If you’re replacing an old boiler or you’ve just moved into a home with a combi boiler and you’re not sure what equipment should be there, the question of whether you need a water tank is completely understandable. Heating systems in UK homes vary enormously depending on when the house was built and what type of boiler was originally fitted. Older properties often have a cold water storage tank in the loft, a hot water cylinder in the airing cupboard, and various bits of pipework connecting everything together.
Then you look at a combi boiler in a modern flat and there’s just the one box on the wall and nothing else. So do I need a water tank with a combi boiler, or is all that equipment just taking up space? This guide explains exactly what a combi boiler needs to work, what you can safely remove, what happens during the transition, and how much everything is likely to cost. No jargon, just practical information that helps you make a sensible decision.
The quick answer
No, a combi boiler does not need a cold water storage tank or a hot water cylinder to function. It draws water directly from the mains and heats it on demand. If you already have a combi boiler and there’s an old tank sitting in your loft, it’s almost certainly redundant and can be removed. If you’re switching to a combi from an older system, part of the installation process involves decommissioning and removing that old equipment.
What a combi boiler actually needs to work
A combi boiler is connected directly to your mains cold water supply. When you open a hot tap, cold mains water enters the boiler, passes through an internal heat exchanger, gets heated very quickly, and arrives at your tap as hot water. When you switch the heating on, the same unit heats a separate loop of water that circulates through your radiators. That’s the whole system. No stored water, no tank, no cylinder.
The only water-related component inside the boiler itself is a small expansion vessel, which is a sealed pressure management device that handles the change in water volume as the system heats up. This is not a water storage tank in any meaningful sense. You can’t draw water from it, and it plays no role in delivering hot water to your taps. It’s purely there to manage pressure, and it’s entirely contained within the boiler unit.
What a combi boiler does need is good mains water pressure. Because it heats water on demand rather than storing it, the flow rate and temperature of your hot water depend on how well the mains supply delivers cold water into the unit. If your property has low mains pressure, a combi might not deliver the hot water performance you’d expect, and this is worth checking before installation.
Why some homes have tanks even with a combi boiler
This causes a lot of confusion, and it’s worth explaining clearly. In many UK homes, especially older ones, there are tanks and cylinders left over from a previous heating system that weren’t removed when the combi boiler was fitted. Leaving them in place is sometimes done to save time and cost during installation, but it can cause problems down the line.
An unused cold water tank in the loft is a genuine liability. Stagnant water sitting in a tank that’s no longer connected to the system can become a health concern. The tank itself takes up loft space, and the associated pipework is just extra joinery that could develop faults. Similarly, an old hot water cylinder in the airing cupboard that’s no longer connected to anything is just taking up space. Leaving equipment in place when it serves no purpose is not ideal, and most engineers who do a proper job will decommission and remove redundant components as part of a combi installation.
If you’ve moved into a property and you’re not sure whether the tank in the loft is connected to the current heating system or not, this is something a plumber or Gas Safe engineer can check very quickly. It’s worth knowing for certain rather than guessing.
The three main boiler system types and what they each need
Understanding the alternatives helps put the combi question in context.
Combi boiler systems need no tank and no cylinder. The boiler connects to the mains and the central heating circuit. Full stop. These are compact, straightforward setups that work well in smaller properties with one bathroom and moderate hot water demand. For a one or two bathroom home with up to four occupants, this is usually the most practical arrangement.
System boiler setups don’t need a cold water tank in the loft either, since they draw directly from the mains, but they do work with a hot water cylinder. The cylinder stores pre-heated water so that multiple taps and showers can be used simultaneously without the pressure dropping. System boilers are a better fit for larger homes where two or more bathrooms might be in use at the same time, or where a family has high morning hot water demand.
Conventional (regular) boiler setups need both a cold water storage cistern in the loft and a hot water cylinder. These are the oldest and most space-intensive setups, still common in houses built before the 1990s. They can handle high hot water demand but involve more components, more maintenance points, and the inefficiency of storing water in an open-topped tank in a cold loft.
What happens when you switch from a tank system to a combi
This is where costs start to become relevant, and it’s important to go in with realistic expectations.
If you’re replacing an existing combi boiler with a new combi, the process is straightforward. The old unit comes out, the new one goes in the same position, and assuming the pipework is in reasonable condition, the job is usually done in a single day. Costs for a like-for-like combi replacement including the new boiler typically run from £1,500 to £2,800 depending on the boiler output, the brand, and where in the UK you’re based.
