3-Port Valves
3-Port Mid-Position Valve
In short
A 3-port mid-position valve is one valve with three pipe ports. The water comes in at AB, from the boiler. Its terminals: E, N, W, G, O. The parts list also has it as Pre-wired 3-Port. Units based on makers’ products sit under it in the parts list, such as the Danfoss HSA3 (brown heating core) and the Danfoss HSA3 (white heating core).
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 3-PORT MID-POSITION VALVE is one valve with three pipe PORTS. The water comes in at AB, from the boiler. Port A goes to the heating, and port B to the hot water cylinder. It is the valve of a Y-PLAN system.
It rests at B, the hot water, held there by its spring. A live on its white terminal, W, drives it toward A, the heating. A live on its grey terminal, G, means hot water is not wanted. With white and grey both live, it goes on to A.
In the middle, at MID, both ports are open: heating and hot water together.
Only at A does its END SWITCH join white to orange and fire the boiler, for heating alone.
The cut-away above is live. It shows the paddle, the motor and the two switches as the valve moves.
Terminals
| Terminal | What it is for | On the Y-Plan example |
|---|---|---|
| E (Earth) | The earth (E). | Wiring centre block 3 |
| N (Neutral) | The neutral (N) for the motor. Without it, the motor cannot run. | Wiring centre block 2 |
| W (White) | White: the heating CALL. A live here drives the valve away from B. | Wiring centre block 5 |
| G (Grey) | Grey: hot water NOT wanted. It comes from the programmer's hot water off (HW OFF), or the cylinder thermostat's satisfied contact. With white live too, it takes the valve to A. | Wiring centre block 7 |
| O (Orange) | Orange: out of the end switch at A. It goes to the boiler's switched live (SL). | Wiring centre block 8 |
Read from the wired example circuits: Y-Plan. A wiring-centre block carries whatever is landed on it; the numbers are the habit the lessons teach.
What the meter reads
White only: the valve stops in the middle, at MID. The boiler then fires from the call of the cylinder thermostat (stat).
White and grey: the valve reaches A, and the orange goes live.
Grey alone, from rest: nothing moves. But grey alone HOLDS a valve that is already at A. That is how this simulator models the valve: the makers' sheets test only white alone, and white with grey.
Dead, across W and N the meter reads the motor's WINDING: about 2.40 kilohms in this simulator. Across G and N it reads open here.
Units based on makers’ products
In the parts list these sit under the 3-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 HSA3 (brown heating core)
- Based on the Danfoss HSA3 (white heating core)
- Based on the Drayton MA1
- Based on the EPH Controls B322PF
- Based on the Honeywell V4073A
- Based on the Salus SBMV38
- Based on the Siemens CMV322
- Based on the Sunvic SD2701
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.”
- Heating stops once the cylinder is hot (Y-Plan)
“The heating's fine on its own. With the hot water on as well, the radiators go cold once the cylinder's hot.”
- Hot water only when the heating is on too (Y-Plan)
“We only get hot water when the heating is on as well.”
Where it connects
Lessons that use it
- Reading a valve
Foundation · Reading a circuit
- The 3-port valve
Foundation · The parts
- The Draw Warm Method: the parts on the wall
Foundation · The Draw Warm Method: Y-Plan
- Block 1: the permanent lives
Foundation · The Draw Warm Method: Y-Plan
- Tracing a demand: the Y-Plan
Foundation · Testing
- The same on the Y-Plan
Intermediate · Swapping controls
- The practical: a swap from memory
Intermediate · Swapping controls
- Inside a 3-port head
Intermediate · Actuator anatomy
Wiring diagrams it is part of
- Y-Plan
Y-Plan
- Y-Plan with a frost thermostat
Y-Plan
Also in 3-port valves
Read the explanation
- Y-Plan explained: what white, grey and orange do on a mid-position valve
A Y-Plan’s mid-position valve: white calls for heating, grey says hot water is not wanted, orange fires the boiler. None of its cores is a permanent live.
Practise it in the simulator.
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