# Yes, another Newbie :-)

#### TheManWithNoName

Joined Dec 16, 2013
2
Hello everyone

Can any one please point me in the right direction... I'm a new hobbyist, and taking up electronics after many moons in absence.

I have a caravan, with only one ventilation hole. I wish to mount 2 small 12v PC fans with a set up to have one fan suck fresh air into the caravan for around 15 mins, then turn off, and then the other fan set to extract air from the caravan to the outside.

I know I can simply wire a 12v relay that when powered (using a 12v timer set on 15min increments) can start one of the fans, when the timer switches off the power to the relay, it would switch on the other fan...

What relay do I need?.. Im lost...

Many thanks for any help...

#### Sensacell

Joined Jun 19, 2012
2,850
If the caravan has only one hole... you will have a hard time moving ANY air through.
Needs an IN hole and an OUT hole for fans to work.

#### TheManWithNoName

Joined Dec 16, 2013
2
If the caravan has only one hole... you will have a hard time moving ANY air through.
Needs an IN hole and an OUT hole for fans to work.
yes, i know, but unfortunately thats not an option...hence need to fit 2 fans working in 15 min increments...

#### crutschow

Joined Mar 14, 2008
27,395
The point is, any air going in needs a way to also get out. You can't readily do one for awhile and then do the other. In any event a small PC fan will move only a small amount of air compared to the volume of air in the caravan. If you are trying to remove hot air from the caravan they will have little effect on that.

#### wayneh

Joined Sep 9, 2010
17,153
Getting your caravan to essentially "breath" is not a valid strategy. You'll be very disappointed in the final result. Unlike your chest, the walls of the caravan cannot compress and expand in any appreciable amount. A small fan will not be able to pull any significant vacuum on the exhaust cycle or develop any pressure on the inhale. The scheme simply will not move much air.

Dividing the existing vent into "in" and "out" channels would give a far superior result.

BTW, what the heck is a caravan? I assume it's not a Dodge Caravan?

#### inwo

Joined Nov 7, 2013
2,419
Can you clarify the timing?

Both fans (in an out) run for 15 then off?

Or fans run all the time. One on for 15 while other is off,then the other on for 15 other off?

#### #12

Joined Nov 30, 2010
18,222
caravan is a British word for some kind of camper or recreational vehicle.

You can just have one fan blow air into the vehicle and hope there are enough cracks to let the air out somewhere else.
My car can blow generous amounts of fresh air in with all the windows rolled up, so it must be finding an escape route somewhere.

#### inwo

Joined Nov 7, 2013
2,419
caravan is a British word for some kind of camper or recreational vehicle.

You can just have one fan blow air into the vehicle and hope there are enough cracks to let the air out somewhere else.
My car can blow generous amounts of fresh air in with all the windows rolled up, so it must be finding an escape route somewhere.
Good idea. In or out!

That's the air exchange system in my house and house is extremely tight.

Joined Jul 18, 2013
23,342
What relay do I need?.. Im lost...
If you have an auto wrecker close by, you can usually buy a handful of relays for a couple of .
Max.

#### wayneh

Joined Sep 9, 2010
17,153
caravan is a British word for some kind of camper or recreational vehicle.
Ah yes, I recall learning that here before.
You can just have one fan blow air into the vehicle and hope there are enough cracks to let the air out somewhere else.
I do agree that a single direction is all that's needed. Reversing it on a time schedule will not improve performance.

I believe a fan blowing in will move a bit more air than one blowing out (same ∆P, denser supply), but the position of the vent relative to natural air flow is probably more important in choosing the direction of the fan. You'd obviously want to blow hot air out a vent near the top.

#### #12

Joined Nov 30, 2010
18,222
I'd say, "blow in" because you know where you're blowing in from. If you blow air out, a lot of the incoming air might be coming from very near the engine compartment. You wouldn't want to suck in air that has been heated by a recently used engine...or would you? I hear it's a bit chilly in England.

#### wayneh

Joined Sep 9, 2010
17,153
Good point - exhaust fumes are an issue to consider as well.

#12

#### vk6zgo

Joined Jul 21, 2012
677
A "Caravan" in Brit (& Australian) parlance is what would be called a "House trailer" in the USA.