Every gas boiler heats radiators in much the same way. Where the three types part company is hot water: a combi makes it instantly and only while a tap is open, a system boiler heats a stored cylinder under mains pressure, and a regular boiler heats a cylinder fed from tanks in the loft.
That single difference decides how many showers can run at once, how much cupboard and loft space you keep, and how much of your existing pipework survives the change.
The three types side by side
Combi
One appliance, no cylinder, no tanks. Hot water is made on demand from the incoming main.
- Best for flats and houses with one bathroom, or two used at different times
- Frees up the airing cupboard and loft entirely
- Hot water performance is capped by your incoming mains flow rate
- Two showers at once will usually feel weak
System
Boiler plus an unvented cylinder. Pump and expansion are built into the appliance.
- Best for two or more bathrooms, or a household with simultaneous demand
- Mains-pressure hot water at every outlet, no loft tanks
- Needs cupboard space for the cylinder and its safety valves
- Cylinder needs an annual check on the expansion and discharge arrangement
Regular (heat only)
Boiler, vented cylinder and cold feed and vent tanks in the loft.
- Often the pragmatic keep-what-works choice in older, larger properties
- Gravity-fed hot water, so shower pressure usually needs a pump
- Tolerant of very poor incoming mains flow
- Loft tanks take space and can freeze in an uninsulated roof
The mains flow test that settles most arguments
Before anyone recommends a combi, the incoming cold main should be measured with a flow cup — litres per minute at the kitchen tap, plus static and working pressure. A combi that is rated for a strong shower needs the water to arrive in the first place; the appliance cannot invent flow that the street does not supply.
Where flow comes back weak, you have three honest options: keep stored hot water, upgrade the incoming supply where that is feasible, or accept a slower fill and a modest shower. Choosing a large combi and hoping is the one option that reliably disappoints.
How to work through the decision
- 1
Count simultaneous demand, not bathrooms
Two bathrooms used at 7am and 7pm behave like one. Two used at the same time behave like four taps.
- 2
Measure the incoming main
Flow rate and working pressure at the kitchen tap set the ceiling for any combi.
- 3
Decide what space is worth to you
A combi hands back a cupboard and a loft. For some households that is the whole point of the job.
- 4
Check what the change costs in pipework
Combi conversions mean decommissioning tanks, capping feeds and making good. System conversions mean a new cylinder and its safety pipework.
- 5
Then choose the appliance
Brand and model come last, once the type and output are settled by the house.
Questions people ask
Can a combi run two showers at once?
Realistically no. A combi splits one stream of heated water between open outlets, so both showers drop in flow. Households that genuinely shower simultaneously are better served by a stored cylinder.
Do I lose the airing cupboard with a system boiler?
No — you keep it, because that is usually where the cylinder goes. You do lose the loft tanks, since an unvented cylinder runs directly off the main.
Is a regular boiler outdated?
The type is not obsolete. In a large older house with poor mains flow and existing gravity pipework, replacing a regular boiler with another one is often the most cost-effective and least disruptive choice.
Which type is cheapest to run?
Running cost depends far more on controls, insulation and how well the system is balanced than on the type. A combi avoids cylinder standing losses; a well-insulated modern cylinder loses very little.
If you take one thing from this: let the water main and the pattern of household demand pick the type, then pick the appliance. Doing it the other way round is how people end up with a boiler that is technically excellent and daily frustrating.