I'd have thought using a relay as they now do would stop issues of arcing in
the switch which can become conductive. have you actually proof its become
wet?

Might be worth looking into it further by looking at the wiring and what
current the relay is taking and also trying to stop any kind of arcing with
a snubber network. A few days even for this sounds wrong to me.
Brian

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"Fredxxx" wrote in message
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On 22/11/2017 20:55, Cynic wrote:
I've got a friend who has a Firebird oil fired combi about 15 years
old. It's been a reliable unit with the exception of suffering a few
failures of the 1/2" flow switch that detects when a hot tap is
opened and starts the pump feeding primary hot water to the plate
heat exchanger. There was a component change of type of switch from a
microswitch to a reed type a few years ago accompanied by a slight
circuit change so the reed switched the coil of an octal plug in
relay. The paddle operated reed switches in a plastic body are prone
to failure in the nc (on) state which results in the central heating
pump being inhibited. I replaced one for her a few days ago and it's
failed again already. It shows continuity in the on position but has
a resistive path in the off position. I'm guessing it's got water in
the internal (sealed) connections. Whatever the reason it's holding
the relay energised. I'd like to find a robust suitable 1/2" flow
switch capable of switching a grundfos 15-60 and rated for mains cold
water pressure, say 6 bar or more.. Anyone able to suggest a suitable
make and model, economically priced and available in the UK?

I'm wondering if all the failures have been down to inductive loads.
Perhaps a VDR or R/C suppression (or both) should be considered before
choosing your next micro-switch or similar?

A branded microswitch should be very reliable, if treated well.