Combi vs System Boiler: Which Is Right for Your Home?
TLDR: Combi boilers provide instant hot water without a tank and suit smaller homes (1-2 bathrooms). System boilers store hot water in a cylinder and deliver high flow to multiple outlets simultaneously. Combis cost less upfront and save space; systems handle higher demand better. The right choice depends on your home size, number of bathrooms, and hot water usage patterns.
The Boiler Decision Every Homeowner Faces
Your boiler's given up the ghost. The heating engineer's just shaken their head and told you the compressor's gone—or the heat exchanger's corroded—or whatever mechanical failure has finally ended its life. Now you need a new one, and suddenly you're faced with a decision that feels more complicated than it should be.
Should you stick with a combi? Switch to a system boiler? What even is the difference, really?
This is one of the most common questions British homeowners face. Get it right, and you'll have reliable hot water and efficient heating for 15-20 years. Get it wrong, and you'll either run out of hot water when you need it most or pay for capacity you'll never use.
This guide explains exactly how each boiler type works, what they cost, and how to choose the right one for your home. We've also included real experiences from homeowners across Britain who've made this decision themselves.
How Each Type Works
Combi Boilers
A combi (combination) boiler provides both central heating and hot water from a single, compact unit. There's no separate hot water cylinder or cold water tank—everything happens inside the boiler itself.
When you turn on a hot tap, cold water from the mains passes through a heat exchanger inside the boiler. The boiler fires up, heats the water almost instantly, and delivers it to your tap. When you turn the tap off, the boiler stops heating.
Key characteristics:
- Compact size: Fits in a kitchen cupboard or utility room. No need for cylinder or tanks.
- Instant hot water: No waiting for a tank to heat up. Hot water available whenever you need it.
- Mains pressure delivery: Hot water comes at mains pressure, providing good shower performance without a pump.
- One outlet at a time: Flow rate is limited by the boiler's capacity. Running multiple hot taps simultaneously reduces flow to each.
- No stored hot water: If the boiler fails, you have no backup hot water.
Combis account for roughly 70% of all new boiler installations in the UK. They're the default choice for most homes.
System Boilers
A system boiler heats water and stores it in a separate insulated cylinder, usually located in an airing cupboard. When you turn on a hot tap, pre-heated water flows from the cylinder at high pressure and high flow rate.
The cylinder refills with cold water as you use hot water, and the boiler works to heat the new water back to temperature. Modern cylinders are well-insulated and lose very little heat when standing.
Key characteristics:
- Requires a cylinder: Usually 150-250 litres, located in an airing cupboard or utility space.
- High flow rate: Can deliver 15-25+ litres per minute to multiple outlets simultaneously.
- Multiple showers at once: Two or three bathrooms can run simultaneously without significant flow reduction.
- Stored hot water reserve: Even if the boiler fails, the cylinder retains hot water for a while.
- Compatible with renewable energy: Cylinders work with solar thermal panels and heat pumps for future-proofing.
- Limited by cylinder size: Very long showers or back-to-back baths may exhaust the hot water supply, requiring wait time to reheat.
System boilers are the preferred choice for larger homes and families with high hot water demand.
What About Regular (Conventional) Boilers?
Regular boilers (also called conventional or heat-only boilers) require both a hot water cylinder AND a cold water storage tank in the loft. The cold water tank feeds the cylinder by gravity, and the boiler heats water as needed.
These are older technology and are being phased out in favour of system boilers. The main difference: system boilers take water directly from the mains (eliminating the need for a loft tank), while regular boilers rely on gravity-fed cold water.
If you currently have a regular boiler, you can usually upgrade to a system boiler relatively easily. The installer removes the loft tank, connects the cylinder to the mains, and fits a new system boiler. This is a common job that typically costs £3,000-£4,500.
Real Homeowner Experiences
Numbers and specifications only tell part of the story. Here's what British homeowners say about their boiler choices.
Sandra and Tom, Leicester (3-bed semi, 1 bathroom): "We went with a combi when we replaced our ancient back boiler five years ago. Best decision we made. The house is just the two of us most of the time, and we've never once run out of hot water. The Vaillant cost £2,400 fitted, and it sits in the kitchen cupboard where the old airing cupboard used to be. We gained a whole cupboard's worth of storage. The only time I notice any issue is when my daughter visits with her partner—if two people try to shower at the same time, the second person gets a bit of a trickle. But that's maybe four times a year."
