Cold water tank replacement W9

What is a cold water storage tank

A cistern in the loft that stores cold water fed from the mains, supplying cold water to taps, toilet cisterns, and the hot water cylinder via gravity. Found in traditional vented (gravity-fed) plumbing systems common in mid-20th century London houses and older mansion blocks. Modern properties with unvented cylinders or combi boilers have no tank.

Why cold water tanks fail

Old polythene and fibreglass tanks become brittle with age and UV exposure, leading to cracks and splits. A failed ballcock valve causes the overflow to run into the guttering. Internal corrosion, algae growth, or an uncovered tank causes water discolouration. Tanks that do not comply with Water Regulations (requiring a close-fitting lid, insulation jacket, and screened vent pipe) create a Legionella risk — particularly serious in HMOs and commercial properties.

Tank materials and sizing

Old tanks in London are typically 50-gallon (230-litre) rectangular polythene or galvanised steel cisterns. Modern replacement tanks are one-piece flexible polythene (Ecosure or Granby brands) that can be passed through loft hatches even if the tank is larger than the hatch opening. Tanks connect via compression fittings to 22 mm or 15 mm copper pipework. All new tanks include an integral ballcock, lid, and overflow.

Legionella compliance

Regulation 15 of the Water Supply (Water Fittings) Regulations requires cold water storage cisterns to have lids that prevent light and insects entering, insulation to prevent freezing and to keep water below 20 degrees C (warm water favours Legionella growth), and screened vent and overflow pipes. Landlords with HMO properties have a legal duty to control Legionella risk and document compliance.

Loft insulation conflict

Cold water tanks in lofts should have insulation on the sides and top but NOT underneath. The floor void beneath the tank should be left uninsulated to allow warmth from the living space below to prevent the tank from freezing in cold weather. Incorrectly insulating under the tank is a common mistake that leads to frozen cisterns in winter.

Converting to mains pressure

Removing the cold water storage tank entirely is possible when converting to an unvented hot water cylinder or a combi boiler system. The tank and its associated pipework are decommissioned, cold water is fed directly from the mains, and the loft space is reclaimed. This work requires a survey to assess pipework and pressure suitability. Contact Prestige Engineers for a loft tank decommission assessment.