Residential ducted system advise needed

[kiwisteve74]

Hi guys

i have over 15 years experience in the service and installation of split system air conditioners, however I have done little in the way of ducted systems and on the occasions I have installed them the system has either been spec'd by an air flow engineer or the ducting has been installed by a separate contractor. I have been given an older Mitsubishi Electric SEZ-KD60VA unit including plenums but I'm a little unsure on ducting, supply and air return sizing. I intend heating 3 small bedrooms in my own home and this is what I have.

air supply plenum = 3 x round 200mm spigots.
air return plenum = 2 x oval 355 x 325 spigots with adjustable baffles and 1 x 100mm fresh air spigot.

my question is, will these plenums do the job? how do I calculate the size of the return air grill? Is it crucial or can I use the baffles to fine tune? I assume that my air supply outlets will be 200mm items as per the plenum? Is there a calculation I should be doing for flow rates etc?

Any advise would be much appreciated

thanks

steve

[frank]

Hi Steve

Looking at at the data sheet for your model http://www.mitsubishielectric.com.au/assets/LEG/MEE07K559_W.pdf, it's got an airflow of 640CFM or 0.3m3/s in new money.

With 3 x 200dia spigots this airflow will be 0.1m3/s in each duct giving a velocity of 3.18m/s...nice and quiet.

The unit can only cope with 50pa external resistance, so you will have to keep your duct lengths as short and bend free as possible. Use insulated flexible duct, as the ducts will sweat in cooling mode.

Connecting the return air ducts to the 2 x oval spigots will be OK. If you are returning the air from 3 separate rooms/ 3 grilles then each grille will need to be at least 200 x 200mm to allow the 200 dia ducting to connect to them. If you are using just a single return air grille, then you will need at least a 350 x 350 neck size to keep the pressure drop as low as possible.

Have a look at the attached grille sizing charts

[install monkey]

use steel galv ducting - insulated with typically 25mm duct wrap- foil backed insulation- try to use flexi duct only from duct to plenum box- short lengths- insulated flexi does cause increased resistance due to its insides being ribbed;)

[kiwisteve74]

Thank Frank and monkey, that's exactly the kind of info I needed. Something to ensure I'm going in the right direction.

The bedrooms are all off a common hall way and I intend using one large air return in the hall as it gives me a large hole to pass the unit into the ceiling space through. I can get a custom built hinged egg crate style made but need to work out best size for the system. I will only ever use the system for heating and intend using flexi insulated ducting feeding 200mm adjustable round defusers. The length of the ducts depends on my next question which is the location of the supply outlets. The rooms are only small and each run will be under 3 meters.

All bedrooms have glass windows at one end, do I put the outlets as close to the centre of the ceiling and hope for an even distribution or do I place them as far as possible from the air return ( by the windows) so that the warm air is sucked from one side of the room to the other?

[frank]

The further away from the return grille will give you a better spread of the heat.

One thing to remember though, If you put the return grille in the common hall/landing and close the bedroom doors, you will prevent the return air escaping from the bedrooms and the efficiency of the system will go down.

[install monkey]

best to duct the return air from the landing onto the unit- otherwise your using air from the loft space- which can decrease the performance as the loft can be typically 45deg in summer and 17 deg in winter:D

[kiwisteve74]

The further away from the return grille will give you a better spread of the heat.

One thing to remember though, If you put the return grille in the common hall/landing and close the bedroom doors, you will prevent the return air escaping from the bedrooms and the efficiency of the system will go down.

This is a valid point frank and I have considered putting transfer vents above the doors or skimming the bottoms of the doors.

Alternatively

Can an I put a 200mm round defuser in the centre of the ceiling in each room and a small square egg crate air return with a 200mm neck in the corner of each room? I'm a bit concerned that the warm air will short cycle between each vent instead of spreading around the room? Keep in mind my rooms are only 3.8m x 2.5m with a 2.7m ceiling height. I could place the supply outlet by the window which is another 1.5 meters away from the air return if you think its a better option?

Im im just not sure how the warm air will flow in the room?

[install monkey]

in heating mode the air will always collect at the ceiling height, by skimming the doors or fitting a transfer grille- then 1 common return air on your landing will allow air to collect in the room improving warm up rate

[Brian_UK]

I will only use it for heating
Yeah right, you wait until they find out that it can cool down their hot, stuffy bedrooms in the summer.

Might be worthwhile fitting the drain to the indoor unit jus in case. ;)

[kiwisteve74]

Yeah right, you wait until they find out that it can cool down their hot, stuffy bedrooms in the summer.

Might be worthwhile fitting the drain to the indoor unit jus in case. ;)


I always fit the drains regardless, ya never know when someone will bump the controller to cooling. I live in a part of the world where cooling is definitely not needed. The lowest outside temp we get in winter is around freezing point, we had 2 frosts on the lawn last winter. This summer is nearly over and we only had about 3 days over 25 Celsius.

[kiwisteve74]

in heating mode the air will always collect at the ceiling height, by skimming the doors or fitting a transfer grille- then 1 common return air on your landing will allow air to collect in the room improving warm up rate

Thats what I thought, pump the hot air in high and suck it out on the opposite side of the room low. Common sense tells me this will allow the warm air to fill the room before it's sucked out. Is this common sense or is there a trap to this?