This one has stumped me for a while and I can't seem to find an answer.

Why are commissioning sets always specified as being in the return leg of a LTHW system?

Is there a logical reason or just tradition?

I think you may get an answer if you put in some US terminology? What is a commissioning set? and what is LTHW? what return leg? is this water? air?

Being Baltimoron background and not a Brit, I'd say it also depends on whether it's single coil or multiple coil, and 2-way or 3-way valves. Commissioning set would be the flow valve, Pete's plug, and drain. When setting the pump, you will need to know presure and flow. If on 3-ways, the flow is measured and trim set for the pump. If on 2-ways (and not using a recirc including temp control) the commissioning set would include a dP instead of a Pete's plug so the pump can run off VFD. 3-ways can also run off temp control, so commissioning set would include Pete's plug.

control valves should be located at the lowest pressure point in the system. this gives the greatest possible range and finest resolution, in other words better control.

maybe this "commissioning set" is our autoflow device, which typically contains the manual air vent. air vent always goes on the coil outlet, or upper connection. this is to prevent air binding.

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