How far can flexible ducting be ran from misubishi ducted system

[Chris2005]

Hi All

I was asked today to give a price on a ducted system which I dont normally get involved in.

At the moment they have ceiling suspended units in the building but they want them taken out and a ducted unit to be installed for neatness.

But they want to install both units in a store room on the top then run flexible ducts to ceiling on that floor then down to bottom floor.

What is the max distance flexi pipes can be run from unit without losing its efficiency

And what would be the fire regulations on this matter.

I have told him to put in 2 cassttes and forget about the ducted system but he wont listen.

Regards

Chris

[sedgy]

hi chris,
if you are adamant about this then quote a ridicuas amount , it sounds like he is testing you , get back in controle, or he will walk all over you .
sedgy.< and subcrontract it<

[Chris2005]

Hi sedgy,

The problem is I dont do ducted so my knowledge is limited,

I would like to tell him if its possible or not to cover myself.

Thanks

Chris

[Kev The Tool]

hi, in my experience i have always been led to believe that flexi should only be used for final connections or to overcome obstacles ie: beams and cable tray etc. i always go in spiral or square duct for maximum efficiency. i could be wrong but i have never had any problems from clients.

[Chris2005]

Hi Kev,

Whats the max distace for the spiral duct from the indoor unit.


Thanks

[superswill]

Chris,

i am no expert in design but heres my best 10 pence worth,

take the air flow rating from the manufactures spec in the mode you will be using it: eg 1 L/HR in Low 2 L/HR in Med 3 in L/H High

run that though a duct loss calculator: http://www.freecalc.com/ductfram.htm as an example

now you end up with with the air changes for that space based on room volume and sir from the ducts being supplied, calculate this against the required duty required for the space and TAD DAR easy as that!!

nice download on this:

http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT%20Kharagpur/Ref%20and%20Air%20Cond/pdf/R&AC%20Lecture%2038.pdf

for my own piece of mind does that sound right?

[Chris2005]

Hi superswill

Thanks for the calculator, I will give it ago

Regards

Chris

[frank]

The ducted unit will come with a set airflow volume, as Paul said.

If you add 2 flexiducts to the supply and 2 to the return air, then the airflow will be split between the ducts, but, the split will not be equal unless the duct lengths, bends, diameters etc are equal.

So, first problem is: how do you regulate the air volume to give equal amounts into each room. As you say 1 room is on one floor and the other room is on the floor below. I would guess that the flexi ducts would be of different lengths - which means you will get different volumes of air into each room.


If you add control dampers into the flexi ducts to try and balance the flow rates, you will be introducing external static resistance to the system.

If you look at the specs for the ducted unit, you will see that it gives a volume flow rate against an ESR.

Heres the second problem:
If you add lots of ductwork, grilles and control dampers, the total resistance of these must be below the figure given by the manufacturer, or the unit's fans will be over powered and unable to push the air through the ducts to the grilles to give any cooling duty to the rooms.

With low air flow, you risk the evaporator coil freezing.

Problem 3:
Ducted units have a return air sensor in the back behind the filter and near the fan scroll inlet. This is the sensor that controls cooling. If you duct the air to different rooms, the air coming back will be a mix of the each room air at this point. So, if the rooms are not identical in size, the ducting supplying air into the rooms is not identical and the heat load in each room is not identical, you will get poor control temperatures in each room.

This can be overcome by fitting a remote air sensor, but you can only fit 1 - so which room do you fit it in?

Tell him this and if he still want to go ahead, get him to accept design responsibility - in writing!

The short answer is, yes, it can be done but the end result will be poor performance and a control nightmare.

By the way, flexi ducting gives a resistance per meter about 2.5 times that of metal duct - minimum. And, due to the shape of the flexiduct (internally) compared to relatively smooth metal duct, fluff and other airbourne contaminents tends to build up on the ridges at an alarming rate, reducing the airflow and increasing the static resistance.

[Brian_UK]

You will have to use insulated ductwork to help maintain conditions and handling lagged flexi can be a right pain.

Don't forget the fire protection requirements of the system as well. Fire dampers through the different floor levels and maybe walls.

Tell him he needs a properly designed air handling unit and ducted system.

[al]

flexi duct should only be used for the final connection to the outlet box, 1m max, figures are from specs supplied by local consultants.

if you go through a floor or fire rated wall then a fire damper must be installed and left accessable afterwards.

Solid ducting is your best bet, take the spec of the indoor unit to a ducting company and let them size and fit if for you, it will be more expensive than a cassette.

BTW if there is room for a cassette why not replace them with ducted units in these rooms instead?

alec

[Chris2005]

Hi All

Thanks for the replys,

For some reason the builder wants to put two units in a store room I was only chatting to him breifly today,

But I think he was more thinking of one unit running two seperate rooms to save cost which I told him wouldnt work.

I will phone him in the morning and tell him its a ducted system on each floor if possible or just forget about it,


Many Thanks everyone at least I no the pit falls know.

Regards

Chris