James and Rachel, Bristol (4-bed detached, 3 bathrooms): "We made the mistake of fitting a combi in our last house, which had two bathrooms. It was constantly frustrating—someone would be in the shower and another person would run the kitchen tap, and the shower would go cold. When we moved here, we insisted on a system boiler. We've got a 250-litre cylinder in what was the airing cupboard. Three teenagers and two adults, three bathrooms, and we've never had a problem. The cylinder cost us extra—about £4,200 all in compared to maybe £2,800 for a combi—but worth every penny."
Margaret, Edinburgh (2-bed bungalow): "I'm on my own now, and my plumber said a combi would be perfect. He was right. I don't need a big hot water tank taking up space, and the water's always hot when I need it. My gas bills are actually lower than they were with the old system boiler because I'm not heating water that just sits in a tank. I paid £2,100 for a Worcester Bosch combi, fitted, and it's been completely reliable for three years now."
David and Claire, Manchester (5-bed Victorian terrace, 2.5 bathrooms): "We'd never have considered a combi for this house. There's six of us living here—four kids—and the morning rush for the bathroom is mental enough without worrying about hot water running out. Our system boiler with a 300-litre cylinder handles everything. We can have two showers running and someone filling the bath, and there's still plenty of hot water. The cylinder's in the cellar, so it doesn't even take up living space. Cost was higher—about £4,800 total—but there was no other option really."
Detailed Cost Comparison
Here's what you'll actually pay for each boiler type in 2025:
| Boiler Type | Boiler Cost | Installation | Total Typical Cost |
|---|---|---|---|
| Combi (budget) | £700-£1,000 | £800-£1,200 | £1,500-£2,200 |
| Combi (mid-range) | £1,000-£1,500 | £1,000-£1,500 | £2,000-£3,000 |
| Combi (premium) | £1,500-£2,500 | £1,000-£1,500 | £2,500-£4,000 |
| System (mid-range) + cylinder | £1,200-£1,800 | £1,500-£2,500 | £2,700-£4,300 |
| System (premium) + cylinder | £1,800-£2,500 | £1,800-£2,800 | £3,600-£5,300 |
Why system boilers cost more:
- The cylinder adds £400-£800 to the equipment cost
- Additional pipework and fittings required
- Installation takes longer (often a full day vs. half a day)
- Cylinder installation and mounting adds labour time
Brand considerations:
Worcester Bosch, Vaillant, and Viessmann are considered premium brands with excellent reliability and warranty coverage. Ideal, Baxi, and Glow-worm offer good mid-range options. Budget brands may save money upfront but often have shorter warranties and potentially higher repair costs.
Hot Water Performance Comparison
This is where the real difference shows. Understanding flow rates helps you choose correctly.
Combi Boiler Flow Rates
Combi boilers are rated by their hot water output in litres per minute (l/min). A 24kW combi typically delivers 9-10 l/min; a 30kW combi delivers 12-14 l/min; a 40kW combi delivers 15-17 l/min.
What do these numbers mean in practice?
- A comfortable shower needs 8-10 l/min minimum
- A power shower or rainfall head may need 12-15 l/min
- Running two taps simultaneously halves the flow to each
- A 24kW combi can run one good shower OR fill a bath—not both comfortably at once
If two people try to shower at the same time with a typical combi, each shower gets maybe 5-6 l/min—which feels distinctly underwhelming.
System Boiler Flow Rates
System boilers draw pre-heated water from the cylinder at mains pressure. A well-designed system with a large unvented cylinder can deliver 20-25+ l/min.
What does this mean in practice?
- Two showers running simultaneously still get 10-12 l/min each
- You can fill a bath while someone showers without noticeable impact
- Kitchen taps, dishwashers, and bathrooms can all run at once
- The limitation is cylinder capacity, not flow rate
A 200-litre cylinder provides roughly 200 litres of usable hot water before you need to wait for reheating. For context, a typical shower uses 50-80 litres; a bath uses 80-100 litres.
Which Suits Your Home?
