Vent-Axia Lo-Carbon Tempra: How to Reduce the Lowest Trickle Speed

Problem

Solution

• When pulled on, everything behaves as normal, that is:
• trickle behaves as supplied (and it can still be set to fast trickle by removing jumper JP2, as explained in the installation manual);
• if you have a model with humidistat and/or timer, these will still turn on boost mode as normal, but (obviously) the pull-cord will no longer turn on boost mode.
• When pulled off:
• the fan drops to its lowest possible trickle speed (about half the noise of trickle as supplied, and consumes 2.5W of power instead of the 3.2W as supplied);
• boost mode is disabled, even if you have a humidistat or timer

Risks

• Obviously, these mods will be likely to invalidate any warranty.
• Nonetheless, you can revert it to look as new - no permanent changes.
  
• There's always a chance it might reduce the life of the motor, but I'm pretty sure it will actually put less stress on it (as supplied there is a slight buzz in the motor noise which goes away when switched to the very low trickle speed).
• I can't get at the microswitch on the pull-cord, so I'm not sure it's rated for the new voltage I'm expecting it to switch. However, as it's only switching less than 5V, which is what it was switching before, that is highly unlikely.

Applicability

• Models with humidistat, timer and pull-cord (HTP or LHTP) [my model];
  
• Models without humidistat, but with timer and pull-cord (TP or LTP);
• For models with just a pull-cord (LP or P), boost mode will not be available at all, but the pull-cord will switch between very low and normal speed trickle.

Modifications

• Turn off fan at the isolator.
  
• Remove the front casing (loosen 2 screws at bottom - see installation manual if necessary),
• Remove the plastic cover over the control circuit inside (remove another 2 screws).
• To quickly hear how the modification will end up sounding, unscrew and disconnect the yellow ('Y') wire from the MOTOR terminal block, and turn on at the isolator.
  If you're happy, turn off the isolator again and continue...
  
  
• Mark one of the two blue wires connected to the PULL CORD terminal block in case it's important which is which (I'm certain it isn't, but I used tippex on the top one).
• The terminal block is tagged 'PULL CORD' on the circuit board and in Fig 20 reproduced from the installation manual below
  

• Disconnect the fan motor's yellow speed-control wire from its terminal block ('Y' on the 'MOTOR' terminal block in the diagram above)
  
• Instead connect it via the pull-cord switch, as follows
  
• Disconnect the two blue wires from the PULL CORD terminal block;
• Connect one blue wire (I chose the bottom one) into the Y terminal of the 'MOTOR' terminal block
• Connect the other blue wire and the yellow wire together, making sure the flying joined wires are insulated so they cannot touch other terminals (I used an extra insulated terminal block I had available)

As supplied	Modified wiring on bottom left blue terminal block

• Test:
• Replace covers (to follow safety precautions to the letter) and turn on at the isolator;
• The pull-cord should toggle between trickle speed as supplied and a noticeably lower trickle speed.
  
• Replace covers (if you haven't already).
  

Aside: Timer vs. Humidistat

You might have noticed from the photos that I have not connected the switched live from the lights (the middle terminal of the top-left blue terminal block is not connected). That's because I consider that there's no point having a humidistat to turn on boost, but then forcing it to boost even if humidity is low, just because someone's turned on the lights. However, if you want to reduce odours as well as humidity, you might want to allow your lights to turn on boost as well.

Aside: How I worked out this approach

If you're interested,

• I measured the voltage between the 3 wires of the fan motor:
  

  	boost off	boost on
  red - black	23.8V (DC)	23.8V (DC)
  yellow - black	2.8V (DC)	4.1V (DC)

  
  
• from this, I guessed that black is 0V and yellow seemed to be controlling the motor speed;
  
  
• One at a time, I disconnected:
• red:- fan stopped
• yellow:- fan reduced to a lower trickle speed, whether on boost or not
• I checked that the wires to the PULL CORD terminals just went to a simple switch by disconnecting them then checking that:
• they had no voltage, and
  
• the resistance between them went from zero to infinity when I pulled the cord