for any electricians out there, can I run more than one fan off a speed controller

your fans will have an electrical rating, same as the speed controller

depending on these, its highly possible

for example, i've a 3amp sms controller, but the fan on it, an 8" rvk, is 0.51amp,

so i could have five of those 8 inchers on that controller

I don't see any reason why a suitably-rated variac-type controller couldn't control the speed of multiple fans wired in parallel.

cool I;ll be using a couple of 3 A controllers to run a 6 inch out and a 4 inch in,
with a few thermostats to make a new multi speed controller

you 'can' do it, i.e. it'll be safe, but it most likely wont be any good, as dropping the voltage by say 30volts to one size of fan, will have a drastically different effect on a much smaller fan. try it first though, as you might find a happy medium. i can't remember which way round it went when i tried, whether the smaller one needs more of a voltage drop than the larger one, but you could end up with positive pressure in the tent if it works out this way. either that or to drop your exhaust fan to a sensible level, at the same voltage the intake fan is barely moving any air, and you end up with massive negative pressure

I use an Ecotechnics Evolution controller which has seperate connections for in and out fans. It seems to maintain a reasonably consant negative pressure at various speeds.

I think its the best thing I have used. Made a system myself which worked nearly as well but cost more and was not as plug n play.
Worth a look if u investing a few quid anyway I would say.(All my friend have em now and rave about em.)
I have no connection with the company.

does it have separate knobs for each fan though? i found i had a much larger drop in speed for one size of fan for a given drop in voltage.

No.You set a minimum percentage of fan speed.But it does seem to have seperate circuitry for each fan.(I'll open it next week and have a good look). Probably assumes intake slightly smaller than exhaust and varies voltage differently for each.(I'll stick a multimeter on it next week when I get time-I'm curious now)
I have tried it with different sizes of fan and never had noticable pressure discrepency.(Intake always smaller than exhaust obviously)

Further to what I said earlier its well thought out and by the time had bought all the bits, I don't think you could make one better for the money. Has a thermostat/humidistat and a contactor for three kilowatts of heat.(Min temp setting turns on heater) Also simple for not so technically minded.
I like good design and I like this. Only time will tell as to reliability etc but so far so good.

cool, hopefully bob will have the same experience. as for reliability, simple electronic controllers like this rarely have problems, and if they do the circuitry is so simple it's usually just a case of solder on new components till it works again

cheers guys that was just the info i was looking for very usefull.
Although I am now going down a different route
I think its a lot simpler to have a ruck temp controlled exhaust fan, and augment it with an intake fan
that only comes on if the loft temp outside the box exceed a temp set on a thermostat. I've plenty
of time to work on that before next summer
I just installed one of the ruck fans for a friend and it works a treat
combined with an on off thermostat/humidistat and a thermostatic heater

wouldn't recommend a ruck fan combined with an intake fan, as your intake fan will have to be slow enough not to cause positive pressure on the ruck's lowest speed, which means it'll act as a restriction when the ruck is on any higher speed than it's lowest speed..... not good, better off with just a passive intake.

yes i understand, the active intake will only come on in extreme circumstances.
The passive intake is from the loft and the roof tiles are uninsulated, i've seen temps of
well over 30 degrees in that space.and in that situation the air is drawn in from an external source
suplementing the passive intake, this would only happen when the extractor is running at a temp
that would trigger its maximum speed.