I wish to de-noise my render using the compositor but I found half-way that I have no environment or emission layer selected. As rendering takes time, I wish to render only that two in order to be able to combine the passes again later on after I adjusted the noise of my indirect passes. Question is, how to render only some given passes without rendering the combined pass?
Thanks, plan to test it as soon as the current render finishes 
No luck, the combined pass appear in the render result and it doesn’t say it would take less time to render 
Environment and emission are merely split-out components of the combined pass, they are not generated in a separate step and thus cannot be rendered independently. However, since they tend to require lot fewer samples, you can re-render with something like 16 samples and get those two passes, and use them with your higher-sample render that you already have.
Render out with fewer passes as mentioned. But if you gave visible emission like TV screens or something, I’d make sure to add a material index for that as well so you can obtain a mask. The z buffer from the original render may be sufficient to mask in the environment.
Thanks for the hints, lads/lasses.
I did just that, made a separate scene and changed all the materials to a cheap one and rendered all again with like 20 samples. Finished quickly and could composite the whole lot 
thanks for all of your help.
So far as I have ever known, “you are stuck with ‘combined.’” :spin:
Put your five bucks on the table and wait it out. You have no choice.
I asked this question a couple of years ago. The response from those more technical that I was that Blender will calculate the Combined pass whatever is selected. No way round it. The selections are only on the output.