Hot Water Cylinder Replacement Guide

A hot water cylinder is one of the longest-lived things in a house, which is why most people only think about it when it fails. Knowing the signs early is worth something, because a cylinder that lets go while you are out is a serious amount of water in one place.

Signs Yours Needs Replacing

• Rusty or discoloured hot water while the cold runs clear. Internal corrosion. See our guide to brown water.
• Water no longer staying hot. Either failed insulation or heavy scaling on the coil.
• Weeping or staining at the seams, particularly around the immersion boss or the top dome.
• Noticeably slower recovery than it used to have.
• Age. Copper vented cylinders often last 25 years or more; modern unvented units are typically rated for 15 to 25.
• Frequent immersion element failures, which often means the water chemistry or scale inside is aggressive.

A cylinder that has begun weeping does not stop. It gets worse, and it usually fails properly at the least convenient time.

Vented or Unvented?

Vented — the traditional system

Fed by gravity from a cold tank in the loft. Simple, cheap to replace, and it will keep working during a mains interruption because the tank holds a reserve.

The drawback is pressure. Hot water pressure depends entirely on the height of the tank above the outlet, which is why upstairs showers on a vented system are usually weak without a pump. Our guide to the loft tank explains the arrangement.

Unvented — mains pressure

Fed directly from the main, so hot water arrives at the same pressure as cold. Strong showers without a pump, no loft tank, and the loft space is freed up.

What it requires:

• Adequate incoming mains flow. Typically at least 20 litres a minute. A poor supply makes an unvented cylinder a disappointment.
• A discharge route for the safety valves, run to a safe visible point.
• Annual servicing of the expansion vessel and safety valves.
• An installer holding the unvented hot water qualification, verifiable via the Chartered Institute of Plumbing and Heating Engineering or the relevant scheme. This is a legal requirement under building regulations, because a failed unvented cylinder is a pressure vessel.

Before committing, measure your flow rate — time how long a fully open kitchen cold tap takes to fill a ten-litre bucket. Under about 20 litres a minute and an unvented cylinder will underperform.

Thermal store

Less common. Stores heated water and passes mains water through a coil to heat it on demand. Gives mains-pressure hot water without the pressure-vessel regulations, but is bulkier and less efficient at storing heat.

Rough Costs

• Vented cylinder, like-for-like: £600 to £1,200 fitted
• Unvented cylinder, like-for-like: £1,500 to £2,500 fitted
• Vented to unvented conversion: £2,000 to £3,500 — new pipework, discharge route, and removing the loft tank

Costs move with cylinder size, access, and how much pipework has to change. A conversion is a bigger job than a swap because the whole distribution changes from gravity to mains pressure.

Sizing It

Too small and you run out mid-shower. Too large and you pay to keep water hot that nobody uses.

A rough guide for a household with one bathroom is 120 to 150 litres; two bathrooms, 180 to 210; three or more, 250 upwards. Adjust for how many people shower at similar times — that matters more than the number of bathrooms.

Servicing an Unvented Cylinder

The Energy Saving Trust recommends annual checks on stored hot water systems. This is not optional and it is often neglected. The expansion vessel inside loses its air charge over time, and once it does, pressure has nowhere to go and the safety valve discharges — often the first sign anyone notices is water running from a pipe outside.

Annual service should cover the expansion vessel charge, the temperature and pressure relief valve, the tundish, and the thermostat. Skipping it is what turns a 20-year cylinder into a 10-year one.

Frequently Asked Questions

Can I fit an unvented cylinder myself?

No. It requires the unvented hot water qualification under building regulations. An uncertified installation is unlawful, uninsurable, and genuinely dangerous — these are pressure vessels.

Will an unvented cylinder improve my shower?

Usually significantly, provided the incoming mains flow is good. If your supply is poor, it will not — check the flow rate before deciding.

How long should a cylinder last?

A traditional copper vented cylinder often 25 years or more. An unvented unit typically 15 to 25, and heavily dependent on whether it has been serviced annually.

Why is water dripping from a pipe outside near my cylinder?

That is the discharge from a safety valve, and it means the cylinder is releasing pressure. Most often a failed expansion vessel charge. It needs attention promptly rather than being ignored.

Getting It Specified Right

The most common cylinder complaint we see is an unvented unit fitted to a mains supply that cannot feed it. We cover Milton Keynes and the surrounding area for cylinder replacement and hot water system work, and we measure the flow before recommending anything. Get in touch.