and vent pipes in the loft freeze, a hot water cylinder could explode as it heats up. It is therefore essential to insulate pipes in unheated areas well, and to route as much of the pipework as possible through heated areas of the house.


  • Before a hot water cylinder can be removed, it will need to be drained down. First turn off the boiler and immersion heater, then shut off the cold feed to the hot water cylinder from the cold water storage cistern. There should be a valve on the supply pipe from the cold water storage tank to allow you to do this.
  • Open all hot water taps to drain down the system. If water continues to run from hot taps, you will have to turn off the mains water supply to the tank, and then drain the cold water tank. This will cut off the water supply to the cylinder.

“The water supply may be off for sum time. So ensure there is plenty of water available for drinking and cleaning by filling kettles and saucepans.”

  • To drain the cylinder, locate the draincock. This should be at the base of the cylinder, near the point where the cold water supply enters. Attach a hosepipe to the draincock, and secure it in place with a clip or piece of wire. Run the hosepipe to a lower point outside and open the small square nut on the draincock – you should hear water begin to run. There may be a few dribbles of water around the nut, but if water runs quickly through the hose it will suck in air rather than leak.
  • If the draincock won’t work. The washer has probably been baked on or the outlet is full of limescale. Another way to drain the cylinder is to siphon the water through the top by disconnecting the vent pipe. A small amount of water might escape, so have a towel ready to mop it up. To siphon out the water, push a hosepipe down into the cylinder and partially fill the hose from a garden tap. As the hosepipe begins to drain, it will also draw out the water in the cylinder.
  • Once the cylinder is completely empty, drain the water from the boiler or a low point on the radiator circuit. Open any motorised valves by sliding the manual lever against the spring to the open position.
  • When the heating and primary hot-water circuits have been drained you can undo all the connections on the cylinder.

Removing and refitting an immersion heater

  • Turn off the electricity supply to the immersion heater at the consumer unit by removing the fuse or MCB (miniature circuit breaker). Undo the cover on the immersion heater and disconnect the three wires. Loosen the flex clamp and pull the supply flex through.
  • Use an immersion-heater spanner in an anti-clockwise direction to loosen the heater (3). If it won’t undo, don’t force it or the cylinder will buckle. Use a hammer and gently tap the spanner handle. If this doesn’t work, use gentle heat from a blowlamp or hot-air gun to warm the brass and soften any linseed oil-based sealant that has been used.

“When you fit an immersion heater, make sure that the sealing is placed on the underside. If you use additional sealant, make certain it is a non-setting variety that is suitable for drinking water. This prevents bacterial growth.”

  • When fitting an immersion heater doesn’t use too much force or the cylinder could buckle. Once the cylinder has been filled with water, check for leaks and tap the spanner slightly to tighten it. When you are sure that there aren’t any leaks, you can connect the wires first – if you need to tighten the heater, you won’t be able to get the spanner on without disconnecting it again.
  • The live supply must be connected to the thermostat that fits in a dry pocket. Set the thermostat to 60ºc.
  • Make sure that the heater is securely connected to the earth wire.
  • If the flexible cable from the wall switch to the heater needs renewing, make sure heat-resistant PVC or butyl rubber flex is used. If you use 1.5mm² flex, the length should not exceed 1 metre, otherwise use 2.5mm². The supply cable from the fuse box to the switch must be 2.5mm².

Fitting the cylinder

  • Try to arrange all the pipework so that it is easily accessible (4): if there are awkward connections to be made behind the cylinder, try to attach the fittings and fit short lengths of pipe so that the cylinder can be moved into place pre-plumbed. Connections to those pipes can then be made in a more accessible position.

Cold feed

  • The bottom connection must be made with a thread of PTFE tape and be generous with it, or you may find the connection leaks. The thread should be so tight that a spanner is needed to complete it. However, don’t tighten it so far that you damage the relatively thin cylinder walls.
  • Incorporate a draincock in the lowest connection so that the cylinder can be emptied in the future.
  • Secure the pipe to the wall and take it up to the gate valve. The gate valve must be above the cylinder level.

Vent pipe

  • Wrap the top threaded connector with PFTE tape, and screw it into the hole in the cylinder so that the elbow is in the right position to take the pipe to the vent pipe. The 22mm (¾in) pipe must rise to the open vent that hooks over the top of the cold water. On no account must any valve be fitted to the vent pipe.
  • Avoid running a pipe over the top of the immersion heater or it will be impossible to change the heater if it burns out.

“Don’t run the vent pipe directly up from the middle of the cylinder because hot water will tend to rise and fall inside the pipe and draw heat out of the cylinder.”

Hot supply to taps

  • The branch to hot taps is taken from the vent pipe close to the cylinder. It must fall away slightly towards the taps so that any air in the pipe is automatically released to the vent. If the pipe runs upwards even slightly, you may have problems with air locks.

Shower connections

  • If you have a pumped shower, you may want to fit a separate supply. This can be done with a special dip pipe flange (Surrey Flange) which screws into the top of the cylinder (5).

Primary coil connection to the boiler circuit

  • On a gravity system, which uses the weight difference between hot and cold water to circulate water from the boiler to the cylinder, it is essential that the flow enters the side of the cylinder at the top of the coil. On fully pumped systems it doesn’t matter.
  • To connect the primary pipework to the cylinder, you can use 1in make threaded connections or purpose-made cylinder unions (which look like large tap connectors). If the system if fully pumped, 22mm (¾in) pipes will be adequate. Where the pump only circulates radiator water, 28mm (1in) pipework and connections are needed. Do not reduce the size of pipework going into the cylinder.

Check that the top primary connection is level or slightly rising from the  cylinder to the vent pipe (6). Some systems have a venting mechanism at this point so that pipework can drop down to a vent elsewhere.

  • The bottom connection returning to the boiler is connected in the same way. On fully piped systems there is often a valve at this point to balance the water between the radiator circuits and the cylinder.

“Do not connect copper pipes to galvanised iron pipes (silver coloured) because electrolytic action between the two will cause the pipes to corrode and block. If you need to keep old pipework, connect plastic hot-water pipes to it”

Preventing scale

  • If you live in a hard water area, fit a scale-reducing device to the cold feed of the cylinder (7). Even a small build up of scale will add several pounds to your fuel bills and slow down the rate at which the hot water cylinder will heat up.

Attaching a cylinder thermostat

  • If there is a clamp-on thermostat on the old cylinder, you will need to fit it to the new cylinder. Cut a small rectangle put of the insulation on the opposite side to the boiler primary pipes. The height of the thermostat should be about one-third up the cylinder.