Add sensor for fixed target flow temperature

[lwis]

Useful for reporting given we don't have any live values for flow temperatures!

[BottlecapDave]

While this looks fine, I was wondering if this along with minWaterSetpoint and maxWaterSetpoint would be better represented as a water heater.

People can then see all three of these values, as well as set the values easily using the service you've already exposed.

[lwis]

@BottlecapDave I think the water_heater type is supposed to be used for DHW heaters, i.e. WATER zone from the API (which would probably be better represented as water_heater than climate).

These configurations are for space heating flow temperatures, and I'm not sure there's a good fit for those in HA.

[BottlecapDave]

These configurations are for space heating flow temperatures, and I'm not sure there's a good fit for those in HA.

That's fair enough. It should probably be exposed then as a standard number entity. I feel like if we're wanting to expose the value for reading and it's available to be set under an existing service, then it should be exposed as an entity that can be both read and set.

I think the water_heater type is supposed to be used for DHW heaters, i.e. WATER zone from the API (which would probably be better represented as water_heater than climate).

I have been thinking that over the last couple of weeks, and might convert to a water heater entity and deprecate the existing version. I've raised #1171 to cover this.

There is an open issue around boosting not working for water, but not sure if this has been incorrectly exposed or if the API call should be updated to not supply the target temperature (as it doesn't seem to be returned in the data). I've asked for more information on the matter, but you might be able to weigh in.