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Pipe length between supply and demand side?

asked 2025-07-03 03:04:21 -0500

updated 2025-07-03 10:33:38 -0500

Hello,

I’m wondering how (indoor) pipe lengths should be accounted for inbetween the supply and demand side nodes. If my understanding is correct, there needs to be a pipe object on the supply outlet and another pipe object on the demand inlet. Corresponding HVAC diagram:

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Are these supposed to represent the same pipe segment (as with constructions between adjacent thermal zones the rule is to model the same construction twice, but in the corresponding reverse order), or are they separate?

If in reality the total segment would have a length of L, should both be modelled L=L1=L2, or L1=L/2 & L2=L/2 respectively?

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Thanks!

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answered 2025-07-08 05:38:58 -0500

ASHRAE Appendix G modeling methodology does not allow modeling of pipe losses, and most modelers and UI's by default will use Pipe:Adiabatic instead of Pipe:Indoor when pipes are required in PlantLoops. You should use Pipe:Indoor only when you want to explicitly model piping losses. As far as the pipe length - EnergyPlus has no requirements that the pipes be of equal length

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The question has little to do with compliance modelling according to ASHRAE 90.1, this came up during a discussion with a mechanical engineer colleague exactly for the purpose of modelling pipe losses, as energy certification calculations in my country do account for pipe losses. Although in most cases these calculations are being simplified to a large extent, we would like to understand it more in depth to include pipe losses in our dynamic models.

So the total length of L1 and L2 should be equal to L, or one of the pipes can be adiabatic and the whole length can be on one side. Thanks!

furtonb's avatar furtonb  ( 2025-07-08 05:56:28 -0500 )edit
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It really depends on where the losses will be experienced in your building. What is the length of the pipe between the plant and the demand equipment? What is the quality of the insulation on that side? Between the plant and the demand equipment, there will be a larger temperature difference (in most buildings) between the fluid and the space temperature. Less dT on the return side.

The reason to model both L1 and L2 as Pipe:Indoor would be if there were different insulation characteristics, or if the pipes were exposed to different zones.

kwalkerman's avatar kwalkerman  ( 2025-07-08 06:02:32 -0500 )edit

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Asked: 2025-07-03 03:04:21 -0500

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Last updated: Jul 08