Combi, System or Regular: Which Boiler Suits Your House
Most people choosing a boiler start with the price list. The property should decide it, and two things about the property decide almost all of it: how many bathrooms get used at the same time, and how much water your incoming mains can actually deliver.
Get those two answers and the choice usually makes itself.
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The three types
Combi
No cylinder and no tanks. The boiler heats water on demand as it passes through, so hot water is effectively unlimited in duration but limited in rate.
Suits: most one and two bathroom homes where two showers are not used simultaneously, and properties with a decent incoming mains supply.
Advantages: compact, no cylinder taking up a cupboard, no loft tanks, and no stored water sitting hot waiting to be used.
The limitation, and it is the whole story: a combi can only heat water as fast as your mains delivers it. Open two outlets and the flow divides between them. A bigger combi does not fix a weak mains supply, because the constraint is the water arriving rather than the heat available.
System
Works with a hot water cylinder, usually an unvented one, which stores heated water at mains pressure. No loft tanks.
Suits: homes with two or more bathrooms in genuine simultaneous use, and households where several people shower in the same half hour.
Advantages: simultaneous demand is not a problem, because stored water does not care how many outlets are open. Good pressure at every tap.
The trade-offs: a cylinder needs space, and stored water eventually runs out and needs time to reheat. Sizing matters, and so does reheat rate.
Regular, sometimes called heat only
The traditional arrangement: a cylinder plus one or two tanks in the loft, with hot water pressure coming from the height difference between tank and tap.
Suits: properties where the existing pipework would not take the higher pressure of a sealed system, and situations where a straight like-for-like replacement is the right call.
The limitation: gravity gives low pressure by design. That is why showers on these systems are frequently disappointing, and worse the higher up the house you go. It is not a fault developing, it is inherent to the arrangement.
The two questions that decide it
How many bathrooms, used how?
Not how many bathrooms exist. How many get used at once.
Two bathrooms where one is a rarely used en-suite behaves like one bathroom. Two bathrooms in a household of four teenagers behaves like considerably more than two.
- One bathroom, or two never used together: a combi is usually right.
- Two or more in genuine simultaneous use: a system boiler and a properly sized cylinder is usually right, regardless of how large a combi you could buy.
What is your mains flow rate?
This is the number almost nobody is given and it decides everything on a combi.
It is measured in litres per minute at the kitchen cold tap, and it is not the same as pressure. You can have good pressure and poor flow, which is exactly what a narrow or partly blocked supply pipe produces.
Any installer recommending a combi without measuring it is guessing
If your mains delivers modestly, fitting a high output combi buys you output you cannot use.
The conversions people ask about
- Regular to combi. Popular, because it frees the airing cupboard and the loft. Only sensible if the mains flow supports it and the bathroom count suits it. Involves removing tanks and altering pipework, so it is a bigger job than a straight swap.
- Regular to system with an unvented cylinder. Frequently the better answer in a house with more than one bathroom. You keep stored hot water but gain mains pressure at every outlet, which usually transforms the showers. See our unvented cylinder page.
- Combi to system. Less common, and the right move where a household has outgrown its combi, typically after adding a bathroom.
Two things that apply whichever you choose
- Radiator sizing. Part L now requires new and fully replaced wet heating systems to be designed for a maximum flow temperature of 55°C rather than the higher temperatures older systems ran at. A radiator delivers less heat at a lower temperature, so radiators sized decades ago are frequently undersized now. See our radiator page.
- Hard water. In this area, a scale reducer on the cold feed belongs on every installation, and several manufacturers require one for the warranty to stand. See our hard water guide.
Frequently asked questions
Is a combi always cheaper?
The appliance often is. The installation may not be, particularly if you are converting from a cylinder system and removing tanks. And it is the wrong choice if your bathroom count or mains flow does not suit it.
Will a bigger combi give me a better shower?
Only if the mains can supply the extra. If the incoming main is the constraint, a bigger boiler changes nothing.
Can I keep my cylinder and just change the boiler?
Yes, that is a system or regular replacement. Whether the cylinder is worth keeping depends on its age and condition.
What is the difference between a system and a regular boiler?
A system boiler works with a sealed, mains pressure cylinder and no loft tanks. A regular boiler works with a vented cylinder fed by tanks in the loft, giving low pressure.
How do I know which I have now?
No cylinder anywhere means a combi. A cylinder plus tanks in the loft means regular. A cylinder with no loft tanks means a system with an unvented cylinder.
Does the size of my house decide it?
Less than people expect. Bathroom count and simultaneous use decide it, along with your mains flow rate.
What to do about it
Get the flow rate measured
Call 07514 312117 or send a WhatsApp with a photo of your existing boiler and cylinder if you have one. We measure before recommending anything. Gas Safe registered 981362, CIPHE member, fully insured.