Hot water is at the heart of everyday life, from morning showers to cooking and cleaning. Choosing the right hot water system for your home isn’t just about convenience—it affects your energy bills, efficiency, and comfort too. With options like combi, system, and conventional boilers available, knowing which setup suits your household can feel confusing. This guide breaks down the main types of hot water systems, their benefits, and what to consider before making a decision, so you can invest with confidence.
What Is a Combi Boiler?
Mains pressure combination boilers — more commonly called combi boilers — have become one of the most popular hot water systems in the UK. These units combine a high-efficiency water heater and central heating boiler in a single compact appliance, usually mounted on an external wall.
Unlike traditional systems, a combi boiler heats water directly from the mains as soon as you turn on a tap. This means:
No need for a separate hot water storage cylinder.
No cold water tank in the loft.
A more space-saving, modern setup for smaller homes.
Because they rely on mains pressure, combi boilers provide a strong and consistent water flow. A typical output is around 20 psi (1.8 bar, or 18m head), though this may vary by model (always check your boiler’s manual). In practice, this pressure should give you a reliable flow rate for taps, showers, and everyday household use.
In short, a combi boiler is a compact, energy-efficient choice ideal for homes where space is limited and hot water demand is moderate.
Mains Pressure Unvented System:
A mains pressure unvented system is a common setup in UK homes, especially where there’s no space for a cold-water storage tank in the loft. Instead, this type of system combines a standard boiler with a separate hot water storage cylinder, usually located in the airing cupboard.
Here’s how it works:
The system stores mains-fed water in a large, strengthened hot water cylinder.
The stored water is heated either by:
Immersion heaters fitted into the side of the cylinder, or
A wall-mounted central heating boiler.
When you turn on a tap or shower, cold water from the mains flows into the cylinder, pushing hot water out through an outlet pipe to your fixtures.
Because the cylinder is sealed and has no open vent, it stays pressurised, delivering hot water at mains-like pressure throughout the home.
Key points to know about unvented systems:
Typical pressure ranges between 1.5 – 1.8 bar (check your system manual for specifics).
They provide strong, consistent flow rates to multiple taps and showers at the same time.
Great for larger households with higher hot water demand.
Require professional installation and annual servicing to meet safety regulations, as the cylinders are pressurised.
In short, a mains pressure unvented system combines the convenience of stored hot water with the high pressure of a direct mains feed, making it a reliable option for busy homes that need plenty of hot water on demand.
Low Pressure Gravity System
A low-pressure gravity system is one of the most traditional domestic water setups in the UK, especially common in older properties. This type of system is easy to recognise as it typically features:
A cold water tank (cistern) in the loft.
A hot water cylinder (often in the airing cupboard) on the first floor.
The system works by using gravity to create pressure. Water flows down from the cistern into the hot water cylinder, then out to taps and showers. The pressure and flow rate depend on two main factors:
Mains pressure supplying the cold cistern.
Height difference between the cold water tank and the water outlet (tap or shower).
How the pressure is calculated:
Roughly 1 metre of vertical drop = 0.1 bar of pressure.
For example, if your loft tank is 5 metres above your showerhead, you’ll get about 0.5 bar of water pressure.
This means the higher the cistern above the outlet, the stronger the flow.
Key points about low-pressure gravity systems:
Common in homes built before modern high-pressure systems became the norm.
Can sometimes result in weaker flow rates, especially for upstairs showers.
Often benefit from shower pumps or other booster solutions to improve pressure.
Flexible in design, as they can be paired with vented hot water cylinders.
In summary, while a gravity-fed system may not always deliver the powerful flow of a mains-fed or unvented system, it remains a reliable and simple solution in many UK homes. With the right setup—or by adding a pump—it can still provide excellent performance.
