Extract fan on flow switch

[matty m]

Hi ,
Just wondering if anyone would be able to help.

I will be starting alterations of a bathroom soon, The client would like an extract fan in zone 1 of room which he wishes to operate via the flow of water running to shower.

At the moment i am planning to install a S.E.L.V fan with the transformer in a space under the bath(only accessible with use of a tool !). I was then planning to use a flow switch with wiring into an adaptable box with din rail, and mounting a relay/contactor in said box to switch the 230v supply to fan transformer when flow switch operates.

does anybody know if there is a better way i,e if there are flow switches which could perhaps take the small load of an extract fan and be connected perhaps inbetween the positive conductor entering the fan enclosure and the actual positive terminal of the fan.

would be gratefull for any help, many thanks

 

[Steve]

i cant remember if it was ban who rigged his fan up to the pullswitch fo the shower via a relay circuit. He might point you in the right direction for pics if you ask nicely!

 

[matty m]

many thanks cheers

 

[ban-all-sheds]

What sort of shower?

Gravity mixer, pumped mixer or instant-heat electric?

 

[Lectrician]

Most flow switches will switch atlest an amp....ample for a fan.

 

[matty m]

thanks to you all, problem solved

 

[bathjobby]

And for the benefit of the rest of us, he ain't telling how. Glad we could help matty !

 

[Adam_151]

You can do what you want, safely, but it requires the use of items which are out of the ordinary.

The problem you have to solve is that the cable to the shower is protected by a large MCB - 45 or 50A is not uncommon, but even a small shower is likely to be 40A, and that is way too big to protect the 1mm² or 1.5mm² cable you've used for the fan. If you could wire to the fan in 6mm² or 10mm² then you'd be OK, but you'll struggle to get 2 cables that size into the terminals of the switch. So you end up needing some kind of junction box for the cables, which can be done - TLC sell a 60A JB with terminals that will take 2 x 10mm² cables. Then you need to go from there to an FCU (or two if the fan is a timed-overrun one) in 6/10mm², and then to the fan in1/1.5mm²

I've got just such an arrangement of a fan switched by the shower, but instead of a junction box I used DIN-rail mounted connection blocks and a fuseholder, and a contactor so that I could use the lighting circuit to supply the fan:

It actually didn't get built quite according to plan, as I didn't have enough space on the rail for a block for the fan neutral, but as the contactor was a DP one, I was able to use one of its terminals instead.

Under construction:

(can you spot the deliberate mistake?

)

All installed. No way to get the items to sit upright on the rail:

But when the cover is in place you can't see them.

 

[matty m]

many thanks ban all sheds for time and effort put into your response.
For the benefit of bathjobby and anyone else interested.
i intend to take a supply from local 6 Amp lighting circuit with an isolation point outside of the bathroom. run this supply to the fan transformer located in permissable zone.I then intend to connect a flow switch into an outgoing conductor of the fan transformer.
as i have been informed, most flow switches can handle 1 amp, so using the power triangle I=P/V I=11wats/12 Volts I= 0.91Amps.

so in theory as water is running along the cold water feed to the electric shower, this will close the contact of the flow switch hence allowing fan to operate.

the reason the client wishes to have the fan operating like this is because the bathroom has had problems or is prone to problems with damp and he wants to ensure that whenever shower is used the fan comes on, i realise that a humidistat fan would be ideal, but is not practicle in this situation.

cheers

 

[Lectrician]

If you put the flow switch in the tranny supply, it would only need to switch 0.047amps

Assuming the switch is rated to 240v ofcourse

 

[matty m]

Thanks lectrician, a very good point, i will do just that.

 

[ban-all-sheds]

Are there any issues with a flow switch that is connected to both an extraneous-conductive-part and a SELV supply? Any possibility of a fault removing the S from the ELV?

If you switch the primary, make sure the contacts are rated for reactive loads...

 

[Lectrician]

Don't think it could be that reactive.....they indend them for domestic circuits

I would never like the idea of switching the output of the tranny for a fan, or any ELV Lights.

A selv supply can supply pretty much anything you like - you can supply a socket outlet in galv trunking from a selv supply if you wished......not not you would really want to. I don't see how this is too different from a switch connected to earthy pipework

 

[ban-all-sheds]

Don't think it could be that reactive.....they indend them for domestic circuits

Yes, but with the expectation that the primary will be switched by something rated at several amps, not just 1A.. I didn't say it would be a problem, just that it should be checked...

I would never like the idea of switching the output of the tranny for a fan, or any ELV Lights.

It just feels wrong....

A selv supply can supply pretty much anything you like - you can supply a socket outlet in galv trunking from a selv supply if you wished......not not you would really want to. I don't see how this is too different from a switch connected to earthy pipework

True....