additional in-line fan

[chloebear]

I have an installation with 3x in-line fans with timers in seperate bathrooms with back draft shutters and all connected to a single duct and extract. The question is would it be possible to wire up another fan mid way in the duct to come on when any of the others are switched on.
The issue I keep coming up with in my head is that if all the switch/line wires are connected together at the mid fan this would cause a problem - think I some kind of relay maybe - anyone come ocross this in the past or have any good suggestions??

 

[Richard]

We had to install a heat recovery system recently. Basically it ran normally, but when either bathroom or the humidistat switched, it went into boost mode.

Back to your point.....problem was, that when you switched either bathroom, the other light would have come on. To prevent this, as you correctly said, we put the bathrooms through two seperate relays to prevent feedback. We drew a diagram, which is on the van, but i will try and photocopy it and stick it up online.

Are the bathrooms supplied from the same circuit as I can see problems there?

 

[chloebear]

A diagram would be good and yes the bathrooms are on the same cct.

 

[Des 56]

I have answered this type of question on here before,it can work for yours
Operate the 3 existing fans off double pole pull cords

Feed the fan through the one side of the pull cord,fan switch wire on load side.
Off that load side,loop a wire to then feed the other half of the switch,that then feeds a switched live to the fan
Do the same for all three fans
As long as they are fed off same mcb, all new fan switch wires will not back feed through an unused fan, because the double pole isolator will be in the off position

 

[chloebear]

tried following your descriptions, but there seem to be a few words or steps missing - any chance you could review it?

 

[Sintra]

Have a look at this pdf it shows how to do it with 2 switches but same principle for 3.

View attachment Common fan switching.pdf

 

[thomsonminis]

i got relay yhat took 8 switch wires in to control 1 load came from william wilsons could get make number on monday if thats any use cant remember exact price but dont think iw over expensive

 

[chloebear]

So for the sake of clarity - the attached diagram should work??

View attachment Fans.pdf

 

[Knobhead]

Electrically it’s no problem, but have you considered airflow?

Say the 3 fans have a capacity of 100CFM each
To deal with all 3 fans running together the in line fan needs a slightly higher capacity say 350CFM otherwise it acts as a choke.
Now turn off 1 fan, the inline fan is still trying to shift 350CFM, it will make up the shortfall by dragging air through the back draft shutter of the fan that’s supposed to be stopped. That fan will now be wind milling and the other two will be being dragged.

The net result is anyone switching on a bathroom fan will start extraction in the other two bathrooms.

The only answer is larger ductwork or go for one central fan.

Sorry.