How-to
Combi vs System vs Conventional Boilers: Which Do You Have?
Learn to identify whether you have a combi, system or conventional boiler — and why it matters when you suddenly have no hot water.
Published 1 July 2026
When your hot water disappears, the first thing a plumber will ask is: “What type of boiler do you have?” It sounds like a trick question if you have never thought about it, but the answer shapes everything — from where to look for the fault to how quickly it can be fixed. This guide helps you work it out in a few minutes.
The Three Main Types
1. Combi (Combination) Boiler
A combi boiler heats water directly from the mains on demand. There is no hot water cylinder and no cold water tank in the loft — just the boiler itself, usually mounted on a kitchen or airing-cupboard wall.
How to spot one:
- One compact unit on the wall
- No cylinder in an airing cupboard
- No water tank in the loft
- Hot water arrives quickly but flow rate depends on mains pressure
Combis are the most common type in modern TW-area homes, particularly flats and smaller terraced houses.
No hot water from a combi? Common causes include a pressure drop (check the gauge on the boiler — it should read around 1–1.5 bar), a faulty diverter valve, or a lockout error code on the display.
2. System Boiler
A system boiler heats water and stores it in a sealed hot water cylinder (usually in an airing cupboard), but it draws cold water directly from the mains — so there is no tank in the loft.
How to spot one:
- Boiler unit on the wall
- A cylinder in the airing cupboard, often with an immersion heater element on the side
- No cold water tank in the loft
- Hot water can run out if the cylinder empties
System boilers are common in larger TW-area homes where multiple taps and showers need to run simultaneously.
No hot water from a system boiler? The cylinder may have lost its charge, the motorised valve may have stuck, or the boiler itself may have locked out. Check whether the immersion heater (if fitted) works as a temporary backup.
3. Conventional (Regular or Heat-Only) Boiler
A conventional boiler works with both a hot water cylinder and a cold water storage tank — typically a black or blue plastic tank sitting in the loft.
How to spot one:
- Boiler on the wall (often older, sometimes floor-standing)
- Cylinder in the airing cupboard
- A tank or tanks in the loft
- Hot water pressure is lower, fed by gravity
These systems are more common in older TW-area properties — Victorian and Edwardian terraces, for example — and are increasingly replaced during refurbishments.
No hot water from a conventional system? Start by checking the loft tank has water in it and the ball valve is not stuck. A faulty motorised valve, a failed pump, or a boiler lockout are also frequent culprits.
Why Does It Matter for Diagnosing Problems?
| Boiler type | Hot water storage | Cold water source | Key failure points |
|---|---|---|---|
| Combi | None | Mains | Diverter valve, pressure, error codes |
| System | Sealed cylinder | Mains | Motorised valve, cylinder stat, boiler |
| Conventional | Vented cylinder | Loft tank | Ball valve, pump, motorised valve, boiler |
Knowing your setup means you — or a plumber arriving at your door — can go straight to the most likely cause rather than working through the whole system from scratch. It also helps you give accurate information when you call, so the engineer can bring the right parts.
Quick Checks You Can Safely Do Right Now
- Look at your boiler display. Error codes are model-specific but are often explained in the manual or a quick search online.
- Check the pressure gauge (combi and system boilers). Below 1 bar is a common cause of no hot water.
- Check your airing cupboard. Is the cylinder warm to the touch? A cold cylinder points to a heating-circuit or valve issue.
- Check the loft (conventional systems only). Is there water in the tank?
Do not attempt to open the boiler casing, interfere with gas pipework, or top up pressure repeatedly without understanding why it keeps dropping. These are jobs for a qualified engineer.
When to Call Us
If you have run through the checks above and still have no hot water — or if you smell gas, see water leaking from the boiler, or simply are not sure what you are looking at — please do not wait. A loss of hot water can escalate into a more serious fault, particularly in older systems.
Emergency Plumbers TW are available 24 hours a day, 7 days a week across Twickenham, Richmond, Teddington, Hounslow, Feltham, Staines and the surrounding TW postcode area.
Call us now on 07725 479493 and we will talk you through what you are seeing before we arrive.