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Author Topic: Should ceiling fans suck or blow?  (Read 4890 times)
Jeremy
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« on: February 24, 2009, 02:50:14 AM »

I'm talking about simple ventilation fans here, not air conditioning - should they blow fresh air into the bus or suck hot air out? Or would the ideal be to have some sucking and some blowing to have a flow of air through the bus? I've got a total of five openings in my roof, and I'm not sure how to configure them for best effect. In brief, I have two large opening skylights in the middle of the roof, one towards the front and one towards the back. The skylights don't have fans fitted, although I know it is possible to do so. In addition, there is a fairly small fan-powered vent in the middle of the roof about two-thirds of the way along the bus. The fan in this vent 'sucks', and the vent is of the 'permanently open' mushroom-type (which makes me wonder whether I will lose lots of heat through it).

Lastly, there are two large vents located in the centre of the bus, but at the sides of the roof above where the baggage racks used to be. These large vents opened on an air-operated system controlled by the driver, and had very powerful fans fitted to them that blew air down ducting in the baggage racks to the vents above each seat. I have removed the big fans and the ducting, and want to fit much smaller fans to the vents, but I cannot decide whether the new fans should suck or blow - although on one side of the bus the vent is located immediately above the kitchen area, so I guess that one should suck at least.

What do you think?

Jeremy
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« Reply #1 on: February 24, 2009, 03:27:56 AM »

Hey Jeremy, most RV vent fans are reversible. You walk in the RV and it has been closed up all day in the sun. Maybe not a problem in the UK?  It is real hot inside so you want to suck/blow the hot air out.

 Now you have me thinking, I cannot remember any reason to suck/blow outside air into the bus?? I do not recall ever doing it.

  http://www.fantasticvent.com/products/products.html     Link to RV vent manufacturer you will see most are reversible.
                                     HTH  Jim
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Jeremy
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« Reply #2 on: February 24, 2009, 03:59:32 AM »

Thanks for the reply. I had thought of reversible fans, but am not sure how it affects the type of fan to buy - I was actually leaning towards AC fans (cheap, efficient, silent, and fit the space well), but I have not seen these in a reversible format. I expect there are 24v DC fans out there that would do the job, but I've not come across them yet (although I have not really researched the market yet)

Jeremy
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« Reply #3 on: February 24, 2009, 05:43:23 AM »

Jeremy,

Reversible will leave you the most options since conditions will change whenever you travel or where your sitting.  Seems you would want the ducted one to lean toward blowing down the duct.

Just my tucense. Smiley
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« Reply #4 on: February 24, 2009, 05:52:38 AM »

I'd like to take Jeremy's question a little deeper and discuss usage of reversible Fantastic-like roof fans.  I have two installed (bathroom and bedroom).  I frequently use them on very warm days that are not quiote to the point of needing A/C.  

I almost always run mine in the exhaust direction.  If I come into the bus after it has been closed up and I need to cool things down a bit, I crack a couple windows open and turn the fans on. My rationale is that the heat is going to be up near the ceiling. I want to get that sucked out the fan openings, rather than pushed through the bus and out a window.

Sometimes I may only open the windows on one side of the bus, depending on an outside wind direction or the side that is out of the sun and cooler.

There have also been times when I have considered setting one fan to exhaust and the other pulling air in.  My thought would be to try to set up a directional flow from one fan to the other. Typically the bathroom fan would be set exhaust with the bedroom pumping in air to backfill.   That might be used to get rid of moisture after a shower, or to ventilate after some other bathroom activity I won't get into discussing.  

What do others do with their fan(s)?

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« Reply #5 on: February 24, 2009, 05:58:49 AM »

I've got 3 reversible  Fantastic roof fans and I love 'em!  Hot bus? Pull air in from the rear, pust air out from the front.  Vise versa.  You definitley want to be able to suck air out of the shower/bathroom area.

You could use single direction fans since you have so many options.  Draw air in throught the floor vents (powered or not) and pull air out from the roof.  That would cover most situations, and you could do that with AC powered fans.  Good luck!

Glenn
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« Reply #6 on: February 24, 2009, 07:29:31 AM »

I've always had the most success using fans to exhaust air out rather than blowing in.
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H3Jim
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« Reply #7 on: February 24, 2009, 07:50:08 AM »

I have two Fantasitc fans, and although they are reversible, I have alwasy used them to exhaust the air up and out.

If I had a ceiling fan, I'd probably want it to be reversible, so I could try it both ways, but I bet if I found it worked better in one direction, thats the one I'd always use.  The ones in my house seem to work better blowing up rather than down.

I am considering putting a ceiling fan above the bed, and as it turns out, one of the original bus esdape hatches would be just above and in front of where I  mount the fan.  I suspect that having the fan blow up, and the hatch open would be the best, get that cooler air coming in all the windows.

