Mcws flow switch

[Cynic]

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 t he 1/2" flow switch that detects when a hot tap is opened and starts the pu mp 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 sw itched the coil of an octal plug in relay. The paddle operated reed switche s 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 posit ion but has a resistive path in the off position. I'm guessing it's got wat er in the internal (sealed) connections. Whatever the reason it's holding t he 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 a vailable in the UK?

[Fredxxx]

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.

[Graham.]

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

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.

[Fredxxx]

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.

[Michael Chare]

I use one of these:

but it might well be difficult to mount inside a boiler. I like it because there is nothing to leek, unlike the previous device I had.

There is a wide choice of flow switches on ebay.

Worcester Bosch use a flow switch in their Heatslave oil boilers. The switch is not very big. You might find one to look at an appropriate supplier.

[Roger Hayter]

Indeed in my similar vintage (2003) Firebird the relay was fitted from manufacture and was a standard 'octal' 11 pin type with a mains coil. My flow switch only failed once, and that was also water leaking through the paddle seal and through the electrics to the outside world. I am unconvinced that a much better one can be easily found, but I am biassed because it was probably the only part of the boiler that only failed once.[1]

[1] Apart from the terminal rusting through of the main thermal store which is integral with the boiler heat exchanger.

[Roger Hayter]

As long as you run another cold feed to the filling loop the supply to the hot water heat exchanger can come from outside the boiler via the flow switch and a flying lead from the external flow switch can be connected into the control box (which is the existing arrangement, but a shorter lead).

[Michael Chare]

I paid £24.43 for that flow switch in Feb 2011. They are now more than

3 times that!

[Brian Gaff]

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

[Cynic]

Actually looking at the switch innards is virtually impossible without cutt ing the plastic assembly to pieces. I could understand the contacts of the reed welding but surely the result w ould be a closed circuit regardless of the position of the actuator paddle magnet. Moving the paddle gives closed circuit or resistive path instead of closed/open which is what it started with when first installed. I'll probably end up taking the switch back as defective but what's to say another won't be as crap.

[Cynic]

That's correct. It's a 240 volt ac circuit.

[Cynic]

Seems ideal but a lot of money for the unit. I could quite easily modify the mcws pipe to allow a vertical upwards mount.