Is a combi boiler better than a hot water cylinder?

Neither is inherently better — the right choice depends on your home. A combi boiler is better for smaller homes with one bathroom, where space saving and simplicity matter. A cylinder system is better for larger homes with two or more bathrooms, where you need strong hot water flow at multiple outlets simultaneously. Both are efficient with modern equipment.

How much does it cost to switch from a cylinder to a combi boiler?

Switching from a cylinder-based system to a combi boiler in Exmouth typically costs £2,500 to £4,000. This includes the new combi boiler, removing the old cylinder and loft tank, modifying the pipework and fitting a magnetic filter. The exact cost depends on the amount of pipework that needs rerouting and the boiler brand chosen.

Can a combi boiler supply two showers at once?

A standard combi boiler (24–30kW) struggles to supply two showers at full flow and temperature simultaneously. You will notice a drop in both flow rate and heat. Larger combis (35–40kW) cope better but still cannot match a well-sized unvented cylinder for multi-outlet performance. If two or more showers running at the same time is important, a cylinder system is the better option.

Do I still need a hot water cylinder with a system boiler?

Yes. A system boiler heats water and sends it to a cylinder for storage — it cannot heat water on demand like a combi. The cylinder stores the hot water until you need it. This is actually an advantage: the stored water is available at high flow rates and the immersion heater in the cylinder provides backup if the boiler fails.

How long does a combi boiler take to heat water?

A combi boiler heats water almost instantly — typically within 2 to 5 seconds of opening a hot tap. There is a brief delay while cold water in the pipes clears. The trade-off is that the flow rate is limited to what the boiler can heat in real time (10–15 litres per minute for a typical 30kW combi), whereas a cylinder releases stored water at whatever rate the pipework allows.

Will a combi boiler work with a heat pump?

Not easily. Heat pumps produce heat at lower temperatures and work best when they can store energy in a hot water cylinder over time. A combi boiler heats water on demand at high temperatures, which is the opposite of how a heat pump operates. If you plan to install a heat pump in the future — increasingly common as Exmouth homes decarbonise — keeping or installing a cylinder now saves you a costly conversion later.

How do I know if my mains pressure is high enough for a combi boiler?

A combi boiler typically needs a mains flow rate of at least 10 litres per minute to perform well. You can do a rough test at home: time how long it takes to fill a 10-litre bucket from your kitchen cold tap. If it takes under a minute, your mains pressure is likely adequate. A plumber will do a proper flow rate and static pressure test before recommending a combi installation.

Combi Boiler vs Hot Water Cylinder: Which System Suits Your Home?

Thinking about replacing your boiler or hot water system? The choice between a combi boiler and a hot water cylinder affects your space, water pressure, running costs and future options. This guide compares both systems to help you decide what works best for your Exmouth home.

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Head-to-Head Comparison

How combi boilers and cylinder systems compare across the factors that matter most when choosing a hot water system.

How It Works

Combi Boiler

A combi (combination) boiler heats water instantly when you turn on a tap. Cold mains water passes through a heat exchanger inside the boiler and comes out hot in seconds. There is no stored hot water — it is heated on demand every time.

Hot Water Cylinder

A system or regular boiler heats water and stores it in an insulated cylinder (usually in the airing cupboard). When you open a hot tap, pre-heated water flows from the cylinder. The boiler refills and reheats the cylinder automatically.

Hot Water Performance

Combi Boiler

Good for one outlet at a time. Flow rate is typically 10–15 litres per minute. If someone runs a shower while another person turns on the kitchen tap, both will notice a drop in flow and temperature. Not ideal for homes with multiple bathrooms in use simultaneously.

Hot Water Cylinder

Excellent for simultaneous use. A well-sized cylinder (150–300 litres) can supply two or three showers at once without losing pressure or temperature. Once the stored water runs out, you wait for the cylinder to reheat — typically 20 to 40 minutes.

Space Requirements

Combi Boiler

Very compact — the boiler is wall-mounted (usually in the kitchen) and there is no cylinder, no cold water tank in the loft and no airing cupboard needed. Frees up significant space, which is a big advantage in smaller Exmouth homes, flats and terraced houses.

Hot Water Cylinder

Requires an airing cupboard for the cylinder and (for vented systems) a cold water tank in the loft. Unvented cylinders eliminate the loft tank but still need cupboard space. The trade-off is better hot water performance in exchange for the space used.