Key Benefits of Each Hot Water System
System Type
Efficiency
Installation
Running Costs
Best For
Combi Boiler
Heats water on demand, no storage heat loss
Compact, no tanks needed
Lower (only heat what you use)
Small homes, limited space
Unvented System
Stores hot water at mains pressure
Needs cylinder & professional install
Moderate (stored water reheated)
Large homes, multiple bathrooms
Gravity System
Reliable but less efficient
Loft cistern + hot water cylinder
Higher (heat loss from storage)
Older homes with existing setup
Mains Pressure Combination (Combi) Boilers
Space-saving, efficient, and lower running costs.
Best for smaller properties.
Mains Pressure Unvented Systems
High pressure is great for multiple outlets at once.
Ideal for family homes with more than one bathroom
Low-Pressure Gravity Systems
Common in older homes, it works reliably but at lower pressure.
May need upgrading or pumps to boost performance.
What to Consider Before Choosing
When selecting a hot water system, think about how it will fit your home and lifestyle:
Household size – Larger families with multiple bathrooms may need the capacity of an unvented system, while a small flat might suit a compact combi boiler.
Water demand – Consider how many showers, taps, and appliances are used at the same time. High demand requires higher capacity.
Available space – Combi boilers save space as they don’t need tanks or cylinders, while gravity or unvented systems need more storage.
Budget – Upfront installation costs vary widely. Combi boilers are often cheaper to install, while unvented systems can be more of an investment.
Energy Efficiency and Running Costs
Energy efficiency should be a key factor in your decision. Modern systems are designed with eco-friendliness and long-term savings in mind:
Combi boilers – Highly efficient because they only heat water on demand, with no heat lost from stored water.
Unvented systems – Offer high performance and pressure but may have higher heat-loss costs if insulation isn’t optimal.
Gravity systems – Often less efficient, especially in older homes, but can be upgraded with pumps and better insulation.
Eco benefits – Choosing an A-rated boiler or cylinder can cut energy bills and reduce your carbon footprint.
Hot Water System Maintenance Tips
Regular care keeps your system running smoothly and avoids costly repairs:
Annual servicing – Always book a qualified engineer to check your boiler or cylinder.
Check for leaks – Small drips around pipework or tanks can quickly become bigger problems.
Insulate pipes and tanks – Reduces heat loss and improves efficiency, especially in older gravity systems.
Flush and clean – Periodically flush out systems to prevent scale build-up, especially in hard water areas.
Monitor pressure – Keep an eye on system pressure gauges to spot issues early.
What are the main types of hot water systems in the UK?
The most common are combi boilers, system boilers, and conventional boilers. Each has different benefits depending on your home size and hot water demand.
Which hot water system is best for a small home?
A combi boiler is usually best for smaller homes or flats. It heats water on demand, saving space by not requiring a hot water cylinder.
Do hot water systems affect energy bills?
Yes. Choosing an efficient system can lower your energy bills. For example, combi boilers are efficient for smaller homes, while system boilers suit larger properties with higher demand.
How long does a hot water system last?
With proper maintenance, most hot water systems last 10–15 years. Regular servicing helps extend lifespan and keep efficiency high.
Can I switch from one hot water system to another?
Yes, but it depends on your home’s layout and plumbing. Switching may require extra installation work, so it’s best to get advice from a qualified heating engineer.
Choosing the right hot water system is about finding the balance between efficiency, household needs, and long-term value. Whether you opt for a space-saving combi boiler, a high-performance unvented cylinder, or a traditional gravity-fed setup, each option has strengths that suit different homes. By considering factors like water demand, available space, and running costs, you can make a decision that keeps your home comfortable and efficient for years to come.
Combi boilers: best for smaller homes, instant hot water, no storage tank needed.
System boilers: ideal for larger homes, can supply multiple taps at once.
Conventional boilers: traditional setup with tank and cylinder, good for high-demand households.
Energy efficiency: modern systems reduce bills and carbon footprint.
Maintenance matters: servicing extends lifespan and reliability.
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