The Haber Process consumes Nitrogen and produces Ammonia at full rate as long as there is any steady supply of Hydrogen(from electrolysis or atmo processor), no matter how small.
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Also, if Hydrogen runs out and the steady supply is interrupted, Haber Process continues eating Nitrogen and power but produces nothing.

Confirmed working on 1.20.17. Thanks

It should be fixed next release, please verify that it does

Tested on 1.20 release. The process refuses to work with "insufficient storage", even though I have room for Ammonia in both liquid and gas form.
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Though strangely the ISRU process window correctly identifies spare Ammonia capacity.