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Published by Mo's Plumbing and Heating
What Is a System Boiler?
A system boiler heats a cylinder of hot water and stores it, rather than heating water on demand as you draw it. That one difference explains everything else about it: why it needs a cupboard, why it copes with two showers at once, and why it is the right answer in some homes and the wrong one in others.
The Short Answer
A system boiler heats your radiators and heats a separate hot water cylinder, which stores hot water ready to use. A combi does away with the cylinder and heats water instantly as it flows through. A heat only boiler, sometimes called regular or conventional, is the older arrangement with a cylinder and usually tanks in the loft as well. System boilers suit homes that draw a lot of hot water at once; combis suit homes that draw it in short bursts and want the cupboard back.
What a Boiler Actually Does
Worth starting here, because the differences between types make much more sense once the common part is clear. Every gas boiler is doing the same basic thing: burning gas to heat water, and moving that hot water where it is needed.
Gas burns at a burner. The heat passes into a heat exchanger, which is a set of passages carrying water past the hot gases so the heat transfers into the water without the two ever mixing. A pump pushes that heated water round the circuit. The waste gases leave through the flue, which is the pipe or terminal you see on an outside wall.
The heating side is the same on every type: hot water is pumped round the radiators, gives up its heat into the rooms, and returns cooler to be heated again. That loop is sealed and it is the same loop whatever badge is on the boiler.
The hot water side is where the types diverge, and it comes down to one question: do you heat water in advance and store it, or heat it at the moment you turn the tap on. Everything else follows from that answer.
How a System Boiler Differs
A system boiler has most of its components built into the unit, including the pump and the expansion vessel, which is why it is called a system boiler: much of what used to hang around the boiler is inside it.
It heats two things. The radiator circuit, directly. And a hot water cylinder, usually in an airing cupboard, by pumping hot water through a coil inside that cylinder so the stored water heats up without mixing with the heating water.
That cylinder is the defining feature. It means hot water is ready and waiting rather than made on demand, so you can run two showers at once without either one weakening, and the flow rate at the tap depends on your incoming supply rather than on how fast the boiler can heat water passing through it.
It also means a finite amount. A cylinder holds what it holds, and once it is drawn down it needs time to reheat. Sizing the cylinder to the household matters as much as sizing the boiler.
Unlike an older heat only system, a system boiler works on a sealed pressurised circuit, so there are no tanks in the loft. That is a genuine advantage in a top floor flat: nothing to freeze, nothing to overflow, and a loft you can actually use.
System, Combi or Heat Only
The short version of the decision, with the detail on each type on its own page.
- Combi: no cylinder, no tanks, hot water on demand. Best for smaller homes and for households drawing hot water in short bursts. Frees up a cupboard. Struggles when two outlets are used at once, and its hot water performance is capped by the incoming mains flow rate.
- System: cylinder, no loft tanks, sealed and pressurised. Best where several people use hot water at the same time, or where the mains flow rate is too modest for a combi to satisfy. Costs you the cupboard the cylinder occupies.
- Heat only, also called regular or conventional: cylinder plus a cold water tank and usually a small feed and expansion tank in the loft. The traditional arrangement, still very common in older property. Most often retained when replacing like for like in a house where changing the system type would be a large job.
Which One Suits Your Home
The honest answer is that it depends on three things, and none of them is the boiler.
How many people draw hot water at the same time. One or two people in a one-bathroom flat are well served by a combi. A household where two showers run on a weekday morning is not, and no amount of paying for a bigger combi fixes that, because a combi produces one good hot outlet at a time by design.
What your incoming mains delivers. A combi can only heat water as fast as it arrives, so a modest flow rate caps its performance permanently. A cylinder system fills slowly and delivers quickly, which is why it is often the answer where the supply is limited.
And whether you can spare the space. A cylinder needs a cupboard, and in a flat that cupboard is frequently the only real storage there is. That is a genuine cost rather than a technicality, and it is the reason plenty of people choose a combi knowing the trade-off.
There is no universally better type, and anyone telling you otherwise is selling one. The right question is which trade-off suits your household.
What Goes Wrong With Each
Useful to know before choosing, because the failure modes differ and one of them may matter more to you than the other.
On a combi, a boiler fault takes heating and hot water together, and there is no backup. You are without both until it is fixed.
On a system or heat only setup, the cylinder usually has an immersion heater, an electric element that can heat the stored water independently. So a boiler fault leaves you cold but often still able to get a bath. In a Scottish January that is not a small consideration.
Cylinders have their own failure modes: a failed immersion, a leaking cylinder, or a coil that has furred up so the boiler heats it slowly. And systems with loft tanks add the possibility of a tank overflowing or freezing, which is one of the arguments for a sealed system boiler over an older heat only arrangement.
Worth knowing where your immersion switch is before you need it, since it is often in the airing cupboard or on a wall nearby rather than anywhere obvious, and it is frequently on a separate circuit. Finding it during a breakdown in January is a worse time to look than finding it now.
Where a Cylinder Still Wins in Glasgow
The general trend has been towards combis, and in most of this city's one and two bedroom flats that is the right call. There is a specific and sizeable set of Glasgow properties where it is not, and know whether yours is one before defaulting to a combi on a replacement.
The clearest case is the subdivided townhouse and the larger converted flat with more than one bathroom. Big West End and Southside properties have been carved into generous flats and duplexes over the years, and a good number ended up with two bathrooms or a bathroom plus an en suite. Those households genuinely do run two outlets at once on a weekday morning, and that is the situation a combi handles worst and a cylinder handles best.
Shared and multiple-occupancy housing is the same argument with more force. Where several unrelated people are getting ready at the same time on the same supply, stored hot water is not a luxury, it is what makes the property work.
The other factor is the incoming supply itself. In an upper floor tenement flat fed through older pipework and a shared riser, the flow rate arriving at the property can be modest, and a combi's hot water performance is capped by exactly that figure. A cylinder sidesteps the problem, because it fills slowly over time and then delivers at whatever rate the outlet allows. That is why a property measuring poorly on flow is often better served by keeping a cylinder than by fitting the biggest combi that will fit the wall.
When to Call Someone Out
Worth a conversation before deciding a type rather than after, because the choice comes from the number of simultaneous outlets, the measured mains flow rate and whether you can spare a cupboard, and the first two want checking on site. The survey is free on installations and replacements and includes both measurements. Mo and Muhammad are Gas Safe registered for domestic work, and if a combi would disappoint you in your property we will say so rather than fit one, since that is a complaint waiting to happen rather than a sale worth making.
Boiler Installation in GlasgowCommon Questions
A system boiler heats a stored cylinder of hot water; a combi heats water instantly as you draw it and has no cylinder. That single difference drives everything else. The combi frees a cupboard and gives unlimited hot water in short bursts; the system copes with several outlets at once and is not limited by how fast water arrives at the property.
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How Does a Combi Boiler Work?
A combi heats your radiators, and when you open a hot tap it stops heating the radiators and diverts all its output into heating the water flowing through to that tap instead.
What Is a Condensing Boiler?
A condensing boiler extracts extra heat from its own exhaust gases by cooling them until the water vapour in them turns back into liquid.
What Is a Heat Only Boiler?
A heat only boiler heats water and nothing else.
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