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Jim Stewart
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« Reply #8 on: February 24, 2009, 08:55:21 AM »

On my bus it isn't possible to open the windows, but the idea of having all the ceiling fans/vents 'sucking', and to have fresh air coming in at a low level somehow is a good one. I will also have AC incidentally, but only in the form of a fairly small unit mounted in one of the bays (actually a portable office-type AC unit that I already have). I don't expect to use the AC very often, but with it's big windows my bus is a bit of a greenhouse on hot days, which is why I'm concerned to have a decent ventilation system, and I'm at the stage of needing to make some decisions as I am currently working on the ceiling.

Thanks for the replies so far

Jeremy
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« Reply #9 on: February 24, 2009, 09:22:42 AM »

NOt sure how strong the sun is there, but in southern California, the sun coming in those big windows puts a lot more heat in than anything but the 9 tons of bus airconditioning could deal with.  When I'm parked, three roof airs would not be able to keep up with it.  I put front to back awnings on both sides, and that makes a HUGE difference in the amount of heat I have to deal with.  Just awnings and open windows is enough sometimes.
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« Reply #10 on: February 24, 2009, 09:27:33 AM »

We have one of the roof vent fans that is variable speed and reversible with a thermostat.  Seems to be very ineffective as fans go.  We carry a 24 inch box type el cheapo fan.  We set it against what ever window we want and blow out or in depending on conditions.  Many times more effective than roof mounted fan.

Typically if we come 'home' to a hot bus we put it in the bedroom and open a window/s in the front.  If we don't want to run the air all night we will put it up front and open a bedroom window.  Bedroom hot from driving during the day, (engine heat) put it in the bedroom and open the other bedroom window and move lots of air to cool it down quick.  

If I could come up with a good way to build a spot to put a box fan enclosure in the new bus I would.  Beats the roof mount fans and a bunch cheaper. The only likely spot is the rear cap.  Hoping that the much better insulation in the new rig will make it less of an issue.

Don 4107
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« Reply #11 on: February 24, 2009, 11:51:28 AM »

To me, this boils down to the basics of air circulation which are that a fan is an air pump.

Any pump needs and intake and an outlet.  If all fans exhaust the bus will be evacuated and will collapse or if all fans intake the bus will expand and explode (just kidding!)!

The bus is the air chamber so it needs both inlets and outlets.

As already posted, the main idea in a bus is to vent the heat from the ceiling which includes the bath and kitchen.

So, for hot weather in a bus, the ideal setup is inlets at the floor and outlets in the ceiling. Ceiling fans therefore should exhaust, the problem is where to have intakes.

Most passenger buses have some type of lower fresh air intake designed into the original heating  system. I use this on my 4104 although most all my windows are original and still open so I have a choice.

It is also possible to use one ceiling fan for exhaust and another fan or vent for intake but this is not nearly as efficient as having a floor intake. In this case it is not necessary to run both fans, just the exhaust. Or, if parked, just leave the door open and exhaust with all ceiling fans.
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« Reply #12 on: February 24, 2009, 12:04:14 PM »

I had two Fantastic Vents in my travel trailer plus a regular vent with fan in the bathroom.

If I turned on only one vent with all other vents and windows closed the fan would struggle because it couldn't move any air.  It was fine if the other vent or a window was open.
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Jeremy
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« Reply #13 on: February 24, 2009, 12:27:10 PM »

I suppose in many ways we should not even be discussing this as it should be a question that has already been answered by the original designers of the bus. Thinking about my situation, there are indeed air intakes at floor level, where the air was again ducted along both sides of the bus to serve every passenger. This system has both 'inside intakes' (to recirculate the existing air) and 'outside intakes' to bring in fresh air. The reason why I had not thought of this system in conjunction with the ceiling vents before is because the floor system is built around a couple of huge heat exchangers, so in my mind I had catagorized it as 'just' a heating system - but I suppose it will also act as an intake of cold air to replace that extracted by the ceiling fans. Obvious when you think about it, but it has taken this thread to make me realise it.

So the simple answer to my original question is that all the ceiling fans should suck.

Jeremy
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« Reply #14 on: February 24, 2009, 12:33:55 PM »

Hi Jeremy,

The conditions in NZ are very similar to the UK, so our experience is probably relevant to your situation.  Our bus came with two large powerful ceiling fans, one in the front & one in the back.  The fans are capable of running at 3 speeds in either direction.  So far we have used them almost exclusively to suck air out and have very seldom used them to blow it in.  Generally we use them on high speed for the first 10 min after we have opened the bus up & then if the weather is warm set them to low while we drive.

In the cooler weather, we found that we lost a reasonable amount of heat through the fans, so we made covers for them to seal them up.  Generally, we only noticed the heat loss, overnight when parked.

We have removed the OTR airconditioning & in NZ, don't think we will need it.  We just open the windows.  When we move back to Australia, we will probably install several rooftop units.

Cheers
    Peter
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Peter
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