Remote flow switch for Bathroom extractor

[Bill Door]

Hi Please be gentle with the newbie.

Need SELV relay or some such thing?

I'm not going to DIY this, but what I need isn't available off the shelf, so guidance welcome to help me describe to sparkie what's needed.

In a rental property there's a 240v extractor. Tenants keep switching it off because of noise / draught and other silly excuses, and we end up with mould and rust. Tried humidistat, but set to good extraction it triggers in wet weather and tenants complain about noise.

What is needed is an extractor which actually kicks in when the hot feed to the shower mixer is actually flowing. Although I can;t find anything ready made, a flow switch is easy to find:
https://www.amazon.co.uk/Wal-front-0-110V-Thread-Sensor/dp/B07DCRHTRQ

But how can it be wired in to achieve SELV? I'm guessing something like a door bell. Something with a relay in a box in the loft, with low voltage down to the switch and its feeble wires, switching on the 240v to the fan.


Any ideas most welcome

 

[mfarrow]

https://www.rapidonline.com/measurement-specialities-fs-10-meas-copper-bodied-flow-switch-61-1358

I’ve bought it but yet to use it (just doing an en suite).

Why do you think you need SELV? Is this going to be exposed in the bathroom?

 

[bernardgreen]

A thermal sensor on the hot pipe is another way to switch a fan on. It can be fitted to the pipe without any plumbing needed. The fan will run until the pipe cools.

This will close at 30°C and re-open at 20 °C LINK

It will switch 230 volts for the Switched Live to the fan but as you say a power supply and relay will be a safer way.

 

[mfarrow]

I forgot Bernard's solution - yes we have that in the other bathroom, just a pipe stat (this one in fact: https://www.screwfix.com/p/drayton-pts1-pipe-thermostat/2395r) set slightly higher at 35 deg to protect against hot weather.

I didn't use it in the en suite as the primary driver is protecting the mirror demister (which are extremely difficult to replace and yield 7 years of bad luck when doing so) therefore the shorter it's on for the better.

Also OP, best having the flow switch on the cold as it avoids thermally cycling the switch. It doesn't state that's a problem in literature but might help longevity.

The Amazon one you found doesn't have a minimum flow rate but the Rapid one does, and is very low (which is good). Many are designed for water burst protection rather than every day use.

 

[flameport]

Another option is a PIR motion sensor in the ceiling.

If possible, put the actual extractor in the loft so that it will be almost silent. Those attached to the wall/ceiling in the bathroom will always be noisy.

 

[bernardgreen]

Fitting the sensor to the hot pipe means the fan only runs when hot water is used and does not run when only cold water is used. (ie no steam produced ).

As mfarrow has said am economically priced flow sensor in a hot pipe will degrade due to hot~cold~hot~cold cycling unless it is one designed for that application. ,

 

[Bill Door]

Unfortunately it's impractical to change to a ceiling version due to the design of the bathroom. The roof-space is very tight too, but the suggestions are much appreciated.

bernardgreen and mfarrow thanks for the pipe heat sensor, that's a great idea!

Out of interest, not sure about SELV being the right thing, but the idea was to use low voltage down to the flow sensor. The hot pipe to the shower is in the airing cupboard, completely outside the bathroom, but reachable by tenants, and the wires being so flimsy, that was the thinking.

But point noted about flow sensor possibility of failure due to hot cycling. The thermostat is a much better idea!


Next thought - do you guys know if the fan isolator is a requirement to be accessible to occupants? I'm thinking of having it moved into the loft so tenants can't switch it off.

 

[bernardgreen]

If you use SELV then you could use the pipe itself as one conductor to the switch and a thick green/yellow ""Earth"" cable as the other conductor to the switch. That way the tenant can be led to believe it is a safety earthing device and as such they may not be inclined to tamper with it.

 

[winston1]

Next thought - do you guys know if the fan isolator is a requirement to be accessible to occupants? I'm thinking of having it moved into the loft so tenants can't switch it off.

As there is no requirement for a separate fan isolators anyway you can put it where you like or not have it at all.