If you’re converting from a conventional system with a loft tank and airing cupboard cylinder to a combi boiler, the job is more involved. The engineer needs to drain and remove the hot water cylinder, drain and remove the cold water tank from the loft, cap off and tidy up the redundant pipework, and then install the new combi and connect it to your mains supply and existing radiator circuit. This typically takes one to two days rather than one, and the total cost including the new boiler and conversion work usually ranges from £2,500 to £4,500. That’s a meaningful sum, but you’re also gaining back the space previously taken up by the cylinder and loft tank, which some people find genuinely valuable.
One thing that often catches homeowners off guard: the cost of making good after the old equipment is removed. Once the cylinder is out of the airing cupboard, you may want to fit shelving or repurpose the space, which adds a small additional cost. In the loft, removing the tank sometimes leaves holes in the ceiling or exposed pipework that needs tidying. These are minor things, but they’re worth factoring into your budget.
When removing the tank is not straightforward
Most of the time, removing an old cold water tank and cylinder as part of a combi installation is fairly routine. But there are situations where things get more complicated.
If the tank in your loft also supplies cold water to certain taps in the property (this is common in older houses where the cold water to the bathroom is fed from the tank rather than the mains), the plumber needs to redirect those connections to the mains supply before the tank can be removed. This adds time and cost but is entirely manageable. A competent plumber will identify this during the initial survey.
If the loft tank or cylinder has been in place for a very long time and the associated pipework is old or corroded, there may be additional work needed to ensure the existing pipework that remains is in good condition before connecting a new boiler to it. Pushing a high-efficiency modern boiler onto old corroded pipework is not a good idea and any engineer worth using will flag this.
In very old properties where all the hot and cold pipework feeds from a gravity-fed tank system, the conversion to a mains-fed combi can involve more pipework changes than a straightforward job. An experienced installer should survey the property before quoting and be upfront about the scope of work involved.
Common mistakes people make during this process
Assuming the loft tank can stay in place without any attention is a frequent error. Some homeowners ask engineers to simply fit the new combi boiler and leave the old tank where it is, either to save money or because they’re not sure whether they’ll need it. Unless the tank is properly isolated, drained, and made safe, it can become a health hazard due to stagnant water, and it can also cause confusion if a future plumber or engineer assumes it’s still part of an active system.
Not checking mains water pressure before committing to a combi is another one that catches people out. A lot of older properties, particularly Victorian terraces and houses in certain areas of the North West and Scotland, have low mains pressure. If the pressure is significantly below what a combi needs to perform well, you might end up with disappointing hot water flow even with a brand new boiler. An engineer should check dynamic mains pressure during the survey visit, before any work is agreed or started.
Getting only one quote is something a lot of people do simply because the process feels overwhelming. Boiler conversion costs vary considerably between installers, and the difference between the cheapest and most reasonable quotes can be £500 to £1,000 on the same job. Getting two or three quotes from Gas Safe registered engineers is always worth the effort, especially on a conversion rather than a straight swap.
Choosing a boiler output that’s too small for the property is also a surprisingly common issue, particularly on conversions where people focus on the removal of old equipment and don’t give enough thought to sizing the new boiler correctly. A combi boiler’s output in kilowatts determines how much hot water it can deliver and how many radiators it can heat. Too small and you’ll have pressure and temperature issues. An engineer should calculate the required output based on your number of radiators and bathrooms, not just replace the old boiler with whatever happens to be in the van.
How to approach this decision properly
Here’s what usually matters when you’re trying to work out whether you need a tank with a combi boiler, or what to do with the tanks you already have.
Start by establishing what you currently have. If you’re not sure what’s in your loft or airing cupboard, have a look or ask a plumber to check. Knowing whether your existing tank is active or redundant is the foundation of any decision.
If you’re planning a boiler replacement, have the engineer or installer survey the property before quoting. A proper survey takes around thirty minutes and should cover your mains pressure, the condition of existing pipework, what equipment needs to be removed, and whether there are any complications specific to your property.
Consider your household’s hot water demand honestly. A combi works brilliantly for a couple or small family in a home with one bathroom. If you have two bathrooms and teenagers who shower every morning at the same time, a system boiler with a cylinder might genuinely suit your household better than a combi, even though it means keeping a cylinder in place.
Factor the full cost of the conversion into your budget, not just the boiler price. The boiler is often only half the total cost on a conversion job. Understanding the complete figure upfront avoids unpleasant surprises mid-way through the project.
Read more: Why does my Combi boiler keep losing pressure?
Do I need a water tank with a combi boiler? No, you don’t, and if one is currently sitting in your loft alongside a combi that’s already installed, there’s a reasonable chance it’s been left there redundantly and should be properly decommissioned. For most UK homes with modest hot water demand, a combi boiler without any tank or cylinder is the simplest, most space-efficient, and most practical setup available. Getting a proper survey from a Gas Safe registered engineer before any work starts is the surest way to make sure the right decisions get made for your specific property rather than a generic one.
