Unvented Cylinder Installation

What an unvented cylinder installation actually involves

The cylinder is the smallest part of the job. An unvented system is a pressure vessel connected to your incoming main, so most of the work is the control gear around it and the pipework that makes it safe.

On the day, the water is isolated at the stop tap, the old system is drained down and the old cylinder is cut out and carried away. If there is a feed and expansion tank and a cold water storage tank in the loft, both come out and the pipework they fed is re-run. The new cylinder is set on a level base able to carry it when full, then the inlet control group goes on: a strainer, a pressure reducing valve that brings the incoming main down to the cylinder working pressure, a single check valve so heated water cannot push back into the main, and an expansion relief valve. Expansion is taken either by an external vessel or by an internal air bubble depending on the cylinder design.

The safety side is separate from the control side and never shares a discharge. A temperature and pressure relief valve sits in the cylinder shell, piped to a tundish mounted where it can be seen, and from the tundish a discharge pipe runs in metal to a safe visible termination outside. That run is the part of the job that most often dictates where the cylinder can go, and it is the part cheap installs get wrong.

Then the heat sources are connected. On an indirect cylinder that means the boiler primary flow and return to the coil, a motorised valve, a cylinder thermostat and the wiring back to the programmer, usually as an S plan. The immersion heater is wired on its own switched supply. Secondary work follows: the cold distribution is re-fed from the main, a balanced cold take off is provided for showers and mixers that need it, and a drain point and isolation valves are fitted so the cylinder can be serviced without draining the house.

Commissioning is the last hour and it matters. The system is filled slowly, air is purged from the top outlet, the pressure reducing valve is set, the expansion charge is checked against the reduced inlet pressure, the relief valves are proved to lift and reseat, the cylinder thermostat is set, and the benchmark log book is filled in and left with you. Unvented work is notifiable and the installer registers it.

• Flow and pressure test at the stop tap before anything is ordered
• Old cylinder, loft tanks and redundant pipework removed
• Inlet control group: strainer, pressure reducing valve, check valve, expansion relief valve
• Temperature and pressure relief valve, tundish and metal discharge to a safe outside termination
• Boiler primary connections, motorised valve, cylinder stat and controls wiring
• Immersion heater on its own supply as a backup heat source
• Balanced cold supply for showers and mixer taps
• Fill, purge, set, prove the safety valves, and hand over the log book

Unvented cylinders store water above 90 degrees under pressure. If the safety controls fail, the stored water flashes to steam. This is not DIY work and it is not general plumbing work. Only an engineer qualified for unvented systems should install, service or alter one. Our page on what a G3 qualified plumber is covers what that qualification allows and what to ask to see before the work starts.

Will your cold main actually run an unvented cylinder?

This is the question that decides the whole job, and it is the one merchants selling cylinders online never ask. An unvented cylinder does not create pressure or flow. It stores hot water and hands back whatever your incoming main can deliver. If the main is weak, you will have spent a great deal of money to get weak hot water instead of weak cold water.

The test is simple and takes ten minutes. We run the nearest cold tap to the stop tap into a measured jug for a timed period to get litres per minute, then gauge the static pressure with everything closed and the running pressure with taps open. Static pressure tells you what the water company delivers at rest. The drop when you open taps tells you what the pipe between the main in the road and your house can actually carry, which is the number that matters.

Around 20 litres per minute with two or more bar running pressure gives a system that feels genuinely powerful at every outlet. Between 15 and 20 litres per minute an unvented cylinder still works well, though you will notice one shower affecting another if both run at once. Below about 15 litres per minute you are into judgement: sometimes the restriction is a partly closed stop tap or an old ball valve that can be sorted in an hour, and sometimes it is a 15mm lead supply from the road that no cylinder will fix. We would rather tell you that on the survey than after you have paid for a cylinder.

Common causes of a poor reading in London property: a lead or old galvanised iron supply pipe, a 15mm main serving a house that now has three bathrooms, a shared rising main in a converted flat where the neighbours draw off the same pipe, a seized stop tap under the kitchen units, or a builder who buried a service valve and left it half shut. Several of these are fixable, and a mains upgrade from the boundary can transform a property.

• Litres per minute at the tap nearest the stop tap, measured and written down
• Static pressure and running pressure, both gauged
• Diameter and material of the incoming supply where it can be seen
• Whether the rising main is shared with other flats
• Whether the stop tap and any service valve are fully open and not seized

What size unvented cylinder do you need?

