Hydronic system with supply and return connector - which connector gets the pressure?

[HVAC-Novice]

For hydronic devices (boiler, coil etc.) there is a return and supply pipe connector. Both will go to the same pump. The pressuredrop of the device could be assigned to both (half the value), or only return, or only to supply... it shouldn't really matter. but now i wonder if there is a best practice or if there is a non-obvious difference? 

I usually have the pressuredrop (e.g. 1 psi) and determine half of it and assign 0.5 psi to return connector, and 0.5 psi to supply connector. Since i need the supply and return piping system to size my pump that works out to be 1 psi. 

But now I wonder if I skip the step of doing the "half to each connector "and just assign the 1 psi to supply (or  return) and nothing to the other connector. It would end up 1 psi for pump sizing (0psi + 1 psi = 0.5 psi + 0.5 psi). Does it matter at all? Is there some best practice? i think I'm over-thinking this. But I also may be missing something. 

[iainsavage · Accepted solution]

For pump sizing in a circulation system it doesn’t matter.

[dyp4f · Accepted solution]

Revit probably does not care about that, but if we see it from the flow aspect, there are 3 segments:

1. from the pump outlet --> to the coil inlet (pipe)
2. from the coil inlet --> to the coil outlet (the coil itself)
3. From the coil outlet --> to the pump inlet (pipe)

At the point of coil inlet, the fluid has not yet any pressure drop due to the coil.
It has a pressure (let say: P1) which of course is reduced in comparison to the Pump Outlet Pressure, due to the friction on the (1st) pipe segment.

At the point of coil outlet, the fluid has already an even reduced pressure: P2 = P1 – DPcoil.

So I think the most appropriate point to assign the pressure drop DPcoil , due to the flow passing through the coil, is the outlet connector.

[HVAC-Novice]

Thanks to both of you. Maybe I "standardize" on the outlet (return). That way I save the added "half pressure" parameter. It doesn't have to be consistent between families, just for that family. I think dyp4f's reasoning could be applied to air devices as well. 

For fittings and pipe accessories I never thought about this issue and just assign the pressure to one connector. But for those we stay within the same piping system. But for mechanical equipment I for some reason saw the need to assign half to return and supply. I think many years ago this was my way of dealing with the fact that a piping system always consist of 2 systems (return and supply).

[iainsavage]

My answer was specifically in relation to pump sizing.

If you're doing a more detailed analysis or plotting a hydraulic gradient then obviously the point at which  the loss occurs is more important.

[Kevin.Lawson.PE]

I think you want to assign 1/2 the pressure to supply and 1/2 the pressure to return.  When you run your pressure loss reports, you are going to get two reports, one for supply and one for return. In order to size your pump, you want to be able to just add supply dP + return dP.  

If you don't assign 1/2 pressure to supply and return, the pressure loss reports might return different paths as the critical path.  Let's assume we have 2 VAVs, VAV-1 (coil pressure drop = 5') and VAV-2 (coil pressure drop = 2').  Let's assume the supply and return piping to VAV-1 is 3' and the supply and return piping to VAV-2 is 4'.   The true critical path for this system is VAV-1 with a total pressure drop of (3'+5'+3' = 8'). If you put 1/2 drop on each Revit will see this as 3'(s) + 2.5(s) + 2.5'(r) + 3'(r). However, if you put all of your pressure drop on supply, revit will think the critical path on the Return side is to VAV-2 and will give you 3'(s) + 5' (s) + 4' (r). 

I think that's why the getting into the flow documents tells you to put 1/2 pressure on each connector. 

[HVAC-Novice]

sorry for the late response, I was out of office. 

You give me a lot to think about and using (my original) method of 1/2 to return and supply now makes sense. But for the critical path we still could have different critical paths for return and supply since the control valve is on the return, and strainers, check valve etc. could be on supply or return. So the return and supply still will be different. 

But it likely isn't as bad since the control valves i use all are PICV with 5 psi, so that doesn't' change which branch is critical. but sure a thing I will pay attention to and se if critical paths for supply and return are different.