# Oversized pipework and low flow rates?

Hi Everyone,

I am designing a new ASHP system and was wondering if there is a negative side to oversizing pipework on the main pipe runs for air sources heat pumps?

We recently had 7m of 40mm Uponor EcoTherm insulated pex buried in our garden. The pex runs from the location the Air Source Heat Pump will be installed in the garden then through the foundation and into our plant room.

I have read that the ideal maximum design flow rate for main circuit piping is 1 l/m to reduce noise and the risk of pipe erosion.

My Design
My main pipe run from my ASHP starts in the garden and then runs through the 40mm pre-insulated pex with a flow rate of 0,34l/m at 43,8 Pa/m (43,8 x 14=613pa).

At the opposite end I have 2 UFH manifolds connected in two different rooms by 22mm copper pipe.

My plan is to run pipe from the plant room to the UFH manifolds in 28mm pipework. A total distance of about 6m with 4 x 90 degree bends.

22mm Pipework
0.99 l/m and 155,7 Pa/m
578.5 x 12 = 6942 Pa

28mm Pipework
0.57 l/m and 578.5 Pa/m
155,7 x 12 = 1868 Pa

If I use 28mm pipework my flow rate is 0.57l/m which is lower than the recommended 1 l/m ideal flow rate for the main run to the manifolds

My longest UFH circuit is run in 16mm pex and has a flow rate of 1.67 l/m with the total flow rate for the underfloor heating being 15.28 l/m

Is it better to go with lower flow rates or a higher flow rate and higher pump pressure?

I have calculated that the entire system would require approximately 900 l/h

My plan is to use a Vaillant AroTherm Plus 7kw ASHP that has a maximum flow rate of 1205 l/h.