On 22/11/2017 21:57, Graham. wrote:
On Wed, 22 Nov 2017 12:55:35 -0800 (PST), Cynic
coalesced the vapors of human experience into
a viable and meaningful comprehension...

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?


It sounds like the back-EMF of the relay coil has a tendency to weld
the reed contacts together. I would *expect* that to happen! which is
odd given that it's a manufacturers modification.

What is the voltage across the relay's coil when it is energised?

If I am right about the cause, a silicon diode wired across the coil
with the cathode (marked terminal) connected to the positive side (ie
the diode is reverse biased) will snub the reverse EMF.

Check there isn't a diode in that position already.

Unless Cynic comes back with different information, I would expect the
relay solenoid supply would be AC. If so, you wouldn't want a diode
across the winding.