dr120 vent help

so it goes filter-vent-light-vent-fan vent out the chimney this is all six inch then i have a 4 inch intake vent in the bottom of the tent.. now i dont want my exhuast fan running all the time.. what do i do

put it on a timer then

I use a mercury thermostat and a relay that goes off when it is energized. When the thermostat cuts in, the reverse relay will cut out the desired fans. I have taken standard relays and bent twisted and restuctured so they would act, like backwards.

I have a pic of a relay and circuit but I'm having a problem loading the schematics.
Mercury switched home thermostat