single 90mm : good for 15 metre run

[Post and beam]

My planned Potton is a 200 sq/m house. I think the longest duct run will be from the loft space to the far end of the living room. This will be 15 metres as best as i can estimate it.

A previous quote and design ( from a company i dont intend to use) said i need a house total of 60 litres/second.

Will a single 90mm duct be sufficient? If its good for the living room run then it should be good for everything else.

[JohnMo]

Our bedroom is circa 15m and has two outlets, you are likely to need 2x90, otherwise you will be setting fan to higher speed to get the flow to where you need it, with increased noise etc

[Post and beam]

Do you have 2 x 75mm or 2 x 90mm?

[Nickfromwales]

Choose where you'd want absolute silence ( yes, ideally everywhere ) and run 2x 92mm ducts. Consider which rooms would be adversely impacted by the increased airflow & audibility, when the system is boosting, and plan around that.

[JohnMo]

9 hours ago, Post and beam said:

Do you have 2 x 75mm or 2 x 90mm

2 x 90mm.  Whole house is 90mm. Double runs in master bedroom and lounge.  Other runs are single.

[Post and beam]

I am having a little difficulty understanding the pressure drop/ airspeed ratio thing.

If i am correct the magic number is less than 2 metres/second to achieve a quiet outlet. So, what should i expect per 15 metre run of 90mm with , say, 2 90 degree bends ?

[JohnMo]

Go on to manufacturer web site download the datasheet it will give pressure drops per meter per bend and by flow rate. It may also state the longest run acceptable.

 

But simple terms.

 

The longer the length of duct the bigger the pressure drop, the more bends, the bigger the pressure drop, more flow the bigger the pressure drop.

 

The bigger the pressure drop the more work the fan has to do to over come it.  So fan speed is increased - more noise, more electric.  So worse case you need a bigger unit to compensate.

 

Flow rate is to do with noise also, more flow more noise.

 

[Nickfromwales]

32 minutes ago, Post and beam said:

with , say, 2 90 degree bends

bends formed in the duct, or mechanical bends?

[Post and beam]

28 minutes ago, Nickfromwales said:

bends formed in the duct, or mechanical bends?

Hi Nick

Mechanical bends. I want to use the Ubink air excellent system.

[Post and beam]

40 minutes ago, JohnMo said:

The longer the length of duct the bigger the pressure drop, the more bends, the bigger the pressure drop, more flow the bigger the pressure drop.

 

The bigger the pressure drop the more work the fan has to do to over come it.  So fan speed is increased - more noise, more electric.  So worse case you need a bigger unit to compensate.

 

Flow rate is to do with noise also, more flow more noise.

 I get the principle, its the actual numbers i need to get my head around.

For example if my living room is 42 sq/m,  its a 15 metre run and i use 90mm round duct. will i achieve a quiet sytem at the required flow rate.

For this size room i actually expect that i will need either a double run or, more likely, 2 single runs to opposite corners of the room.

[JohnMo]

I would work out the flows you need in each room first. Then they are set.  A big room depending on how many people, needs a decent flow.  Although there are two of us and dog, the lounge is set for around 60m3/HR.  In general use CO2 levels are around 600ppm, but when we have 6 people in the room after an hour or so CO2 levels are getting to over a 1000ppm.  So happy with the setting.  Our lounge is 36m2 but 6m tall.

[Post and beam]

_{If i take the 60l/s as a valid whole house figure, is it usual practice to do the following?}

_{With no other previous knowledge to go on i have broken out the volumes of the rooms that require supply and taken their % of the total. Then used that to divide the 60 l/s into per room supply rates.}

_{So, for example my living room is 42 sq/m which is 38% of the total. 38% of 60 l/s is 22.8.}

_{From this ( if correct) i will need 3 x 90mm to keep just below the 2 m/s velocity.}

 

_{Please critique away guys.}