Size by peak demand, not by the number of people in the house and not by what was there before. The question an engineer is really answering is: what is the worst ten minutes this house will ever ask for, and can the cylinder cover it without running cold?

A bath fill is the heaviest single draw in most homes, taking roughly 150 to 180 litres of blended water, which is about 100 to 120 litres from the cylinder once it mixes with cold. A decent mains pressure shower draws around 10 to 12 litres a minute, so two showers running together for eight minutes is a serious ask. Add in the habit of the house: teenagers showering back to back on a school morning are a heavier load than four adults who bathe at different times.

As a working guide, a one bathroom flat is usually comfortable on 120 to 150 litres, a two bathroom house on 180 to 210, and a three bathroom house on 250 to 300. If there is an en suite that runs at the same time as the family bathroom, size up rather than down. The cost difference between two sizes of cylinder is small next to the cost of living with hot water that runs out.

Reheat time is the other half of sizing and it depends on the coil, not the cylinder volume. A cylinder with a large surface area coil will recover a full store in around half an hour from a decent boiler. A small coil on the same volume can take twice as long, which makes a nominally adequate cylinder feel undersized every evening. If a supplier is quoting only litres and not the coil rating, they are selling you a tank rather than a hot water system.

Where the cupboard physically will not take the size you need, a taller slimline cylinder often solves it, or the cylinder can be relocated. We have moved plenty of cylinders out of a cramped airing cupboard and into a utility room or a garage, which usually also shortens the discharge route and makes future servicing easier. If you want to work the sizing through yourself first, our guide to what size hot water cylinder you need takes the same demand figures step by step.

One caution about the numbers on a manufacturer datasheet. Published hot water cylinder sizes describe the nominal store and nothing about how quickly it comes back, so two cylinders carrying the same figure can behave completely differently in a house that showers twice in an hour. Read the volume and the coil rating together, and treat the volume on its own as marketing rather than specification.

• One bathroom, no en suite: 120 to 150 litres is typically enough
• Two bathrooms: 180 to 210 litres
• Three bathrooms or simultaneous showers: 250 to 300 litres
• Ask for the coil rating and reheat time, not just the volume
• Size up if anyone in the house runs a bath while a shower is on

Converting from a vented cylinder to unvented

A vented to unvented conversion is the most common version of this job in London housing stock. You have a copper cylinder in an airing cupboard fed by a cold tank in the loft, showers that dribble on the top floor, and a small feed and expansion tank next to it for the heating. The conversion removes both tanks and re-feeds everything from the main.

The pipework consequences are worth understanding before you commit. Your cold taps are currently fed from the loft tank at low pressure. After conversion they are fed at mains pressure, which is better water and better flow, but it means the cold distribution pipework and the taps themselves now see full pressure. Old capstan bath taps and worn washers that were holding back a two metre head will sometimes weep or whistle once the pressure trebles. Ceramic disc taps and any reasonably modern fitting are fine. We check the outlets during the survey and tell you which ones are likely to complain.

Any shower pump in the airing cupboard or under the bath becomes redundant and comes out, along with negative head sensors and their wiring. Gravity fed shower mixers designed for low pressure often perform badly on mains pressure and are usually worth replacing at the same time, because a low pressure mixer on a high pressure supply tends to feel harsh and lose temperature stability.

Balancing matters more after a conversion than before it. Any mixer shower or mixer tap needs its hot and cold from sources at similar pressure, so the cold leg for those outlets is taken from the balanced cold connection on the cylinder or from an equivalent point after the pressure reducing valve, not from a random tee off the main. Skipping that is why some conversions end up with showers that jump temperature whenever a toilet fills.

The loft space you free up is real. So is the weight you move: the load goes from a tank in the roof to a cylinder on a floor, which is one of the things a survey checks rather than assumes. If you are still deciding whether to convert at all, our comparison of unvented and vented cylinders sets out what each type does well and where each one struggles.

If you are keeping the existing boiler, the conversion also changes how the heating is controlled, because a gravity primary system has to be converted to a fully pumped S plan or Y plan arrangement with a motorised valve and a cylinder thermostat. Anyone quoting a cylinder swap on a gravity primary system without mentioning that has not looked at the heating.

What makes a property straightforward, and what makes it awkward

Straightforward looks like this: an airing cupboard on or near an outside wall, a 22mm or 25mm incoming main in good condition, a stop tap that works, a boiler already fully pumped, and space to stand the cylinder without cutting joists. That install goes in inside a day and the discharge pipe runs two metres to a gulley.

