Passive ventilation on a hot day?

Charlie Evans

New Member
Joined
Jun 27, 2019
Hello All,

I'd like to create some secure ventilation. When I'm in the van I can open windows. But when I leave the van on a hot day I often feel I can't leave the windows open. So I'm thinking about installing one or more vents in the roof and floor:

roof and floor vents.png

I don't have space for a full roof-light and I can't run wires to a powered vent (not without the wires being out in the open). I've considered autonomous solar vents, though I've seen mixed reviews about them and I would prefer a vent that can fully seal at roof-level. So, I'm considering passive ventilation via 'UFO' vents that can seal: https://www.vetus.com/media/magentominds/sasdocument/20130129144840_0.pdf or https://aquafax.co.uk/product/6-04481-ecs-ventair-shut-off-vent-cw-ss-cover

But would passive ventilation work? On a cold day it would work fine for sure:

static ventilation on cool day.png

But would it work in the sun on a hot day, when the outer surface of the roof gets very hot? Presumably the steel 'UFO' cover would get hot too:

static ventilation on hot day.png

There would be convection across the roof, but I think the air outside the van immediately in contact with the metalwork would be hotter and less dense that the air inside. So I'm afraid that nothing would move without a fan.
Can anyone tell me from experience, or from design principles, whether a passive vent like this, coupled with a floor mounted inlet, would help to cool my van on a hot day?

Thank you for any help,
Charlie
 

BigStevie

Active Member
Joined
May 17, 2017
Location
Highlands
Hi Charlie,

I suspect that you're going to get some differing experiences and ideas from others, I'll give you mine.
An opening roof vent for me is a must in any van, large or small. I'd be interested to know what size van you have?

In various VW Transporter builds I've usually put in a 280mm square vent such as the Fiamma (these can be bought with or without a fan). Vents such as the Fiamma also vent when shut and draw air through when on the move. Now on a larger van, I've a fan assisted vent in the bathroom area and a similar unit above the kitchen space.

My feeling is that if you want to let the hot air out, you need an opening in the roof. You mention you don't have room for a roof vent, sounds like a busy roof! Again, I think knowing the size of van/volume of air is the first step.

Not a lot of help I'm afraid, type of van would be a help to advise. Stevie
 

Charlie Evans

New Member
Joined
Jun 27, 2019
Hi Stevie,

Thanks very much for your reply. My van is a MWB Citroen Relay - L2H2, so not enourmous:

IMG_20180426_221017.jpg

I chose to not fit a full roof-light, because the popular models I found would stand too high for my roof-bars. Also, I put my van together for winter sports, so I didn't want a big cold-spot in the ceiling. I have four top-hung Seitz windows, so ventilation when cooking or sleeping hasn't been a problem. It's just on hot sunny days when I leave the van somewhere public that the van can get very hot (despite reflective window mats).

I realise that these mushroom/UFO vents are small, but I hope that with an inlet low down, through the floor ideally, some of the hottest air would go. Which would be better than none at all, as is the case now when all my windows are closed!

My main question is, when the air around the passive vent outside the van is hotter than the air inside, would any air move at all?

Cheers, Charlie
 

BigStevie

Active Member
Joined
May 17, 2017
Location
Highlands
Hi Charlie, Honestly, I don't know. Others here on the site may have more experience and have something to add.
I still think an opening roof vent is the way, you can have an insulated block to fill the gap beneath the vent in winter if you're worried about having a cold spot.
As the van's mainly for winter sports.....

Stevie
 

tacr2man

Member
Joined
Dec 13, 2017
The answer may be to fit the vents as you suggested and then fit a solar powered fan on an inlet at low level , therefore this would cause a pressurization effect inside the van to push the hot upper air out thru the vent . @Charlie Evans ps on really hot days the air close to ground is warmer than 2mtrs above .
 

Chris Shaw

Active Member
Joined
May 30, 2017
This is the view of my roof Relay L3H2. The smaller one in the foreground is directly over the bed. We keep that on the first notch at night to help stop condensation.

The small one in front of the solar panels is in the loo cubicle. it's a fiamma with a fan, use as appropriate;)

IMG_4793.JPG
 

Roadrunner

Active Member
Joined
May 29, 2016
Location
Glengormley
I had solar vents for a boat,they had a fan in them and worked well in hot sunney days,do have a look on some boat shop forums or ebay to find what you require.
 

Wissel

Active Member
Joined
Oct 19, 2017
Location
Cornwall
I have a similar issue.

The old Vito we have ready to convert is an ex crew bus, so already has windows but they don't open. I have a spare mini Heki that I'll fit, but would like lower vents to aid air flow.

Was thinking along the lines of using two silent PC fans to draw air in, but need exterior covers.

Any ideas?
 

Roadrunner

Active Member
Joined
May 29, 2016
Location
Glengormley
I have a similar issue.

The old Vito we have ready to convert is an ex crew bus, so already has windows but they don't open. I have a spare mini Heki that I'll fit, but would like lower vents to aid air flow.

Was thinking along the lines of using two silent PC fans to draw air in, but need exterior covers.

Any ideas?
Use a fridge vent or a stainles steel shower one from B/Q
 
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