2-Port Valves
2-Port Zone Valve
In short
A 2-port zone valve opens and shuts one pipe, such as the pipe to the radiators. The zone is the part of the system that pipe feeds. Its terminals: E, N, B, G, O. The parts list also has it as Pre-wired 2-Port. Units based on makers’ products sit under it in the parts list, such as the Danfoss HPA2 and the Drayton ZA5/679-2.
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 2-PORT ZONE VALVE opens and shuts one pipe, such as the pipe to the radiators. The ZONE is the part of the system that pipe feeds.
A MOTOR in its HEAD opens the valve while its brown terminal, B, is live. A SPRING shuts it again when B goes dead.
Inside the head is an END SWITCH. A permanent live on its grey terminal, G, comes out on its orange terminal, O, only when the valve is fully open.
That orange is what fires the boiler. So the boiler can fire only once a valve is open: the INTERLOCK.
Terminals
| Terminal | What it is for | On the S-Plan example | On the Combi example |
|---|---|---|---|
| E (Earth) | The earth (E). | Heating: wiring centre block 3; Hot water: wiring centre block 3 | Heating: wiring centre block 3; Second heating zone: wiring centre block 3 |
| N (Neutral) | The neutral (N) for the motor. Without it, the motor cannot run. | Heating: wiring centre block 2; Hot water: wiring centre block 2 | Heating: wiring centre block 2; Second heating zone: wiring centre block 2 |
| B (Brown) | Brown: the MOTOR. A live here, from the call of a thermostat (stat), opens the valve. | Heating: wiring centre block 5; Hot water: wiring centre block 8 | Heating: wiring centre block 5; Second heating zone: wiring centre block 9 |
| G (Grey) | Grey: a PERMANENT LIVE into the end switch, live all the time the spur is on. | Heating: wiring centre block 1; Hot water: wiring centre block 1 | Heating: wiring centre block 1; Second heating zone: wiring centre block 1 |
| O (Orange) | Orange: out of the end switch once the valve is fully open. It goes to the boiler's switched live (SL). | Heating: wiring centre block 10; Hot water: wiring centre block 10 | Heating: wiring centre block 10; Second heating zone: wiring centre block 10 |
Read from the wired example circuits: S-Plan, Combi boiler with two heating zones. A wiring-centre block carries whatever is landed on it; the numbers are the habit the lessons teach.
What the meter reads
B live, and N there: the motor opens the valve, over about 3 seconds in this simulator. Then O goes live.
B dead: the spring shuts the valve and the end switch opens, so O gets nothing from G.
On an S-Plan both valves' oranges share one wiring centre block, so O can still read live from the other valve. That live comes from the block, not from this valve's end switch.
Dead, the meter's ohms setting across B and N reads the motor's WINDING: about 2.40 kilohms in this simulator. OL there means the winding is open.
Units based on makers’ products
In the parts list these sit under the 2-Port Valve. 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 Danfoss HPA2
- Based on the Drayton ZA5/679-2
- Based on the Honeywell V4043H (5-core)
- Based on the Honeywell V4043H (6-core)
- Based on the Salus PMV22
- Based on the Sunvic SM5203older
- Based on the EPH Controls V222P
- Based on the Siemens CZV222
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.”
- No hot water: heating fine, cylinder cold
“Heating's lovely and warm but there's no hot water - the cylinder's cold.”
- Radiators hot with everything off at the timer
“The radiators get warm when nothing's asking for heat - it's all off at the timer and they're still hot.”
- Boiler fires with everything off, radiators cold
“The boiler keeps firing with everything off at the timer, but the radiators stay cold.”
- Hot water scalding: the cylinder keeps on heating
“The hot water's scalding - far hotter than it used to be, whatever the dial on the cylinder says.”
Where it connects
Lessons that use it
- The Manual, History and the examples
Foundation · Using the simulator
- The words, and why they are what they are
Foundation · Reading a circuit
- Reading a valve
Foundation · Reading a circuit
- Cables and terminations
Foundation · Electricity for heating controls
- The 2-port valve
Foundation · The parts
- The Draw Warm Method: the parts on the wall
Foundation · The Draw Warm Method: S-Plan
- Block 1: the permanent lives
Foundation · The Draw Warm Method: S-Plan
- Polarity: the spur, the centre, a load
Foundation · Testing
Wiring diagrams it is part of
- S-Plan
S-Plan
- S-Plan with a digital room stat
S-Plan
- Wireless S-Plan
S-Plan
- S-Plan with a frost thermostat
S-Plan
- S-Plan Plus
S-Plan
- Combi boiler with two heating zones
Combi
Read the explanation
- Why grey is the permanent live on a 2-port zone valve, and brown is not
A 2-port zone valve: brown drives the motor, grey feeds the end switch all the time, orange fires the boiler. Why grey, not brown, is the permanent live.
- Prove a part with two readings before you condemn it
A motor needs a live and a neutral. Read live to earth, then live to the part’s own neutral, before you change a valve, a pump or a fuse. Then prove the repair.
Practise it in the simulator.
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