A cold water storage tank is the bigger tank, near the small expansion tank, often located in the loft tucked out of site. This tank is filled directly from mains water. The gravity cold water storage tank is in charge of supplying your home's water needs to all of the taps in your house (bath taps, sink taps and showers) except for the cold water tap in the kitchen (as that is usually directly off the mains water). This tank also indirectly supplies the hot water taps, including the kitchen, with their hot water needs. This is achieved by feeding the cold water from the storage tank to the cylinder. The cylinder then heats up the water, which will then branch off to which tap is requesting a hot water source. As water drains out of the cylinder, with the request from the hot taps, the cold water storage tank will refill the cylinder with the amount of water it has lost to the taps. The cold water storage tank is able to fill up the cylinder from pressure as it is usually 3 or 4 metres above the cylinder.

How do I know if my cold water storage tank for mains water needs replacing?

The cold water storage tank is made of polyethylene plastic and might need to be replaced every ten years.

Signs that the cold water storage tank may need to be replaced include;

• showing signs of age with cracks
• cracks may have turned to splits and the tank is prone to leaking
• age can make tank very brittle, 

How much will it cost to replace a cold water storage tank?

The biggest challenge when it comes to replacing a cold water storage tank, is getting the tank inside the loft through the loft hatch and removing the old tank via the loft hatch. As well as the accessibility for the plumber once inside the loft, whether it is safe to stand on loft floor boards and work around the tank with access to pipes.

If the access is relatively easy and the plumber will not have to cut up the old cold water tank to remove it through the hatch, this will result in the cost being lower with only a couple of hours labour time charged on top of the material charges. Cold water storage tanks that are designed size wise to pass through the hatch are referred to as ‘coffin loft tanks’.

Instead of a tank replacement, once the plumber has come through to diagnose the issue, the tank may just need a repair or a patch placed over a visible hole. This will drastically reduce the cost by only needing a 30 min to an hour repair work rather than changing all of the plumbing parts involved in a replacement of the cold water storage tank.

If you are doing a loft conversion and wanting the plumber to move the cold water storage tank to a new location, the plumber might insist on putting a new tank in the desired location. This would be if the existing tank is more than a few years old, as the plumber is reconfiguring pipework, and spending the same amount of labour setting the tank in its new location, they might as well set up a tank that has at least 10 years of life to avoid you having another costly expense in the coming years. Also when moving a brittle older tank, it could cause the tank to crack and leak and it may not be obvious immediately if the leak is subtle. Depending on the size of the tank, it may cost you a few hundred pounds for the tank with a lid, insulation jacket and some of the materials needed for the tank.

Cost to repair or replace a cold water storage tank	Per	Average cost
Replacing a dip tube in the cold water storage tank	Per hour	£45
Fitting a new lid and insulation jacket in the cold water storage tank	Per hour	£35
Replacing the cold water storage tank and removing the old tank	Per day	£260
Moving the cold water storage tank to a new location	Per day	£280
Repairing a hole in the plastic cold water storage  tank or lid	Per day	£40
Relining the cold water tank	Per hour	£180

Can my cold water storage tank be moved to a new location for a loft conversion?

Before you begin planning a loft conversion, if you have a gravity cold water storage tank and living in a maisonette home, you must determine whether the cold water feeds your house only or if it is shared with the house below you. Pre and post war homes, 1930’s - 1950’s, often have a shared system in the loft.

If it is shared, it is still possible to move the tank to a new location but you will need to make your neighbour aware as there will be two days of disrupted water services.

As the cold water storage tank relies on gravity, when moving it to a new location, the plumber will need to ensure it is at least 2 metres above showers and the cylinder of the boiler. This will enable better water pressure.

Older homes tend to have large galvanised steel or copper tanks, when moving the tank to a new position, the plumber will prefer to change this to plastic. The plastic tanks take up less space and the older tank will need to be cut with a welding machine to then be removed through the loft hatch.

Wherever the tank is installed be sure that it is covered with a lid and an insulation jacket plus if boxed in, make sure that there will be access to it for any future issues. 

Can a cold water storage tank be repaired?

Yes, depending on the extent of the damage to the tank. If you have noticed a very slight leak in the cold water storage tank, or that the lid has a hole, this can be ‘patched’ and repaired (in some cases even relined).

A small crack or pinhole that is above water level will be easier to repair as the plumber may just use a sealant to keep dust and insects from entering the tank. If the crack is below water level the plumber will have to first drain down the tank until the hole is exposed. They might use a plastic extrusion welder to inject molten polymer rod into the hole. When the rod has cooled, cut off the protruding bit and sand it down to smoothen the repair. There is always the risk that while repairing a crack or hole that the damage could be made worse, resulting in the need to replace the tank.

Instead of replacing a tank, relining the cold water tank might be an option. This can be a spray-applied Polyurea, (Water Regulations Advisory Scheme) WRAS approved solvent-free polyurethane coating, especially designed for relining drinking water tanks.Other repairs needed might involve just replacing a lid that has become warped or replacing old insulation that no longer does the job. If the water in the tank has become infected by bacteria (perhaps the lid was left off) then you will need a hygiene clean out of the tank, making sure that there is no sign of Legionella bacteria.

Estimates for common plumber jobs;

Rates for other trades