Awkward is not the same as impossible, and most awkward properties are still perfectly good candidates. It just changes the plan and the time on site.

The most frequent complication in London is the discharge route. The safety discharge has to run in metal pipe, fall continuously, and terminate somewhere visible and safe where scalding water cannot reach anyone. A cylinder in a central internal cupboard of a flat, with no outside wall within reach and no gulley below, needs either a long carefully graded run or a different cylinder position. Purpose made discharge arrangements exist for flats with no external route, and they are the right answer in a top floor conversion where the only alternative is an unsafe run.

Other things that change the job: a flat above a shop where the rising main is shared and the reading drops every time the shop draws water, a Victorian conversion where the supply is still lead from the boundary, a listed facade that rules out a visible external pipe, a cupboard that will not take the cylinder height because of a stair bulkhead, and access. A 300 litre cylinder full of water weighs well over 300 kilograms, so the floor under it, particularly on a first floor of a converted house, is something we look at rather than assume.

• No outside wall within reach of the cylinder position
• Shared or undersized rising main in a converted flat
• Lead or old iron supply from the boundary
• Cupboard too short or too narrow for the size the house needs
• Gravity primary heating that has to be converted to fully pumped
• Upper floor position where floor loading needs checking

Indirect, direct, and cylinders for heat pumps and solar

An indirect cylinder is heated by a coil carrying hot water from your boiler, with an immersion heater as a backup. This is what the great majority of London homes need, because the boiler heats a full cylinder far faster and more cheaply than an immersion can.

A direct cylinder has no coil and is heated by immersion heaters only, usually two, with the lower one on an off peak tariff to fill the store overnight and the upper one for a smaller top up during the day. It is the right choice in an all electric flat with no gas, and the wrong choice anywhere a boiler exists.

If a heat pump is in your plans, even a few years out, say so at the survey. Heat pumps deliver heat at a much lower flow temperature than a boiler, so they need a cylinder with a far larger coil surface area to transfer the same energy. Fitting a standard boiler cylinder now means replacing it again later. A cylinder with a heat pump rated coil costs a little more and runs perfectly happily on a boiler in the meantime.

Twin coil cylinders serve solar thermal panels on the lower coil and the boiler on the upper, so the sun preheats the bottom of the store and the boiler only tops up what is missing. They are taller than a single coil equivalent of the same volume, which is a cupboard measurement worth taking early.

On brands, the market is broad and most of the well known names build a sound vessel: Megaflo, Telford, Gledhill, Joule, OSO and others all make cylinders we install and service. The difference in the field is far less about the badge and far more about whether the control gear was set correctly on commissioning and whether the thing has been serviced since.

Hard water and what it does to a new cylinder in London

Most of London sits on hard water, and a new unvented cylinder is a large heated surface for limescale to settle on. Scale does two things. On an immersion heater element it builds as a crust that insulates the element from the water, which makes it run hotter than designed until it fails. On an indirect coil it slows heat transfer, so the cylinder that recovered in thirty minutes when it was new takes fifty a few years later and you assume the boiler is tired.

Two practical measures make a difference at install. The first is a scale reduction device on the cold feed to the cylinder, which does not remove hardness but changes how the scale behaves so it is far less inclined to bond to hot surfaces. The second is not running the store hotter than it needs to be, because scale deposits accelerate sharply with temperature. A store around 60 degrees is the usual balance between scale and keeping stored water safe, and blending valves protect the outlets so a hot store never reaches a tap at storage temperature.

The third measure is not an install decision at all. An unvented cylinder needs an annual service, and in a hard water area that service is what keeps the expansion working, the relief valves free and the immersion element alive. It is a short visit and it is the cheapest thing you will ever do for the system. Our unvented cylinder service page sets out exactly what gets tested on that annual visit and why the expansion side cannot be checked without draining down.

How long an unvented cylinder installation takes, and what to expect at home

A like for like unvented cylinder replacement in an accessible cupboard is usually a single day. A vented to unvented conversion with loft tanks to remove, cold distribution to re-run and heating controls to convert is normally a day and a half to two days. A relocation, a mains upgrade, or a job in a flat needing a purpose made discharge arrangement can run longer, and we say so at the survey rather than discovering it on the second afternoon. Where the existing cylinder still works and you are only replacing like for like, the hot water cylinder replacement page covers that job, and the signs a cylinder needs replacing are set out separately if you are not sure yet.

