Choosing the right boiler type is one of the most important decisions when replacing your heating system. Get it wrong, and you will either run out of hot water constantly or pay for capacity you do not need. This guide explains the three main boiler types, their pros and cons, and how to choose the right one for your property.
Quick Answer: Which Boiler Type Do You Need?
| Property Type | Best Choice |
|---|---|
| 1-2 bed flat, 1 bathroom | Combi |
| 2-3 bed house, 1 bathroom | Combi |
| 3-4 bed house, 2 bathrooms | System (or large combi) |
| 4+ bed house, 2-3 bathrooms | System |
| Large house, 3+ bathrooms, high demand | System or Regular |
| Older property with gravity-fed system | Regular (or convert to system) |
These are general guidelines. Property specifics, water pressure, and usage patterns all affect the final recommendation.
In This Guide
The Three Boiler Types at a Glance
Before we dive into detail, here is what each type does:
| Type | How It Works | Hot Water Cylinder? | Cold Water Tank? |
|---|---|---|---|
| Combi | Heats water on demand directly from mains | No | No |
| System | Heats water and stores it in a cylinder | Yes | No |
| Regular | Heats water via cylinder and tank system | Yes | Yes (in loft) |
Each type has its place. The best boiler depends on your property, household size, and hot water demands.
Combi Boilers Explained
A combination (combi) boiler provides both central heating and instant hot water from a single unit. When you turn on a hot tap, the boiler fires up and heats water directly from the mains, delivering it to your tap in seconds.
How combi boilers work
- Cold water enters from the mains
- When you open a hot tap, the boiler detects flow and fires the burner
- Water passes through a heat exchanger and comes out hot
- No storage, water is heated on demand
- When you close the tap, the boiler stops heating water
Advantages of combi boilers
- Space saving: No cylinder or tank needed, ideal for flats and smaller homes
- Hot water on demand: No waiting for a cylinder to heat up
- Energy efficient: Only heats water when needed (no standby losses)
- Typically simplest installation: Fewer components and less pipework than stored hot water systems
- Mains pressure hot water: Good shower pressure without a pump
- No risk of frozen loft tank: No cold water tank to worry about
Disadvantages of combi boilers
- Limited flow rate: Can only supply one outlet well at a time
- Struggles with multiple bathrooms: Running two showers simultaneously often means reduced flow and temperature
- Depends on mains pressure: Poor mains pressure reduces hot water performance
- No backup hot water: If the boiler fails, you lose hot water completely
- Not compatible with some power showers: Cannot feed a pumped shower designed for gravity systems
Best for
- Flats and apartments
- Small to medium houses (up to 3 bedrooms)
- Properties with 1 bathroom
- Homes where space is limited
- Households with lower hot water demand
System Boilers Explained
A system boiler heats water and stores it in a hot water cylinder (usually in an airing cupboard). The key system components (like the pump and expansion vessel) are built into the boiler, making installation simpler than a regular boiler.
How system boilers work
- Cold water enters from the mains (no loft tank needed)
- The boiler heats water and sends it to a cylinder for storage
- When you open a hot tap, pre-heated water flows from the cylinder
- The cylinder can supply multiple outlets simultaneously (within its capacity)
- The boiler reheats the cylinder as needed
Advantages of system boilers
- High hot water output: Can supply multiple bathrooms at once
- Consistent flow: Stored hot water means steadier performance at peak times
- Mains pressure: Fed directly from the mains (typically better than gravity-fed systems)
- Compatible with solar thermal: Cylinder can be pre-heated by solar thermal
- No loft tank: Less risk of freezing and fewer components than regular systems
- Backup option: Many cylinders include an immersion heater for emergency hot water
Disadvantages of system boilers
- Needs a cylinder: Requires space for a hot water cylinder
- Hot water can run out: Once the cylinder is depleted, you wait for it to reheat
- Standing heat loss: Cylinders lose some heat over time (modern cylinders are well insulated)
- More involved installation: More components and pipework than a combi
- Slightly less efficient in practice: Some energy is lost storing hot water
Best for
- Medium to large houses (3+ bedrooms)
- Properties with 2 or more bathrooms
- Households with high hot water demand
- Homes with space for a cylinder
- Properties considering solar thermal
Regular (Heat-Only) Boilers Explained
A regular boiler (also called a heat-only, conventional, or open-vent boiler) is the traditional type found in older UK homes. It works with both a hot water cylinder and a cold water tank (usually in the loft).
How regular boilers work
- Cold water is stored in a tank in the loft (fed by mains)
- Water flows from the tank to the boiler by gravity
- The boiler heats the water and sends it to the cylinder
- When you open a hot tap, water flows from the cylinder (gravity-fed pressure)
- The loft tank also supports the system’s expansion and feed
Advantages of regular boilers
- Handles very high demand: Large cylinder and tank can suit big properties
- Works with low mains pressure: The loft tank can act as a buffer
- Often suits existing pipework: Like-for-like swaps can be simpler in older homes
- Power shower compatible: Can work with pumped showers
- Good for multi-bathroom homes: High storage capacity if correctly sized
Disadvantages of regular boilers
- Takes up most space: Boiler plus cylinder plus loft tank
- Gravity-fed pressure: Typically lower than mains-fed systems unless pumped
- Loft tank can freeze: Risk if the loft is poorly insulated
- Standing heat loss: More stored-water losses than combi systems
- More components: More parts means more potential maintenance points
- Often avoidable: A system boiler can deliver similar benefits without a loft tank in many homes
Best for
- Older properties with existing gravity-fed systems
- Homes with very low mains pressure
- Large properties with very high hot water demand
- Properties where changing the system is impractical
Side-by-Side Comparison
| Feature | Combi | System | Regular |
|---|---|---|---|
| Hot water storage | None (instant) | Cylinder | Cylinder + tank |
| Space required | Least | Medium | Most |
| Installation complexity | Typically simplest | Medium | Most complex |
| Running efficiency | Highest (no cylinder losses) | Good | Lower (more stored-water losses) |
| Multiple outlets | Poor | Good | Good |
| Water pressure | Mains | Mains | Gravity (often lower) |
| Hot water runs out? | No (but limited flow) | Yes (wait to reheat) | Yes (wait to reheat) |
| Compatible with solar | Limited | Yes | Yes |
| Best for bathrooms | 1 | 2-3 | 3+ |
| Loft tank needed | No | No | Yes |
How to Choose: Decision Framework
Answer these questions to narrow down your choice:
1. How many bathrooms do you have?
- 1 bathroom: Combi is almost always the best choice
- 2 bathrooms: Combi may work, but system is often better
- 3+ bathrooms: System or regular is usually needed
2. How many people live in your home?
- 1-3 people: Combi usually sufficient
- 4+ people: A system boiler gives more flexibility at peak times
3. Do people shower at the same time?
If two or more showers need to run simultaneously (busy mornings), a combi will often struggle. A system boiler with a correctly sized cylinder is usually the right answer.
4. What is your mains water pressure like?
- Good mains pressure: Combi or system will work well
- Poor mains pressure: Regular boiler may be needed, or a pressurisation solution may be possible
5. Do you have space for a cylinder?
- No space: Combi is your only realistic option
- Have space: A system boiler opens up options for higher demand
6. Do you want backup hot water if the boiler fails?
- Yes: A system or regular boiler with a cylinder can include an immersion heater as backup
- No: A combi is fine (but if it fails, you lose hot water completely)
7. What is your existing system?
- Replacing a combi: Usually stick with combi (unless your needs have changed)
- Replacing a regular: Consider upgrading to system (removes loft tank, improves pressure)
- Replacing a system: Usually stick with system
London-Specific Considerations
Installing boilers in London properties comes with particular challenges:
Flats and apartments
Most London flats have limited space. A combi boiler is usually the only practical option because there is nowhere to put a cylinder. The good news is that flats typically have one bathroom, so a combi’s limitations are rarely a problem.
Victorian and Edwardian houses
Many period properties in London were built with gravity-fed systems and still have regular boilers. When replacing, you have a choice:
- Like-for-like replacement: New regular boiler, keep existing pipework
- Upgrade to system: Remove loft tank, keep or upgrade the cylinder, better pressure
- Convert to combi: Remove tank and cylinder, only if demand is low enough
Converting a Victorian house to combi can work for smaller households, but larger families often regret losing the cylinder capacity.
