Replacing a Hot Water Cylinder on a Vented System

A friend of mine has just moved into a rather splendid period property. Large and spacious, it is ideal for his rapidly expanding family. With a modern condensing boiler and a programmable zoning installation controlling the radiators throughout the house, he is confident he can keep his heating bills under control.

The previous occupants also had a large family and found an open vented hot water storage system ideal for their requirements. Where large amounts of hot water are required on-tap, storing hot water in a cylinder is a practical solution to cope with a high quantity demand.

With the property having three floors there is also a good head of pressure available from the cold water feed tank in the loft. This provides a good hot water flow and pressure rate through the cylinder and out of the taps.

However, after taking off the hot water storage cylinder insulation jacket to inspect the tank, my friend noticed that water had been leaking from the indirect heating coil flow pipe leading into the cylinder. Green and white encrustation around the external connection was quite evident along with an indication that someone had tried to stop the leak with a filler compound in the past.

Realising that the copper cylinder was quite old, my friend decided it was time to replace it, rather than try to mend the leak.

Because he has an open vented hot water system, he decided to undertake the work himself. Open vented means that when hot water is heated and expands, the surplus water volume is directed up a pipe leading to the cold water feed tank. If the expansion becomes too great within the system, surplus hot water can be discharged back into the cold water feed tank. This maintains a low and safe working pressure within the system. Any excess of water flowing back into the cold water feed tank is simply expelled from the property via the overflow pipe.

If he had inherited an unvented system, the replacement of a hot water storage cylinder would have been a specialised procedure requiring expert installation.

The original hot water storage cylinder was snugly located in an enclosed cupboard in the bathroom. Although this meant that installing a new cylinder would have to be undertaken in a rather confined space, my friend decided that this location would still be suitable. The installation would simply require the removal of the old tank and the insertion of a new one, plumbing into existing fittings.

New hot water storage cylinders are available in a variety of sizes. Smaller properties can also benefit from installing vented combination cylinders, where the cold-water feed and expansion tank is incorporated into the storage unit. This is designed to save space but can lead to problems with low hot water flow rates.

General domestic vented hot water storage cylinders come in a variety of sizes ranging from 74 litres up to 450 litres. Because the existing cylinder in my friend’s house had a 200-litre capacity, he decided to replace it, like for like, with one of the same capacity and dimensions to ensure that it would fit into the available space.

The new cylinder was coated with a highly efficient foam insulation skin and also had provision for the attachment of a backup electric immersion element in the same position as the existing cylinder. Inlet and outlet fitting locations were also chosen to be identical to try to ensure a problem free installation.

Having obtained the replacement hot water storage cylinder, my friend turned his attention to removing the old one.

Firstly, he shut down the boiler and all the other accessories, pumps, electronic diverter valves and programming installations and then, because there were no additional isolating valves, drained down the central heating system.

He then isolated the current immersion heater element electrical supply.

The existing cylinder now needed draining. This was accomplished by turning off the mains cold feed to the water feed and expansion tank and locating the drain tap, fortunately, plumbed from the rear of the storage cylinder and feeding through to the front.

Next, he disconnected the electrical contacts in the head of the electric immersion heater and then removed the external thermostat device that was secured to the wall of the cylinder.

He was then able to unscrew the cylinder attachment fittings. They were the heating coil inlet and outlet points and the hot water outlet pipe at the top of the cylinder. He was fortunate that they were easy to free and unscrew. However, he could have cut the pipes at appropriate points, ensuring that he could re-plumb when necessary. He also took the precaution of marking the pipe-work to identify which pipe corresponded to which function.

To ensure convenient re-plumbing of the cold-water inlet, in this case, located at the back of the cylinder, he cut this pipe some way above the cylinder. By doing this, he could attach fittings and a length of copper pipe to the rear of the new cylinder prior to moving it into position. This saved him having to struggle to plumb the cold feed into the base of the back of the cylinder when placed in its new location.

He enlisted my assistance to remove the old tank and carry it downstairs. We were quite surprised by the weight of the old cylinder and decided to cut the top off and examine the internal area. This was encrusted with limescale, with a thick coating covering the coils and electric heating element. The base of the cylinder had about six inches of scale and debris, almost clogging the cold water feed inlet. This indicated that the water supplying my friend’s property was particularly hard.

Attention now turned to the new replacement cylinder. To ensure a watertight fitting of a new immersion heater element, a small circular ring of foam insulation was cut from the edge of the element fitting area. The element was then screwed into position, seating perfectly into the cut area and forming a tight seal.

With fittings, and an appropriately measured length of copper pipe attached to the rear cold water inlet point, not forgetting the attachment of a new drain valve leading from the back base coupling to the front area, the new cylinder was inserted into position.

The heating coil inlet and outlet pipes and the hot water outlet pipes were re-attached. The cold-water inlet pipe extension from the back of the cylinder required coupling to the pipe from the cold-water feed and expansion tank.

To facilitate future maintenance work, gate valves were installed at suitable pipe junctions to enable the cylinder to be easily isolated from all water supplies.

The new immersion element was rewired into the mains, and a small piece of insulation foam was cut from the front of the new cylinder to allow attachment of the thermostat.

The mains water was turned back on and the entire system was refilled, not forgetting the addition of inhibitor into the central heating system. The boiler was then re-started and all contributing electrical appliances turned on.

The immersion heater was checked to ensure it was working, radiators were bled to remove airlocks and the system restored to a functioning hot water and central heating installation.

The new hot water storage cylinder and its related plumbing couplings were frequently inspected for a few weeks to ensure that no problems or leaks emerged.

With the new hot water storage cylinder installed, my friend could now relax, confident that a major cylinder failure and water leak issue was unlikely to arise.