Combination Cylinder: Repair, Replace or Upgrade?

What a combination cylinder is and why flats have them

Picture an ordinary copper hot water cylinder with a second, shorter tank sitting on its head. The top section is the cold feed tank, filled from the mains through a float valve. The bottom section is the hot cylinder, fed by gravity from the tank above it. The two are joined as one unit and usually wrapped in the same foam. You will also hear it called a combination unit, a combi tank or, in older buildings, a Fortic, after the brand name that became the generic term.

They were fitted in huge numbers in purpose built blocks and converted flats because a flat has no loft to hold a separate cold tank. Putting the tank on the cylinder solved that in one cupboard. Most are direct, heated only by immersion elements, which suits the all electric blocks where they are most common. Indirect versions with a boiler coil exist too.

Because the cold tank is only a short distance above the outlets, the pressure at the taps is low. That is not a fault. It is how the design works, and it is the main reason people start asking whether to replace it.

Common combination cylinder faults and whether they are worth fixing

Most calls about a combination cylinder are about the parts around the vessel, not the vessel itself. Those parts are cheap to replace compared with the cylinder, so on a sound unit the repair almost always wins.

• No hot water, or lukewarm water: usually the immersion element or its thermostat. Worth repairing.
• Water running from an overflow pipe outside: the float valve in the top tank is not shutting off. Worth repairing.
• Top tank empty and no water from the hot taps: the mains feed to the float valve is off or the valve has stuck shut. Worth repairing.
• Banging or fizzing while heating: scale on the element. A new element usually cures it.
• Damp at the base, or a weep from a seam on the vessel: the cylinder itself has failed. Replace.
• Rust coloured water from the hot taps only: can be a corroding vessel, sometimes the tank lining. Needs looking at before deciding.

The top tank of a combination cylinder holds the water that feeds your hot taps, so its lid matters. If the lid is missing or broken, dust and debris get in. A new lid is a small job and worth doing regardless of anything else.

When replacement is the better call

A leak from the vessel is the clear signal. Copper cylinders fail by pinholing, and once one pinhole has appeared there are usually others on the way. No repair on a leaking vessel is lasting, and the leak will find the flat below before long.

The other signal is a string of separate faults on an old unit: the element replaced more than once, the thermostat renewed, the float valve reseated, and the hot water still slow to recover. At that point each repair is buying a year or two rather than fixing anything. Is it worth repairing an old cylinder at that stage? Our guide walks through that calculation in more detail, and the signs your hot water cylinder needs replacing list the symptoms that mean the vessel is at the end.

Option 1: a new combination cylinder like for like

This is the least disruptive route and often the right one. The new unit goes in the same cupboard, the pipework mostly stays, there is no new discharge pipe to run through a wall and no annual safety service to book. The job can usually be done in a day with the hot water back on that evening.

The trade off is that you keep the same gentle pressure. If the shower has always been weak, it will still be weak. Modern units have much better factory insulation than the old lagged ones, so the new cylinder should hold its heat better and cost less to run, but the pressure does not change with the age of the unit.

Measure before you order anything. Combination cylinders come in a range of heights and diameters, and older cupboards were often built tight around the original. The height of the cupboard, including the space needed above the top tank to reach the float valve, decides what will fit.

Option 2: upgrade to an unvented cylinder

An unvented cylinder runs straight off the mains, so hot water arrives at the taps and shower at close to mains pressure. For many flat owners this is the reason to change, and when it works it is a real improvement. It is also the option with the most conditions attached.

The incoming mains has to deliver enough pressure and, more importantly, enough flow. A lot of London flats, particularly on upper floors of older blocks, share a supply that looks fine at one tap and collapses when two are open. An engineer should measure the flow before quoting. If the mains cannot supply it, an unvented cylinder will simply pass on the weakness.

An unvented cylinder also needs a discharge pipe from its safety valves to a safe point outside, which in a flat can mean a new hole through an external wall. That wall usually belongs to the freeholder, so you may need consent before any work starts. The installation must be done by someone qualified for unvented systems and notified to building control, and the cylinder then needs an annual service. Our comparison of unvented vs vented cylinder systems covers the differences in full, and our unvented cylinder installation page explains what the job involves.

Never let anyone connect an existing combination cylinder, or any vented cylinder, to the mains to improve pressure. It is not built for it. Mains pressure hot water always means a new unvented vessel with its full set of safety components.

Option 3: a thermal store or other alternatives

Where the mains is strong but there is no way to run a discharge pipe outside, a thermal store can give mains pressure hot water without the unvented rules, because the stored water is not under pressure. It takes more space and loses more heat standing still, so it suits some flats and not others. Our page on thermal store vs unvented cylinder sets out where each one fits.

If the flat has gas and a boiler, converting to a combi boiler and removing the cylinder altogether is another route. It frees the cupboard, but it moves the whole hot water load onto the boiler and the mains, so the same flow test applies. For an all electric flat, the choice is usually between a new combination unit and a direct unvented cylinder; our guide to the electric hot water cylinder options covers those in detail.

How to choose between the three

Work through it in this order. First, is the vessel actually finished, or is this a repair? Second, if it needs replacing, is the pressure a problem you want to solve? If not, like for like is the simplest and cheapest answer. Third, if you want better pressure, will the mains support it and is there a lawful route for a discharge pipe? If both are yes, an unvented cylinder is usually the best result. If the mains is good but the discharge route is not, look at a thermal store. If the mains is weak, stay vented and consider whether a suitable shower pump could help.

Size matters in all three cases. A household that runs out of hot water every evening needs a bigger vessel, not just a better one. Our page on hot water cylinder sizes helps you check before you choose.

Get your combination cylinder looked at

We handle immersion heater repair and hot water cylinder replacement on combination units, direct and indirect, and fit unvented cylinders and thermal stores when an upgrade makes sense. We will test the mains flow and tell you honestly whether an upgrade will work in your flat before you spend anything on it. If the unit only needs a new element or float valve, we will say so.

Call 020 4652 5111 and tell us the make and size from the label if you can see it. Our rates are on the pricing page and are agreed before we attend.