I'm fitting a BWT water softener and the installation guides says the drain-off pipe must have a 20mm air gap, what does that mean? I am intending to put it in a stand pipe.
1. Stand pipe will be fine, just don't poke the waste pipe to the bottom of the stand pipe. 2. Depending on how often the regeneration is done, it is possible the trap at the bottom of the stand pipe will dry out. Very unlikely, but if it happens, pour some clean water down to re-establish the seal.