Choose a Combi Boiler If:
- You have 1-2 bathrooms and rarely need to use both simultaneously
- You have limited space and can't accommodate a hot water cylinder
- You live alone or as a couple with modest hot water needs
- You're in a flat or small house where space is at a premium
- You're replacing an existing combi and it's worked well for you
- You want lower upfront costs and simpler installation
- You want instant hot water without waiting for a cylinder to heat
Choose a System Boiler If:
- You have 3+ bathrooms or en-suites
- Multiple people shower in the morning rush at overlapping times
- You have a large family with high hot water demand
- You want to integrate solar thermal panels (now or in future)
- You're planning a future heat pump installation (heat pumps need cylinders)
- You have space for a cylinder in an airing cupboard, utility room, or cellar
- You have mains water pressure issues and want to use a pressurised unvented cylinder
Future-Proofing: Heat Pumps and System Boilers
This is worth considering if you're thinking about the next 10-15 years. The UK government plans to phase out gas boilers in new builds from 2025, and there's increasing pressure on existing homes to transition to heat pumps.
Heat pumps work at lower temperatures than boilers and are most efficient when paired with a hot water cylinder. If you currently have a combi boiler and want to install a heat pump in future, you'll need to add a cylinder—which means finding space and running new pipework.
If you install a system boiler now, the cylinder is already in place. The heat pump simply replaces the boiler and connects to the existing cylinder. The transition is simpler and cheaper.
This doesn't mean everyone should choose a system boiler for future-proofing—if you're in a flat with no space for a cylinder, a combi makes sense regardless. But if you have space and are thinking long-term, a system boiler setup is more adaptable.
Running Cost Comparison
There's a common misconception that combis are significantly cheaper to run because they don't have standing heat losses from a cylinder. This is partly true but often overstated.
Combi advantages:
- No cylinder means no standing losses (though modern cylinders lose only 1-2 kWh per day)
- Only heat water when you need it
- Simpler system means fewer potential faults
System boiler advantages:
- Can heat water during off-peak electricity hours (if using immersion backup)
- Works with solar thermal to preheat water for free
- Larger, potentially more efficient heat exchanger
- Cylinder retains heat even if boiler fails
In practice, the running cost difference between a modern combi and a system boiler with a well-insulated cylinder is minimal—perhaps £20-£40 per year. Choose based on hot water needs, not running costs.
Frequently Asked Questions
Q: Can I upgrade from a combi to a system boiler?
A: Yes, but it's a bigger job than a like-for-like replacement. You'll need space for a cylinder (typically 150-250 litres), new pipework to connect the cylinder, and potentially electrical work for an unvented cylinder. Budget £3,500-£5,500 for a full conversion. It usually takes 2-3 days.
Q: What's the difference between a vented and unvented cylinder?
A: A vented cylinder relies on a cold water tank in the loft and delivers water at lower pressure. An unvented cylinder connects directly to the mains and delivers water at mains pressure—better for showers. Unvented cylinders cost more and require annual servicing but offer superior performance.
Q: How long do boilers last?
A: Both combi and system boilers typically last 12-15 years with regular servicing. Premium brands may last longer. Hot water cylinders often last 20-25 years as they have no moving parts.
Q: What size combi do I need?
A: For a 1-2 bathroom home, 24-28kW is usually sufficient. For homes with higher demand or larger bathrooms, 30-35kW provides better flow. Very large combis (40kW+) are rarely necessary for domestic use.
Q: What size cylinder do I need?
A: As a rough guide: 150 litres for 1-2 people, 200 litres for 3-4 people, 250-300 litres for 5+ people or multiple bathrooms with high usage. Bigger is generally better if you have space.
Q: Which boiler brands are best?
A: Worcester Bosch, Vaillant, and Viessmann consistently rank highest for reliability and customer satisfaction. All three offer 7-10 year warranties when installed by accredited engineers. Ideal and Baxi are solid mid-range options. Avoid very cheap brands—the savings rarely justify the shorter lifespan and warranty.
The Bottom Line
Most UK homes—probably 60-70%—are well-served by combi boilers. They're compact, efficient, and handle typical household hot water needs without problems. If you have one or two bathrooms and don't regularly need multiple hot outlets running simultaneously, a combi is almost certainly the right choice.
But for larger homes with three or more bathrooms, families with multiple children, or anyone who's experienced the frustration of inadequate hot water flow, a system boiler with a properly-sized cylinder is worth the extra investment. The difference in daily comfort is substantial.
When making your decision, be honest about your actual usage patterns. It's easy to underestimate how often you really do need hot water from multiple outlets at once. Ask any parent with teenagers—the morning shower battle is real, and running out of hot water makes everyone's day worse.
And if you're thinking about future heat pumps or solar thermal, a system boiler now sets you up for an easier transition later. The cylinder is the expensive and disruptive part—once it's installed, swapping the boiler for a heat pump is relatively straightforward.