Upfront Cost

Combi Boiler

A new combi boiler supplied and installed costs £2,000 to £3,500 in the Exmouth area. Swapping from a system boiler to a combi (removing the cylinder and loft tank) costs £2,500 to £4,000 due to the extra pipework changes involved.

Hot Water Cylinder

A new system boiler plus unvented cylinder costs £3,000 to £5,000 installed. Replacing just the cylinder costs £400 to £700 (vented) or £1,000 to £2,000 (unvented). Keeping an existing cylinder and replacing the boiler is the cheapest option.

Running Costs

Combi Boiler

Slightly lower gas bills because you only heat the water you use — no energy lost keeping a cylinder hot. However, the difference is modest with a well-insulated modern cylinder (loses only 1–2 kWh per day). A combi saves roughly £50 to £80 per year compared to a poorly insulated older cylinder.

Hot Water Cylinder

Marginally higher if the cylinder is old and poorly insulated. Modern factory-insulated cylinders lose very little heat. You can also use an immersion heater on a cheap overnight electricity tariff (Economy 7) to heat water, which can reduce costs further.

Water Pressure

Combi Boiler

Delivers hot water at mains pressure — good, strong flow at every tap. However, the flow rate is limited by the boiler capacity, so pressure drops when multiple outlets are open simultaneously. A 30kW combi delivers roughly 12 litres per minute.

Hot Water Cylinder

With an unvented cylinder, hot water is also at mains pressure and the flow rate is excellent — a large cylinder can deliver 20+ litres per minute across multiple outlets. With an older vented cylinder, pressure depends on gravity and is often noticeably weaker.

Compatibility with Renewables

Combi Boiler

Limited. Combi boilers cannot easily integrate with solar thermal panels or heat pumps because there is no cylinder to store the energy they generate. If you plan to add renewable heating in the future, a combi makes this harder and more expensive.

Hot Water Cylinder

Excellent. A hot water cylinder is essential for solar thermal panels, heat pumps and other renewable systems. The cylinder stores energy generated during the day for use later. If you are considering renewables, keeping or installing a cylinder is the better long-term choice.

Maintenance

Combi Boiler

Annual boiler service is recommended (and required to maintain most warranties). The boiler does all the work, so when it breaks down you lose both heating and hot water until it is repaired. No cylinder to service, but the boiler is a single point of failure.

Hot Water Cylinder

Annual boiler service plus an annual service for unvented cylinders (checking the expansion vessel and safety valves). If the boiler breaks down, the immersion heater in the cylinder can still provide hot water as a backup — a useful safety net.

Which System Suits Your Home?

The right choice depends on your household size, number of bathrooms and future plans.

Choose a Combi Boiler If

You have a smaller home — flat, terrace or 2-3 bedroom semi
You have one bathroom (or one bathroom plus a downstairs cloakroom)
You rarely run two showers at the same time
Space is tight and you want to free up the airing cupboard
You want the lowest upfront installation cost
You do not plan to add solar thermal or a heat pump

Choose a Cylinder System If

You have two or more bathrooms used simultaneously
You have a larger household (4+ people)
You want strong, consistent hot water at multiple outlets
You plan to install solar thermal panels or a heat pump
You want a backup hot water source (immersion heater)
You already have a cylinder and it is in good condition

What Is Involved in Switching Systems?

Switching between a combi boiler and a cylinder system is a bigger job than a like-for-like boiler replacement. Here is what each direction involves.

Switching from cylinder to combi

The old cylinder, loft tank and associated pipework are removed
Pipework is modified to supply hot water directly from the combi boiler
You gain an airing cupboard and loft space
Total cost: £2,500 to £4,000 depending on pipework complexity
Consider whether your water pressure and flow rate can support a combi — a plumber can test this
You lose the ability to run multiple showers at full flow simultaneously

Switching from combi to cylinder

A new cylinder is installed (usually in an airing cupboard or utility room)
A system boiler replaces the combi, or a separate cylinder is added to the existing system
New pipework runs from the cylinder to hot water outlets
Total cost: £3,000 to £5,000 depending on cylinder type and pipework
You gain the ability to use multiple showers simultaneously and add renewables later
You lose some cupboard or floor space for the cylinder

Combi Boiler vs Cylinder FAQs

Get Expert Advice on Your Hot Water System

The best way to decide is to get a professional assessment. A local heating engineer can test your mains pressure, assess your household demand and recommend the right system. Browse our directory for a no-obligation quote.

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Already decided? See our hot water cylinder guide for repair and replacement costs. Need your existing boiler fixed? Visit our boiler repair page. For a quick cost estimate, try our cost calculator.