Product system impact

Question

Hi everyone,

I’ve created a sequence of 6 processes (Process 1 → 2 → 3 → 4 → 5 → 6), each connected to the next through intermediate flows. When I create a product system using Process 4 as the reference process, the model graph includes upstream processes (1 to 4), and the impact assessment shows relatively higher impact values.

However, when I create a product system using Process 6 as the reference process, the model graph only shows Processes 3 to 6, and the impact results are lower, even though all the processes are properly connected and provide the required intermediate flows.

I’ve verified:

• The intermediate flows have matching names, units, and categories;
• There are no missing providers reported;
• All processes are set to be expanded (not cut-off or terminated);
• I’m using the same LCIA method in both cases.

Still, it seems like Processes 1 and 2 are not being included in the product system when starting from Process 6. Has anyone encountered something similar? Could there be a specific setting or modeling detail I’m missing?

Thanks in advance for any help or insights!

Answer 1

The connection of process 2 to 3 is probably a waste flow?

Comments

Thanks to your suggestions, I’ve resolved the connection issue — now the model graph correctly shows the full chain of processes.

However, I’ve noticed something strange:
When I create a product system using Process 4 as the reference, the impact results are higher than when using Process 6 as the reference, even though all processes from 1 to 6 are properly connected.

My question is:
Do the impact results for a product system with Process 4 as the reference include only Processes 1–4, while a product system with Process 6 as the reference includes Processes 1–6?

If so, I would expect the results for Process 6 to be higher, since it includes more downstream processes — but that’s not the case. Am I misunderstanding how the system boundary is defined based on the reference process?

Thanks again for your help!

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good - how did you resolve it? To your new question, yes, impacts are always calculated "downstream" for products and upstream for waste flows.