The gas venting pump surprisingly doesn't work on ammonia (a gas, at least under the conditions in factorio :). Not sure if this is intended, but which substances can be vented by the pump is a bit hit and miss, maybe this could be documented more clearly (or preferably, fixed).

Basically, the only ones you're supposed to be able to vent are stage 1 gasses, such as hydrogen, oxygen, nitrogen and chlorine, which are all made in pairs... the list was extended mostly to byproducts, like sulfur dioxide, and the list keeps getting bigger as people request it.

Ammonia was never added as a venting option because it was always something you had to make on purpose, but now I guess it's also a byproduct in the chain leading up to making RTGs.

Personally, I have this chain balanced, so the output feeds back to the input, then you don't need to put ammonia in there anymore, but the moment you start using productivity modules, that would break... so I guess I'll consider adding it to the vent list.

Yes, my problem started when using producivity for making RTG ingredients. Also, pure water causes similar issues, but at least there is a chance of re-using that elsewhere (but I guess with god modules there might be too much pure water).

You can use Ammonia elsewhere too, I think it's used for Fertiliser, and to make Nitrogen Dioxide if extended chemistry is turned on.