Boilers & Pump
Combi Boiler
In short
A combi boiler (combination boiler) gives heating and hot water from one appliance. Its pump and its valve are inside it. Its terminals: L, N, E, RT. Units based on makers’ products sit under it in the parts list, such as the Vaillant ecoTEC plus and the Worcester Greenstar 4000.
If this is your home: this page is written for heating engineers. Mains wiring and gas appliances are not DIY jobs; call a Gas Safe registered engineer. If you smell gas, leave the building and call the National Gas Emergency Service on 0800 111 999.
What it is
A COMBI BOILER (combination boiler) gives heating and hot water from one appliance. Its pump and its valve are inside it. It heats the hot water as a tap runs, so it needs no hot water cylinder.
Outside, it needs a supply, a room thermostat (stat) and a timer. A live on its room thermostat terminal, RT, means heat the radiators.
The timer is often the boiler's own plug-in clock, which this simulator does not show.
In England a gas boiler fitted in a home must have time and temperature control. That is a timer and a room thermostat, or one programmable thermostat that is both (BOILER PLUS).
Terminals
| Terminal | What it is for | On the Combi example |
|---|---|---|
| L (Live) | The PERMANENT LIVE (L): live all the time the spur is on. | Fused Spur L and Wireless Receiver L |
| N (Neutral) | The neutral (N). | Fused Spur N and Wireless Receiver N |
| E (Earth) | The earth (E). | Fused Spur E and Wireless Receiver E |
| RT (Room Thermostat) | Room thermostat in: a live here means heat the radiators. Only the switched live of the thermostat (stat) comes here. | Wireless Receiver 2 |
Read from the wired example circuits: Combi boiler with a wireless thermostat. A wiring-centre block carries whatever is landed on it; the numbers are the habit the lessons teach.
What the meter reads
RT live: it fires and runs its own pump, and the radiators heat.
RT dead: there is no heating. The hot water is made at the tap on its own.
Units based on makers’ products
In the parts list these sit under the Combi Boiler. Each is the simulator’s own unit, simplified for teaching, and may differ from the product it is based on: on site, read the unit’s own instructions. DrawWarm is not part of these makers and is not approved by them.
- Based on the Vaillant ecoTEC plus
- Based on the Worcester Greenstar 4000
DrawWarm’s own: Combi.
Call-outs with this part on the path
- No heating: hot water fine, radiators cold
“Hot water's fine but the radiators are stone cold, whatever I do with the thermostat.”
Where it connects
Lessons that use it
- The combi boiler
Foundation · The parts
- A combi's own strip
Intermediate · Reading branded units
- Zones on a combi
Intermediate · More zones
- Weather compensation
Intermediate · Protection and extras
- What OpenTherm is
Intermediate · Bus controls
- The meter reads DC by itself
Intermediate · Bus controls
- Combi programmable stat (page 49)
Worcester wiring
- Combi timer and room stat (page 48)
Worcester wiring
Wiring diagrams it is part of
Read the explanation
- Wiring a wireless thermostat to a combi boiler: the receiver, the link and the room-stat terminals
A combi needs only its supply and a call on its room-stat terminals. The receiver’s relay replaces the link; check volt-free against mains-switched first.
Practise it in the simulator.
Runs in the browser on a laptop or desktop. Cancel any time.