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Old 11-06-2012, 04:28 PM   #1
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DIY Air Supply in Africa


Greetings to all from South Africa

I am 'almost' certain this could fall under the category of HVAC, but if I got it wrong, please forgive me and re-direct me.

Am looking to DIY a ventillation system for our home. Located east coast of South Africa, brick building, concrete tiled roof, temperatures range 10 to 40 celcius ( all year ) and relative humidity usually 70% or higher.

Problem we are having is mould. I have noticed in a few rooms where ventillation is good, the problem is less, so am guessing that a better air flow would reduce the mould presence and smell in other rooms. Due to a number of reasons, we can't leave windows wide open all the time to get the required air flow ( security, excessive winds from the sea, monkeys invade the house, etc ).

My thinking is to use a centrifugal fan ( like this http://za.rs-online.com/web/p/centri...lowers/7013586/ ) 230v, 133 x 270 mm, 890m/h, 86W, 0.64amp.

Since my workshop is elevated higher than the house ( floor of workshop level with top of house walls, and only 3 meters apart ) I was thinking to have a series of air filters on the input to the blower, and then the blower connected to a duct bridging to the house.

Question now is : is it better to run sheet metal duct around the outside of the house ( under the roof overhang ) and feed the air into the 3 bedrooms through the existing air ventillation bricks ( just under ceiling height, about 3 x 5 inches ), or run the ducting into the ceiling cavity, split into 3, and install a vent in each room ?

My intention is to create a slightly presurised house ( at least in the sleeping area where the problem exists ) and to create sufficient flow of cleaner ( reduced mould spore count ) air. The air should follow the path of least resistance and exit down the passageway and out the lounge / dining / kitchen area.

My thinking if this works, is later to add a return feed from the lounge end of the house, back to the workshop. A simple temperature control managed by an Arduino micro controller would determine the air temperatures and, if (for example) I set the preferred temperature at 25 celcius, and the outside incoming air ( through the filters ) is below that, and the return duct exhaust air is warmer, then divert the air through a heat exchanger to use the exhaust air to warm the incoming air.

Any advise or suggestions would be really appreciated.

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Old 11-06-2012, 04:53 PM   #2
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DIY Air Supply in Africa


Monkeys invading the house? Wow, I am at the other end of the world in Canada and we have polar bears to worry about. LOL. We can get to -40 deg C where I am.

I would go with the run the duct in the ceiling in a straight run and then takeoff the 3 runs. It will be easier to balance the air flows into the rooms (put balancing dampers in) and exposed ductwork outside can get corroded, pickup moisture or dirt, needs more elaborate sealing etc. I love the idea of the energy saving ECM motor it has and the speed control should work very well too. Sounds like a good plan you have.

Cheers.

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Old 11-07-2012, 01:53 AM   #3
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DIY Air Supply in Africa


Hi Yuri

Many Thanks for the reply.

Questions if I may :

1. would I be correct to assume that I need solid / fixed duct from the blower to the roof cavity, and then from the 3-way splitter flexible hose to each room vent ?

2. with the outlet of the blower being about 100 x 230 mm, should the fixed ducting be about the same size, or is bigger better ?

3. not being experienced in this field, how would I determine if that blower is powerful enough to push air through the ducting, and what air flow reduction will be caused by the ducts. My concern here is possibly wasting money on a blower / fan that is not suitable.

The spec says 890m/h and I am going to split to 3 rooms, each about 75m in volume.

edit : ( just thought this may influence the answers ) ducting will be about 6 meters solid duct with 2 x 90 degree sections from the blower to the roof cavity splitter, and the after the split, 3 lengths of flex duct, each around 7 meters.


Regards

Last edited by DaveOB; 11-07-2012 at 02:11 AM.
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Old 11-07-2012, 08:26 AM   #4
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DIY Air Supply in Africa


Quote:
Originally Posted by DaveOB View Post
Hi Yuri

Many Thanks for the reply.

Questions if I may :

1. would I be correct to assume that I need solid / fixed duct from the blower to the roof cavity, and then from the 3-way splitter flexible hose to each room vent ?

2. with the outlet of the blower being about 100 x 230 mm, should the fixed ducting be about the same size, or is bigger better ?

3. not being experienced in this field, how would I determine if that blower is powerful enough to push air through the ducting, and what air flow reduction will be caused by the ducts. My concern here is possibly wasting money on a blower / fan that is not suitable.

The spec says 890m/h and I am going to split to 3 rooms, each about 75m in volume.

edit : ( just thought this may influence the answers ) ducting will be about 6 meters solid duct with 2 x 90 degree sections from the blower to the roof cavity splitter, and the after the split, 3 lengths of flex duct, each around 7 meters.


Regards
We use cfm, cubic feet per minute for our fans and sizing in North America so you may want to convert your figures for us or the guys won't really know what you have or can help.

I would use solid duct as flex duct or corrugated flex duct has too much pressure drop and is easy to kink and is less sanitary inside. Dust collects there easier and if that gets damp then mold builds in there. Try use solid round pipe to the rooms as much as possible for the same reasons and put dampers in the takeoffs for balancing.

I don't do duct sizing but there may be other guys here who have a ductulator/calculator and may help. You want to go full size from the blower as far as possible and then takeoff to the rooms. Google: duct sizing, duct sizing calculator and I am sure you can find some info.
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Last edited by yuri; 11-07-2012 at 08:32 AM.
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Old 11-07-2012, 09:32 AM   #5
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Thanks Yuri

OK. When in Rome .... I have converted to imperial sizes :

The spec says 524 cfm (890m/h) and I am going to split to 3 rooms, each about 2650 cf (75m) in volume.

Ducting will be about 20 ft (6 meters) solid duct with 2 x 90 degree sections from the blower to the roof cavity splitter, and the after the split, 3 lengths of solid round duct, each around 21 ft (7 meters).

If anyone that can assist with correctly calculating ( pref with an explanation or formula, so I can learn from this exercise ) the pressure drop would be extremely grateful. I don't really want to risk a beginners mistake with this kind of calculation.

The spec sheet for the fan does have a curve chart for Pa vs. m/h, so I am assuming that if I can get a pressure value for the ducting, I would use this on the Pa side of the chart to determine the air volume that I can expect to get in the rooms.
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Old 11-07-2012, 06:56 PM   #6
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DIY Air Supply in Africa


Google: duct sizing, duct sizing calculator. Lots of info out there. I would use 6" diameter solid galvanized pie for the runs to the rooms if possible. Will reduce pressure loss and the air coming out won't be as loud or whistle.

http://www.engineeringtoolbox.com/si...cts-d_207.html

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Last edited by yuri; 11-07-2012 at 06:59 PM.
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