Supply fan static acting strange

[remp]

I have an new HVAC system consisting of a York fan coil unit (split type) doing 2m3/s.
I also have a return air fan taking air from the room directly through extract grilles and delivering it into the back of the fan coil unit plenum box.

I have the return air fan so as to help the Yorks supply air fan since it is a forward curved unit and has low static capabilities. There is quite a bit of return air ductowrk.
This is where it gets strange.... the pressure in the supply duct goes down when the return air fan is running. When it is off the dischagre pressure is 80pa. When the return fan is ON the supply static is 55pa. I thought it would go up and not down.
I have not measured the air volumes yet but it seem strange.
Any explainations on this?

 

[MintJulep]

Measured wrt where?

At any rate, your return fan is lowering the pressure on the entire return side of the system, and in the space, and in the supply. In effect, you are "pulling" air through the supply.

 

[willard3]

With fans in series. the pressures are additive. You have fans in series.

 

[imok2]

Ib the return fan running backwards?

 

[KiwiMace]

Building on MJ's response: This is easier to visualise with a pressure diagram. Example on Page 35-6 of the 2005 ASHRAE Fundamentals.

Your supply fan needed to create a pressure that would overcome both the supply duct, space losses and return duct ie. 80Pa.

When you add a return fan, the supply fan is equalising with the space pressure 55Pa, and the return fan is looking after the return ductwork. So the static pressure measured in the supply duct (and your space) will drop.

 

[Drazen]

i see no complication, as willard said.

static pressures are addictive for your series connection of fans.

the real question is what you get on flow rates. The flow rates are why you installed your additional fan, and static pressure is secondary by importance in this particular case (alhtough that can be problem if your additional fan is too strong and overcomes the power of york fan. that can be determined by measuring differential pressure of york fan.)

 

[imok2]

You may improve your situation if you remove the return air fan or you can increase the return air flow to suppliment the return side of the supply fan or to put it another way, the return fan is too small

 

[quark]

It is not strange considering constant volumetric flowrate in both cases (i.e before installing the return line fan and after installing it).

The total system resistance is 80Pa (assuming this is the fan discharge pressure at your given flowrate). Now, as you still maintain the flowrate constant and only add an extra fan in the return, the system resistance is shared by both fans and the individual pressures come down, still satisfying the rule of static pressures of fans is additive in series.

My suggestion would be to check whether the supply fan is operating in the unstable zone. You can simply forget this issue if you automatically maintain the flowrate in the system.