The upper point you’ve indicated is where the heat enters the system from the external unit, after passing through the filter. My system has a temperature sensor attached at this same point. So far so good.
The lower point you’ve indicated is the bottom of the low-loss header (aka distribution header) and will be where water returns from your central heating system. This will be the coolest part of the system before it goes out to the heat pump, so might seem a reasonable place to put it.
I see two problems here: (caveat: I’m no plumber)
- this won’t see water returning from the heat exchange for the hot water, as it doesn’t go through LLH.
- due to how low-loss headers work, if the flow through the primary circuit (to the heat pump) is higher than the flow around the central heating system, then the temperature at the bottom of the LLH will be lower than was is actually being used.
On my system, the return sensor is placed after the primary pump (the middle one), behind the 3-way valve. In circulation terms, this is a short distance further from where it is fitted on yours, but also includes water from the DHW circuit.
I would suggest that the return thermister (T2) is placed just before or just after the primary pump (P1).
I think there are some pre-soldered receptacles for the thermisters on the pipes already.
