Airflow issue, balancing problem or should we have used next duct size down?

[back2space]

Hey all,

I have low static ducted units fitted. Downstairs there are 2 units that literally have a return grill and a supply grill (similar to a hotel room kind of install) they are boxed in but there is no ductwork so there is perhaps 5PA of static pressure if that but probably closer to 0. The units are a 1200btu and a 9000btu. They work fine no problems at all.

The upstairs units however are truly ducted as they are installed in the loft space. Both are 9000btu. The master bedroom unit has 1 x round supply outlet and 1 x round return grill and for this room a very short duct run. The duct is 250mm and approx total is 1 meter duct run on supply side and 0.5m on the return side. This unit works equally well in heating or cooling but I have had to boost the static pressure of the fan on this unit to say 20PA.

The 2nd unit though serves two rooms (spare bedroom & bathroom), I use the same style and size return and supply grills. The supply plenum off the front of the unit has two outlets (both 250mm round). There is a mixture of solid duct and small amount of flexi duct, totally duct run for this room is approx. 4m. This unit works well in cooling as cold air falls and spreads across the ceiling. The supply grills are these… https://www.koolair.com/en/catalogue...-de-palas-dvp/ and contain a wax element that responds to the supply air temp. In cooling the blades flatten out to throw the air across the ceiling but in heating the blades point downwards to push the warm air towards the floor (so no having to adjust the angles of the outlets as it does it automatically)

However the airflow from this 2^nd unit seems to be weak. If I close off the bathroom supply damper fully (normally open say 15%) then this unit performs as well as the master bedroom unit and the air can really be felt. The supply duct for the spare bedroom outlet doesn’t have a damper so it is always fully open as this rooms needs as much flow as it can.

The airflow just doesn't seem to push down to the floor enough. I have adjusted the static pressure for the fan to the full maximum 50PA but this just creates lots of fan noise and doesn't seem to do that much.

It seems as if the fan cannot pressurise the duct enough to produce enough velocity for the airflow and in turn the airflow just seems to come out at a slow speed, again fine in cooling but hot air naturally wanting to rise does not help heat the room as well. Ceiling area gets very hot but floor area remains cold.

Would fitting a damper on the other supply duct for the spare bedroom and adjusting this to say 85% open and the bathroom being at 15% open balance it out? Or would this just increase the pressure behind the damper and after the damper the pressure would just reduce and it would be the same? Currently the bathroom only needs a small amount of airflow due to size, but the air flow is very weak as well. Again not sure if fitting dampers on both outlets would achieve anything? The supply grills had a perforated plate in place to smooth out the airflow but this just held back the airflow even further in heating mode so I have removed these.

I can't help thinking that I should have gone for 200mm ducting and supply outlets instead of 250mm. Although I wanted to reduce the static pressure as much as possible and when I installed the system I wanted to reuse some 250mm outlets that I had.

Any help gratefully received.

[Slim R410a]

What is the air volume for the 2nd unit? (9000btu) Or make/model.

[back2space]

It's an LG CL09R.N20 the CL12R.N20 seems to have the same airflow.

10.0high / 8.5med / 7.0low in m3/m. Although putting the static pressure up increases the airflow you can go between 0-50pa

Interestingly the VRF version which is a mid static unit has the following airflow and can be adjusted between 20-147pa.

9.5high / 7.5med / 6.0low in m3/m

https://www.spaceair.co.uk/uploads/f...0One%20R32.pdf

[Slim R410a]

Ok so 167 litres/sec divide by 2 - 84 l/s per duct branch/grille.

Your velocity on each 250dia branch should be around 1.6m/s. So although you will be getting the correct air volume down each grille, the velocity is very low so, like you have noted, have trouble pushing the air down from the ceiling.

I always size at 1.0Pa per m, so for example on that unit I would have a 250dia header duct (3.5m/s), then 2x 200dia (2.5m/s) boots off this to feed each room.
Or, 2x 200dia branches straight off the FCU a supply air spigot.
2x 180dia branches would be approx. 3.5m/s velocity at 84l/sec each.
(this is based on solid spiral wound ductwork with flex connections to grilles at 500mm long)

[back2space]

Thank you Slim that's really useful.

So without enduring a lot of cost as to downsize the diffuses are about £100 each as they are specialist.

The supply plenum on the front of the ducted unit has 250mm spigots, would it be worth putting a 250mm reducer on the front of each one (there are 2 one going to each room) then replacing existing 250mm with 200mm as you have mentioned above, finally I could put a reducer back in on each end to go from 200mm back to the 250mm diffuser outlets?

Or will I not achieve anything this way as the outlets are still 250mm? Or perhaps it would as the air would then come out faster due to having built up velocity in the duct?

[Slim R410a]

Thank you Slim that's really useful.

So without enduring a lot of cost as to downsize the diffuses are about £100 each as they are specialist.

The supply plenum on the front of the ducted unit has 250mm spigots, would it be worth putting a 250mm reducer on the front of each one (there are 2 one going to each room) then replacing existing 250mm with 200mm as you have mentioned above, finally I could put a reducer back in on each end to go from 200mm back to the 250mm diffuser outlets?

Or will I not achieve anything this way as the outlets are still 250mm? Or perhaps it would as the air would then come out faster due to having built up velocity in the duct?

Yes that would be an ideal starting point without changing the grilles.

[back2space]

Do you think both outlets need a balancing damper on? Even though the 2nd outlet is only throttled back to about 15% flow.

Dropped you a PM mate.

[Brian_UK]

When balancing at least one branch should be fully open, this branch is known as the 'index'.

If too much air is coming out of the terminals after the airflow rates have been balanced then the total airflow needs to be reduced back to the design requirement.