Kitchen & Static Pressure

[rjhorton]

In my MVHR retrofit plans (self-designed), I have a 7.5m semi-rigid duct run to the Kitchen where I'm intending to install a filtered extract point - something like this:

https://www.paulheatrecovery.co.uk/product/v125-fb-filtered-kitchen-extract-terminal/

I plan on converting the existing cooker hood fan to be recirculating, but we do use the frying pan a lot so I've assumed a filtered extract is a sensible measure.

While it will never be a passive house, I've considered PH guidelines and I'm therefore trying to achieve flow rates of 46m3/h and 60m3/h for standard and boost levels. I initially assumed the room would require 2x90mm semi-rigid ducts, but even at a standard flow rate this would give roughly 10Pa static pressure in the pipe, and then something like another 10Pa at the vent (based on technical data of the linked vent). This seems a lot if trying to keep to an overall static pressure of 80Pa (PH recommendation for <200m3/h).

I think I can squeeze in a third 90mm duct which would lower the static pressure across the three ducts to around 5Pa, and I've started looking for larger extract terminals. However, I'm not finding much in the way of off-the-shelf components for 3x90 plenums or vents - which leads me to think I may have made a wrong turn and other people are resolving differently.

Is anyone else designing for and achieving the recommended flow rates for Kitchens? Did you need three ducts? How did you terminate them?

 

Thanks

[JohnMo]

34 minutes ago, rjhorton said:

In my MVHR retrofit plans (self-designed), I have a 7.5m semi-rigid duct run to the Kitchen where I'm intending to install a filtered extract point - something like this:

https://www.paulheatrecovery.co.uk/product/v125-fb-filtered-kitchen-extract-terminal/

I plan on converting the existing cooker hood fan to be recirculating, but we do use the frying pan a lot so I've assumed a filtered extract is a sensible measure.

While it will never be a passive house, I've considered PH guidelines and I'm therefore trying to achieve flow rates of 46m3/h and 60m3/h for standard and boost levels. I initially assumed the room would require 2x90mm semi-rigid ducts, but even at a standard flow rate this would give roughly 10Pa static pressure in the pipe, and then something like another 10Pa at the vent (based on technical data of the linked vent). This seems a lot if trying to keep to an overall static pressure of 80Pa (PH recommendation for <200m3/h).

I think I can squeeze in a third 90mm duct which would lower the static pressure across the three ducts to around 5Pa, and I've started looking for larger extract terminals. However, I'm not finding much in the way of off-the-shelf components for 3x90 plenums or vents - which leads me to think I may have made a wrong turn and other people are resolving differently.

Is anyone else designing for and achieving the recommended flow rates for Kitchens? Did you need three ducts? How did you terminate them?

 

Thanks

 

Pressure drop calculation takes the worst case duct, not all the ducts added together.  So when calculating all the shorter lengths you can ignore, the longer lengths you calculate and take the worst one - that is is your pressure drop.

 

So you do during commissioning the longest run the terminal stays fully open and you adjust the shorter duct pressure drops with the adjustable terminal to get the flow you need.

[rjhorton]

@JohnMo

Ahh! Penny dropped! Thanks, that's much easier!
 

So presumably, the air going to each duct is then in proportion to the static pressure of each parallel route? Less pressure more volume?

And balancing is deliberately increasing the static pressure to rooms with lower volume requirements - to decrease the proportion of the volume they receive?
 

On that basis, is the total static pressure referenced by PH the sum of static pressure from the "worst" supply route, worst extract route as well as the intake and exhaust?