Combi vs system vs regular boiler: which do you need?
A combi boiler heats water on demand straight from the mains and suits most homes with one bathroom. A system boiler stores hot water in a cylinder, which suits homes with two or more bathrooms. A regular boiler also uses a cylinder, plus tanks in the loft, and suits older homes or places with low mains water pressure.
That’s the short version. The right choice for your house comes down to how many people live there, how many showers run at once, and what your mains water does when it’s under load.
What’s the difference between the three types?
Here’s how they compare side by side.
| Combi | System | Regular (heat-only) | |
|---|---|---|---|
| Hot water cylinder | No | Yes | Yes |
| Tanks in the loft | No | No | Yes, cold water and a small expansion tank |
| Hot water supply | On demand from the mains | Stored in the cylinder | Stored in the cylinder |
| Best for | One bathroom, smaller homes | Two or more bathrooms | Low mains pressure, older systems |
| Running out of hot water | Doesn’t run out, but flow is shared | Can run out if the cylinder empties | Can run out if the cylinder empties |
| Space needed | Boiler only | Boiler and airing cupboard | Boiler, airing cupboard and loft |
How does a combi boiler work?
A combi does two jobs in one box. When you turn on a hot tap, it fires up and heats the mains water as it passes through. When no tap is running, it heats your radiators. There’s no cylinder to keep hot, so you’re not paying to heat water you don’t use.
The limit is flow. A combi can only deliver so many litres a minute, and it can only work with what comes into the house. Run the kitchen tap while someone is in the shower and both drop. In a flat or a two or three-bed semi with one bathroom, that’s rarely a problem. In a house with teenagers and an en-suite, it can be.
Combis are the most common swap we do across Swindon, particularly in 1930s semis and the smaller homes on the newer estates. Taking out the cylinder frees up a whole airing cupboard, which people are always glad of.
When is a system boiler the better choice?
A system boiler heats a cylinder of stored hot water, usually an unvented cylinder fed from the mains. Because the water is already hot and waiting, two showers and a tap can run together without anyone losing out. Everything sits in the boiler or the airing cupboard, so there are no tanks in the loft.
It suits bigger households, homes with two or more bathrooms, and new-build family homes. Plenty of the four-bed houses around Wichelstowe and Royal Wootton Bassett were built with system boilers for exactly this reason.
The trade-off is space and timing. You need room for the cylinder, and once it’s empty it takes time to reheat. An unvented cylinder also needs checking once a year by someone qualified to work on them, which we usually do alongside the boiler service.
Why would anyone still fit a regular boiler?
A regular boiler (also called heat-only or conventional) is the older setup. There’s a cold water tank in the loft feeding a hot water cylinder, and a small feed and expansion tank topping up the heating. Water pressure at the taps comes from gravity, so it depends on how high the tank sits.
It still makes sense in a few situations:
- Your mains water pressure is low, so a combi or unvented cylinder wouldn’t perform
- Your current system works well and a full conversion isn’t worth the cost or the disruption
- You have solar water heating tied into the cylinder
- It’s a larger older property where the pipework was built around it
We find plenty of these in Victorian terraces in the town centre and in older cottages out in Purton and Chiseldon. Replacing like for like is often the least disruptive option.
Which boiler do I need for my home?
Start with bathrooms and people. As a rough guide:
- One bathroom and one to three people usually means a combi.
- One bathroom and a busy household can still work with a larger combi if your mains flow is strong. If it isn’t, look at a system boiler.
- Two or more bathrooms used at once points to a system boiler with an unvented cylinder.
- Low mains pressure, or a tank-fed system that already works well, points to a regular boiler.
Then check the mains. We measure your incoming flow and pressure at the survey, because that decides what a combi can actually deliver in your house. A reading at the kitchen tap on a quiet afternoon tells us more than any brochure.
Think about the future too. If you’re planning a loft conversion, or you might add a heat pump or solar panels one day, a cylinder keeps more options open. If you’re after the lowest running cost in a small home, a combi usually wins.
Does the type of boiler change the price?
Yes. A like-for-like swap is the cheapest job, and changing from one type to another costs more because of the tanks, cylinder and pipework involved. We set out typical ranges in how much a new boiler costs.
Whatever you choose, the work must be done by a Gas Safe registered engineer. That’s the law. In England any new gas boiler also has to meet Boiler Plus, so it comes with proper time and temperature controls, and a new combi needs an extra energy-saving measure such as a smart thermostat. Your installer notifies Building Control and you get a certificate afterwards.
If you’re still not sure which type fits, that’s what the survey is for. We’ll look at the house, test the water and tell you straight. See our boiler installation page for how it works, or get a free quote.
Frequently asked questions
What is the difference between a combi and a system boiler?
A combi heats water straight from the mains as you use it, so there's no cylinder. A system boiler heats a stored cylinder of hot water, which means several taps and showers can run at once. Both are sealed systems with no tanks in the loft.
Can I switch from a regular boiler to a combi?
Usually, yes, as long as your mains water pressure and flow are good enough and your gas supply can be upgraded if needed. The loft tanks and hot water cylinder come out, which frees up space. It costs more than a like-for-like swap because of the extra pipework.
Which boiler is best for two bathrooms?
For two bathrooms used at the same time, a system boiler with an unvented cylinder is usually the better choice. A large combi can work in some homes with strong mains flow, but showers will weaken if both run together. We check your flow rate at survey before recommending either.
Do regular boilers still get fitted?
Yes. They suit homes with low mains pressure, older systems where changing everything isn't worth it, and some homes with solar water heating. If your current regular system works well, replacing like for like is often the simplest and cheapest option.
How do I know what type of boiler I have?
Check the loft and the airing cupboard. Tanks in the loft plus a cylinder means a regular boiler. A cylinder but no loft tanks means a system boiler, and no cylinder at all usually means a combi.
