I saw your request for help on categories on the main page talk page...
Happy to assist. You can email direct if you want (john "at" internode dot co dot uk)
You will also find me and other diy wiki contributors on the newsgroup uk.d-i-y See the Newsgroup access tips page for details on access to that.
The easiest way to place things into categories when using the visual editor is with the "page options" menu toward the top right of the editor between the "help" and "switch editor" icons. There you can search for and chose an existing category, or simply type the name of a new one and you will in effect create it just by adding an article to it.
It would probably make sense to create a category named "Heat pumps and Aircon" or something similar, since it seems likely we will get more over time. Categories can also be included in others - so the "Heating" category could include the above as well as boilers etc.
OK I have had a play about renaming and adding things to categories. Hopefully I have not messed it up too much! I have tried to follow the normal convention for DIY project articles by naming them to indicate what the project was, rather than the more technical "reference" type articles with less descriptive names. Anyway, here is the new category: