Drayton ZA5 microswitch, better solution ?

[andytw]

2 Neighbours in new bungalows both with same system - S Plan, Grant boiler.
I have replaced stuck microswitches in the Drayton valves, several times, for both neighbours.
I think they just get stuck as turning on the pump and boiler is too much for their rating and the contacts stick.
Sometimes they can be freed with just a tap on the side.
Just wondering if there is a better solution ??
Thanks

 

[MeldrewsMate]

Usually the switched output from the zone valve is only to the 'demand' terminal of the boiler, and it is the boiler which switches the pump on and off; this allows the pump run-on function to work properly. The installer may have incorrectly driven the pump from the Drayton valve's output, though this would rarely be >100 W (eg less than half an Ampere), well within the capabilities of the 3A microswitch. It could be that Drayton have started using cheaper microswitches, and that you could be replacing them with cheap Chinese copies too.
I suggest inspecting the wiring of the system against the boiler manufacturer's schedule.

 

[bernardgreen]

the Drayton valve's output, though this would rarely be >100 W (eg less than half an Ampere), well within the capabilities of the 3A microswitch

Is the 3 Amp rating for inductive loads or only for resistive loads. ?

If the switch is controlling the pump then inrush current as the contacts close may be considerable more than the 0.5 Amp ( 100 W ) the pump takes when it has reached normal speed.

When switching off an inductive load the arc as the contacts open is the prime cause of contact wear.

 

[oilhead]

The start up load of an oil boiler includes the pump, burner motor, transformer, and can often exceed the rating of programmers and motorised valve switches. I find other valves are more durable than the Drayton range.

 

[Rodders53]

Those microswitches can be mechanically sticky rather than contact welding due to arcing (in my limited experience of them in the late in-Laws system which was donkeys years old but working when we left it nearly 7 years ago). Drayton ZA5 was also badged as British Gas item. Something to do with heat, grease from the motor-gearing-actuation mechanism getting on the microswitch actuating button sides, I suspected.

Not to say they haven't been re-engineered down to a price now.

You can find and replace the microswitch with an equivalent (I did so after the mechanical stickiness reoccurred after me cleaning it up). I think they were Burgess XCG3-Z1 like this from RS which are rated for 1/4 hp motors and 6(2) A if I can read the spec sheet correctly. {Drayton heads are claimed 3(1) A so may not be the same switch?.} Care will be needed to ensure mechanical size is identical if a different microswitch is fitted, of course.

AFAIK most Grant boilers have no relays to control the pump separately from the 'call for heat' (mine certainly hasn't). They certainly DO, however have more than one motor involved (circulating pump and oil burner pump etc.,.) ... Which might make the switch contact rating suspect... an extra relay or different make of valve/head could be needed?.