One forum for general plumbing to include heating radiators, pumps and motor valves!
The other for boilers and gas fires and instant water heaters.
Dear diynot my heating doesn't come on. Can you tell me whether it's the boiler or a motorised valve so I can put the question in the right section??!!??
Softus' split looks about right to me.
Water supply systems, like tanks with pump sets, or accumulators, lead to hot water, so straddle the two. I'd put them in with the Heating topics.