Expect the water to be off for most of the first day and the heating off with it. We leave a working cold tap where we can. Old cylinders and tanks are heavy and awkward and they go out through the house, so the route from the cupboard to the front door is worth clearing. Copper is taken away with everything else.

The first heat up is the moment to be present, because the handover is genuinely useful: where the stop tap and isolation valves are, what the tundish is and why it must stay visible, what a drip from the outside discharge pipe means, how to switch the immersion on if the boiler ever fails, and what the programmer is now doing that it was not doing before. It takes ten minutes and it saves a call out later.

For a few days after any system is drained and refilled, small amounts of trapped air work their way out and a tap may spit. That is normal and it settles. Water that stays cloudy, a discharge pipe that drips persistently, or a tundish with water running through it is not normal and should be looked at.

The rate is fixed before we attend and the total depends on how long the job takes. The cylinder itself and any mains upgrade are separate from the labour, and both are confirmed in writing after the survey rather than guessed at over the phone.

What a finished install should look like

If you want to judge the work without being a plumber, look for these things. They are the difference between an install that will still be right in ten years and one that turns into a problem for somebody else by the third winter.

The tundish should be visible without moving anything, because it is the system telling you when a safety valve is passing. The discharge pipe should be metal, fall the whole way, and end somewhere you can see it from outside. The expansion vessel, if external, should be reachable for a service without dismantling the cupboard. Every valve should be labelled or obvious. There should be a drain point so the cylinder can be emptied without siphoning it through a hose out of the bathroom window. The benchmark log book should be filled in, including the commissioning pressures, and left with you rather than posted later.

And you should have been shown the isolation valve for the cylinder before the engineer left. If something ever leaks at two in the morning, that is the single most valuable thing you know about your own house.

• Tundish visible and unobstructed
• Metal discharge pipe with continuous fall to a safe visible outside termination
• Expansion vessel accessible for servicing
• Isolation valves and a drain point fitted
• Benchmark log book completed with commissioning pressures and handed over
• Controls explained, including how to run the immersion if the boiler fails

Choosing the cylinder, and checking whoever is fitting it

Two decisions sit in front of this job and neither of them is about the install itself. One is which cylinder, and the other is who is allowed to put it in. Both are worth settling before a date is booked, because both are much harder to revisit once the old system is on the drive.

On the cylinder, brand matters less than the specification behind the badge, and the specification worth arguing about is the coil, the insulation and whether the safety components are still stocked as spares in ten years. The longer walk through that decision is in how to choose a hot water cylinder, which covers stainless against enamelled steel, coil ratings, and the questions a supplier should be able to answer without looking anything up.

If you have not committed to the sealed type at all, the prior question is which system suits the property. An unvented vs vented cylinder comparison is the right place to start, because a poor incoming main makes the decision for you and no amount of cylinder quality compensates for it.

On the person, unvented work is restricted. The sealed side, the relief valves and the discharge arrangement may only be worked on by someone holding the unvented hot water qualification, and a G3 qualified plumber can show you that card on the doorstep. Ask to see it. An installer who has to go and find it, or who explains why it is not really needed on this job, has told you what you needed to know.

Where the install also touches the boiler, and most conversions do because the primary side has to be repiped and rewired, that part is gas work and sits under a different registration entirely. Knowing how to check a plumber on the Gas Safe Register takes a minute and covers the specific engineer rather than the company name on the van, which is the distinction that catches people out when a firm subcontracts the gas element.

When we would tell you not to fit an unvented cylinder

An unvented cylinder is the right answer for most houses with two or more bathrooms and a decent main. It is not the right answer everywhere, and an installer who says it always is has stopped listening.

If the incoming main is genuinely poor and cannot be upgraded, for example a flat drawing off a shared rising main that the freeholder will not alter, an unvented cylinder will store plenty of hot water and still deliver it slowly. A thermal store or a pumped vented system with a good accumulator can outperform it in that situation.

If the property is a one bathroom flat with no bath and modest demand, a combi boiler often does the same job with no cylinder, no cupboard taken up and no annual service. Losing a whole cupboard to store hot water nobody is going to use is not an upgrade.

If the house has no gas and no off peak tariff, heating a large store by immersion alone at standard daytime rates is expensive, and the conversation should be about the tariff and the heat source before it is about the cylinder.

The survey exists to reach that answer honestly. We would rather test your main, tell you an unvented cylinder is the wrong fit and do something more useful with the money, than sell you a vessel that disappoints you every morning.