You would use PID if you have a temperature feedback from the rooms, but in your case you are not interested so it doesn't matter much. Sure turning off pump when not needed is a good thing. I have colleague that also bought a few netatmo devices to control floor heating of some zones of his house and he is very happy.
Does this help:
http://forum.mysensors.org/topic/748/manual-assigning-node-id-s-for-network-with-repeaters
I agree with you that the serial messages are not perfectly documented, but realise that this site has been made (as far as I know) by a bunch of people that share their 'love' for home automation.....
boozz
When you read the data sheet correctly you can see, that the distance between the pins is different for a AA and AAA battery holder. That`s why I was asking if somebody can recommend a holder that fits.