Combi vs system boiler is the decision that shapes everything else about a heating installation, and it gets made badly more often than any other choice in the process. The default answer in the trade is a combi, because it is cheaper to fit and suits most houses. But “most” is not “all”, and getting it wrong produces a house where two showers cannot run at once and nobody can work out why. Here is how to tell which one your property actually needs.
How They Differ
Combi boiler
A combination boiler heats water on demand. Open a tap and it fires, heating water as it passes through. There is no cylinder and no cold water storage tank — the unit on the wall is the entire system.
Advantages: no cylinder taking up an airing cupboard, no tanks in the loft, hot water at mains pressure, nothing kept hot that nobody is using, and the cheapest installation of the three types.
The constraint: flow rate is finite and shared. A combi delivers a certain number of litres per minute, and running two outlets at once splits it between them.
System boiler
A system boiler heats a cylinder of stored water. The pump and expansion vessel are built into the boiler, so no loft tanks are needed, but a cylinder is — usually in an airing cupboard.
Advantages: multiple outlets can run simultaneously at full pressure, recovery is fast, and it copes with genuinely high demand without complaint.
The constraints: it needs space for the cylinder, it costs more to install, and stored water eventually runs out and needs reheating.
Regular (heat only) boiler
Worth mentioning for completeness. A regular boiler needs both a cylinder and cold water tanks in the loft, and is the oldest of the three arrangements. Most Doncaster homes still on a regular system are candidates for conversion when the boiler is replaced.
The Question That Actually Decides It
Not house size. Not bedroom count. Not budget. The question is: how many hot outlets will realistically run at the same time?
One bathroom, one household, showers taken sequentially — a combi is the right answer, almost without exception.
Two bathrooms with a family who genuinely use them simultaneously on a weekday morning — a system boiler with an unvented cylinder is the right answer, even though it costs more.
Everything else is a judgement call, and it is made on the numbers rather than by preference. Which brings us to the measurement that most quotes skip.
Flow Rate — the Test Nobody Runs
A combi is only as good as the water arriving at your property. It cannot deliver more than the incoming mains supplies, regardless of the boiler’s rating on paper.
The test takes two minutes: run the cold kitchen tap flat out into a measuring jug for exactly ten seconds, then multiply by six. That gives litres per minute.
- Below 10 litres per minute — a combi will disappoint. A system boiler is the sensible route.
- 10 to 15 litres per minute — a combi works for one bathroom, and will feel weak with two.
- Above 15 litres per minute — good supply, a combi will perform properly.
This matters locally. Some of the older streets in Hexthorpe, Balby and central Conisbrough have original supply pipework that simply cannot feed a large combi, and no boiler specification overcomes it. Fitting one anyway produces a disappointed customer and a boiler that short cycles.
Any installer quoting a combi without measuring flow rate is guessing. It is a fair question to ask before you accept a quote.
Which Suits Typical Doncaster Properties
The borough’s housing splits fairly cleanly.
Two and three bedroom terraces and semis, one bathroom — the bulk of Doncaster housing, from the colliery terraces in Bentley to the estate housing in Armthorpe and Intake. A combi, on all but the poorest supplies. Freeing up the airing cupboard and losing the loft tanks is a genuine gain in properties where space is tight.
Larger detached and older properties with two or more bathrooms — Sprotbrough, Cusworth, Bawtry, parts of Tickhill. A system boiler with an unvented cylinder, in most cases. The extra cost buys hot water that does not degrade when a second tap opens.
Older high-ceilinged stock — the Georgian and Victorian brick around Thorne Market Place and central Doncaster. Either can work, but the heat loss calculation matters more here than the boiler type, because room volume rather than floor area drives output.
Let properties — a combi, usually. Fewer components, no cylinder to fail, and less to go wrong between tenancies. Landlords across Mexborough and the Hyde Park streets tend to favour them for exactly that reason.
Cost Comparison
Fitted and commissioned in Doncaster, old unit removed:
- Like-for-like combi swap — £1,800 to £2,800
- System boiler replacement, cylinder retained — £2,200 to £3,400
- Conventional to combi conversion — £2,800 to £4,200
- Conversion to system boiler with new unvented cylinder — £3,500 to £5,000
The gap between a combi and a system installation is real but not decisive. Over a fifteen-year life, £1,000 of extra installation cost is worth paying if it means the heating actually meets the household’s demand. It is a bad saving if it does not.
Five Myths Worth Clearing Up
“Combis are less efficient.” No. Both are condensing boilers at similar efficiency ratings. The practical difference is that a combi does not reheat stored water nobody uses, which is a genuine saving on a low-demand household.
“A bigger combi solves poor pressure.” No. Output does not change what the incoming mains delivers. Flow rate is the ceiling.
“You cannot have a shower pump with a combi.” Correct, actually — and it catches people out. Combis run at mains pressure and cannot be pumped. If you have a pumped shower you love, converting to a combi changes it.
“System boilers need loft tanks.” No, that is a regular boiler. A system boiler runs sealed, with the pump and expansion vessel inside the unit.
“You have to keep your existing type.” No. Conversions in either direction are routine. The right question is what suits the property now, not what was fitted in 1994.
Making the Decision
Work through it in this order:
- Count the hot outlets that will realistically run simultaneously
- Measure the flow rate at the cold kitchen tap
- Check whether there is space for a cylinder, and whether you want that space back
- Have a proper heat loss calculation done for the property
- Compare both quotes with the same six line items itemised
If steps one and two point the same way, the decision is made. Where they conflict — high demand on a weak supply — a system boiler with an unvented cylinder usually resolves it, and that is exactly the case where a cheap combi quote will let you down.
Independent guidance on boiler types and efficiency is available from the Heating and Hotwater Industry Council, which is worth reading alongside any installer’s recommendation.
Getting It Right First Time
The combi vs system boiler question is the one part of a heating installation that is expensive to reverse. Everything else can be adjusted later; the boiler type cannot, not without paying twice.
Our surveys include a flow rate measurement and a room-by-room heat loss calculation as standard, across Doncaster, Thorne, Conisbrough and the surrounding villages. If a combi is not right for your property we will tell you that rather than fit one anyway — see our combi boiler installation page for the full detail on how that assessment works.