Understanding the Honeywell three-port diverter valve…

Firstly a conventional, basic central heating system with radiators and a hot water tank uses a three-port diverter valve to switch the heater water from the boiler to either the radiators, or to the hot water cylinder, or to share it between both. There is a room thermostat which is said to “call for heat” when the house is cold and a hot water tank thermostat which “calls for heat” when the hot water tank needs heating up.

Honeywell is the best known brand of diverter valve but there are others and most of them work in much the same way. Three port diverter valves are fitted to most central heating systems except combi boiler systems. A heating system with a combi boiler does not not having a hot water tank, so doesn’t need a diverter valve.

 All the current brands of diverter valve work in a similar way. The valve takes heated water from the boiler and sends it to either the radiators, or to the hot water cylinder, or shares it between both. It physically switch the hot water coming from the boiler to either the radiator circuit (heating), or to the cylinder circuit (hot water), or shares it between the two circuits if there is demand from both circuits. If there is no demand from either circuit, the valve turns the boiler OFF for economy, which the earliest heating systems did not. 

Mechanically:

The centre pipe connection or ‘port’ on the three port diverter valve receives heated water from the boiler and passes it out through the either the hot water (HW) port, the central heating (CH) port or shares it between both, depending on whether the room thermostat, or the cylinder thermostat, or both are calling for heat. 

The head on the diverter valve contains an electric motor which physically moves the valve from one position to another. If all power is removed from the valve, a spring inside the valve head returns the valve to its default ‘relaxed’ position where all the incoming water will be sent to the HW circuit. 

There is a little lever on the end of the motor head for the heating engineer to use to move the valve manually across to the centre (shared) position when filling the system. This lever is useful for fault diagnosis too, to help determine which position the valve is in. 

Electrically:

There are three wires (or conductors) in the cable from the motor head on the valve which are used to tell the valve which position to move into. There are two further wires, Neutral and Earth making a total of five conductors. (Some brands of valve are “double insulated” so don’t need an Earth wire, and these valves will have only four conductors.)

The three active wires are usually orange, white and grey in insulation colour. These wires all carry 230Vac mains voltage so treat them with great care when diagnosing valve faults. Don’t electrocute yourself. 

When all power is OFF and removed from the valve, the spring in the valve head will move the valve across to its ‘resting’, HW-only position.

There are three positions the valve might take up as follows:

HW-only

CH-only

HW and CH shared

The combinations of power on the three wires which will move the valve to these three positions on a correctly working installation, as follows:

HW-only position:

Orange wire 230 Volts

White wire 0 Volts

Grey wire 0 Volts

Centre position, sharing CH and HW:

Orange 230 Volts

White 230 Volts

Grey 0 Volts

CH-only position:

Orange wire 230 Volts

White wire 230 Volts

Grey wire 230 Volts

and the remaining three active wires controlling the valve position are coloured orange, white and grey. (These three wires carry 230Vac mains voltage in various combinations according to the signals being sent by the programmer and the room & cylinder thermostats so treat them with respect.)

When there is no voltage on any of the wires, the spring moves the valve back to the ‘relaxed’ position, HW. Any water arriving from the boiler when the valve is relaxed will all pass through the valve and out through the HW port. None is sent to the CH (central heating) port. 

The hot water cylinder thermostat sends out a live signal at all times. When the cylinder thermostat is ‘calling for heat’ it applies 230Vac to the orange wire to the valve. The orange wire is also connected to the boiler and tells the boiler to light and the pump to run. Heated water will then circulate around the HW circuit. The valve remains relaxed when only the orange wire is energised so all the boiler water output is concentrated on heating the cylinder. When the HW thermostat is ‘satisfied’ I.e. not calling for heat, it switches power the terminal from the orange wire to the grey wire. This is how the valve knows what to do when the central heating comes on. 

Central Heating: When the central heating room thermostat calls for heat, it supplies power to the white wire on the valve and it moves across to the centre, share, position if the HW is calling for heat too. Or fully across to the CH-only position if the HW is not calling for heat. 

how they actually work is a mystery too most heating engineers I think. This certainly applied to me for many years - I wired them up according to the instructions and they worked, but if a fault was present I found it very difficult to trace. 

All this was fixed about 25 or 30 years ago though when I attended a one-day training course at Honeywell where they taught us how they work, and how to wire them, in proper detail. I’ll try to explain here.

First thing to note is the three port valve with no power applied, relaxes into HW position. (Water from the boiler and pump enters the valve via the centre pipe and all of it is sent to the HW outlet pipe.)

Secondly, the cable into the valve has five conductors including Neutral and Earth, which leaves three conductors actually carrying the operating signals. These are coloured orange, white and grey. 

In a heating system these three conductors are connected as follows:

White to the room thermostat “Call for heat” terminal

Grey to the cylinder thermostat “No call for heat” terminal 

Orange to the cylinder thermostat “Call for heat” terminal

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