Supplementary point: reading this thread sent me to look at my Melcloud results in the 'Hourly Temperature' report to find that it had been regularly cycling through the night because even here in rural Somerset it has been cold enough at night (-5C) here to cool the bedroom with the thermostat to the point where it calls for heat. I've relocated that thermostat to our (much warmer) bedroom and need to keep an eye on the temperature downstairs.

No pretty sure it's the ambient outdoor temp, states in the manual - see attached screenshot

If you set it to 3 degrees....

When outside air temperature gets to 3 degrees, it starts the circulation pumps and monitors the flow (I assume it uses the RWT sensor) temperature . If the flow drops to 15 deg, the ASHP heats it to 20 deg. And repeats ... The 15 and 20 are not adjustable.

This won't use a lot of energy but may be annoying if you want a silent overnight setback.

That's what mine does anyway. 😀 

Surprise on Variable Freeze Stat function. As we know from this thread there are 2 parts to the freeze stat function -

1. To circulate the water so as to displace the colder water in the outside unit and pipework.

2. To reheat the circulating water if it drops below a certain temperature. (Factory set at 20degC.)

And as stated above we had ours starting to reheat at 15c flow temperature and reheating to just above 20c.

However it was -3c last night and I’m monitoring night time reheats at the moment and saw this reheat. So the Freeze Stat Function started a reheat of the circulation water at 20c which is 5c earlier/warmer than expected and took the unit into a defrost 22 minutes later, followed by a reheat up to 40c.

As far as I am aware this was entirely due to the Freeze Stat Function changing its operating parameters based on the protection needs of the system.

the function finished with another defrost then stopped leaving the flow and return temperatures at 30c.

Interesting to note how quickly the temp then drops while the circulation pump continues to operate and it’s -3c outside. 

Incidentally I only have MELCloud and I don’t have specialist data collection on our HP but I’ve worked out a way of approximating the HP OUTPUT just from the MELCloud graphs but it requires an accurate flow rate to calculate it. Here is a graph showing some output values..

Obvs. if someone knows these calcs don’t add up or any thoughts let me know.

The full 24 hours outside and inside temperatures can be seen on this graph.

The anti freeze valves are not supposed to be  insulated but they can be sheltered from wind and rain. This should mean the more exposed thermostat will reach 3C well before the valve does. 
Is that your post on the Facebook page?

I am trying to adjust the freeze stat from 7° tob3°. I have found how to get into this in the settings but it comes up with System shutdown  Y/N.  Do I have to shutdown to adjust this and will it be easy to restore?  

Thank you so much again.

yes and yes.  It will either start up as you exit the menu or you just press the holiday/standby button to wake it up.  Nothing to worry about.