New builds
Modern new-build homes are usually designed for either combi (smaller units) or system (larger homes). The developer will have specified the appropriate type.
Water pressure
London generally has decent mains pressure, but it varies by area and building height. Top-floor flats in tall buildings can sometimes have lower pressure, which affects combi performance.
What About Electric Boilers?
With policy moving towards lower-carbon heating over time, electric heating is becoming more relevant. Here is how electric boilers compare:
Electric boiler basics
- Heat water using electricity rather than gas
- No flue needed (no combustion gases)
- Available in combi, system, and direct styles
- Very high efficiency at point of use
- No flue or combustion (so the electric boiler itself does not require Gas Safe), but any gas disconnection must be done by a Gas Safe engineer and electrics should be done by a qualified electrician
When electric makes sense
- Properties with no gas connection
- Flats where flue installation is problematic
- Small properties with low heating demand
- Supplementing heat pump systems
- Future-proofing where gas use may reduce over time
Considerations
- Electricity typically costs more than gas per kWh
- Running costs are often higher than gas for the same heat output
- You may need an electrical supply upgrade for larger units
- Output can be more limited compared to gas boilers in larger homes
We service and repair electric boilers across London. See our electric boiler services page for details.
Switching Boiler Types
Changing from one boiler type to another is possible but involves additional work.
Regular to System
What is involved: Remove loft tank, potentially upgrade cylinder, modify pipework, and recommission the system.
Worth it? Often yes. Removes freezing risk, improves pressure, simplifies the system.
Regular or System to Combi
What is involved: Remove tank, remove cylinder, cap old pipework, and often upgrade gas supply depending on boiler size.
Worth it? Only if your hot water demand is low enough. Think carefully before losing cylinder capacity.
Combi to System
What is involved: Install a cylinder, run new pipework, and often add electrical for immersion backup.
Worth it? If your household has grown or you have added a bathroom, this can be the right move.
Any boiler type conversion should be assessed by a qualified engineer who can survey your property and advise on the best approach.
Frequently Asked Questions
Which boiler type is most efficient?
All modern gas boilers are condensing boilers with similar efficiency ratings (often around 90-94%). Combis can be slightly more efficient in practice because there are no cylinder standing losses. However, the difference is usually small compared to correct sizing, good controls, and a well-designed system.
Can a combi boiler run two showers at once?
Generally, no. A standard combi can typically deliver around 10-15 litres per minute of hot water. A good shower can use 8-12 litres per minute. Two showers at once usually means reduced flow and temperature to both. If you need simultaneous showers, a system boiler is usually the better choice.
Why does my combi take ages to heat up?
When you first turn on a hot tap, the combi needs to fire up and heat cold water sitting in the pipes. This dead leg delay is normal. If the delay is unusually long or performance is inconsistent, it could indicate a fault such as a diverter valve or heat exchanger issue and should be checked by an engineer.
My house has a regular boiler. Should I change it?
Not necessarily. If the system works well for your needs, a like-for-like replacement is often the most practical option. Upgrading to a system boiler removes the loft tank and can improve pressure, but it involves additional work. A survey will confirm what makes sense for your property.
Is a bigger boiler always better?
No. An oversized boiler can short cycle (turn on and off frequently), which reduces efficiency and increases wear. The boiler should be sized to your property’s heat loss and hot water demand. A proper survey and heat loss calculation is the right approach.
Can I add a second bathroom with my existing combi?
You can add the bathroom, but the combi may struggle to supply both bathrooms well at the same time. Depending on your usage, a larger combi may help, but a switch to a system boiler is often the long-term solution. An engineer can assess your specific situation.
Do I need a pump with a system boiler?
System boilers typically have the circulation pump built in. You might choose a shower pump for a specific shower setup, but basic heating and hot water distribution normally does not require a separate pump.
What size cylinder do I need with a system boiler?
As a rough guide: 150-180 litres for 2-3 bedrooms with one bathroom, 200-250 litres for 3-4 bedrooms with two bathrooms, and 250+ litres for larger properties. Your installer should size the cylinder based on peak demand and recovery rate, not just litres.
Not Sure Which Boiler Type You Need?
Our Gas Safe registered engineers can survey your property, assess your hot water demand, and recommend the right boiler type and size. No pressure, honest advice.