On..

Home

On the durability of the 3 port valve used on heating systems

The 3 port valve used to switch central heating between hot water, heating or both is a curious thing to find in use today as it is entirely electromechanical and contains no micro controller or electronic circuitry except for one diode and one resistor. A disadvantage is that the motor tends to remain fully energised when the system is off and this can shorten the motor life. A simple way to improve durability by choice of programming is given.

Description

A motor drives the valve to one end of travel (central heating, port A). If power is removed the motor turns off and a spring returns the valve to the other end of travel (hot water, port B). For both hot water and heating (mid position) the motor is stalled by feeding it DC through the diode and resistor. A pair of micro switches and a cam in the valve control the switching of the diode in the mid position and the sequencing of the boiler. The motor is a synchronous one so feeding it (pulsating) direct current through the diode locks it to zero frequency (stationary) with sufficient detent force to remain in the mid position against the spring force.

Disadvantages

Whilst the system is fairly simple, it suffers from two disadvantages. One is that a changeover tank thermostat is required which can mean that the wiring will need changing to 3 way + earth when updating older simple systems.

The other disadvantage concerns the durability of the valve motor but can be overcome by a simple alteration to the programming of the time switch. When both hot water (HW) and central heating is on (CH) the valve is in the mid position. When one demand turns off the valve will go to unenergised (CH off, HW on) or full on (CH on, HW off). When this last demand turns off the valve stays in the same position. This means that if CH is the last to turn off when the system goes off for the night, the motor will remain fully energised all night.

Whilst the motor is designed to be run stalled, it will be cooler and last longer if it is turned off completely. Searching on the web soon turns up the cry ‘my valve no longer works, what’s wrong’ and replacement motors are a common spare. The Honeywell spec states that ’Continuous operation of the valve motor at the fully open position is not recommended’ An extra £1 (very approx) electricity cost will result is a 6 watt dissipation over 6 months of nights.

Tip

There is a simple way to stop the motor running continuously stalled when the system is off and that is to program your time switch so that hot water is the last demand each day. On some newer programmers there are 3 slots per day. Programming the 3rd slot for 5 minutes after you have gone to bed will drop the valve back to the fully off position. This assumes that you have used enough hot water for the tank thermostat to come on.

© 2006 Steve Smith

Links:

http://content.honeywell.com/UK/homes/Catalogue/Heating%20Controls/2.21%20V4073.pdf
Honeywell V4073A mid-position diverter valve.
The statement ‘It may be plumbed in at any angle, but must not be mounted so that the powerhead is below the horizontal level of the pipework.’ is probably to prevent any leakage past the o rings reaching the electrics. Contains the operating details on page 2.

http://content.honeywell.com/uk/homes/files/pag109.pdf
A wiring diagram

http://v3.espacenet.com/textdoc?DB=EPODOC&IDX=CA1112718&F=8
Honeywell Patent from 1981
There is a circuit in ‘mosaics’

http://www.diyfaq.org.uk/plumbing/controls/midpositionvalve.htm
How the Mid-Position Three Port Valve works
Note: this site swaps over A and B in the description

The 270K resistor referred to may be included for production testing purposes.