This is part of the Grant's 'frost protection' system (as opposed to the 'defrost cycle' which we're all familiar with in this type of weather!), which runs the system pump continuously once the outdoor temperature goes below 4C, and stays on until it rises above 7C. There are a number of triggers for the system pump for frost protection, but this seems to be the common one.

You have some control over the 4C/7C mentioned above (parameters 43 11 and 43 12) but I've left mine on their defaults. In the cold snap in December my pump ran for over 10 days continuously, but it's just 100W (or thereabouts), so small fry compared to the rest of the system.

Thanks for the reply, and it's what we thought at first but the frost protection symbol doesn't come on. Also, it only started a few days ago (i.e. not in the first cold snap at the start of December). The outdoor temp that triggers this seems to be 5C but just waiting for it to reach this outside to confirm. Another anomoly is the outdoor temp reading on the control panel looking a bit crazy - last night it was 1.5C, then 15 mins later it was 7.5C, then another 15 mins fell back to 2C 🤔 

Parameter 43/11 is currently 0C; 43/12 is 1C.

As Mike suggested, the system pump turned off at outdoor temp 7C but how can this is be if 43/11 is 0C and 43/12 is 1C - and why doesn't the frost protect symbol come on...?

I've just checked on my controller and Frost Protection symbol isn't on, even though Frost Protection is running. The explanation of all of the symbols is possibly not accurate/complete in either the Grant or Chofu documentation? e.g. the 'black tap' symbol flashes if in DHW mode but Comfort temp has been reached (so no DHW heating is actually needed), and this is not mentioned in the docs.

As an aside, I've also noticed that the Controller gives temps to 0.5C granularity, but the modbus interface only to the nearest degree. Which is annoying.

That's interesting - stupid question here but how do you know frost protection is running if the symbol isn't on?

I'm close enough to the pump to hear it running, but the compressor isn't on, just the pump. And it ties in with the outdoor temp being between 3C and 7C.

tbh, I only really noticed it when I started logging all of the parameters to a database and graphing the output, so could see this 100W of background usage when nothing else was happening, and it tied in to outdoor temp in the right range.

"there are no stupid questions" 😀 

Thank you for making me not feel stupid 😌 I will try to continue in a similar vein.

If we assume the pump here is coming on in protection mode, the next question is why does it do this at 7C when the parameters are set to 0C/1C?

I've checked the other defrost trigger types (in addition to outdoor temp) and they're both enabled: room temp (10C, hysteresis 1C); outgoing water temp (1.5C, hysteresis 1.5C).

I've noticed, though, that there are two parameters for hysterisis of flow water temperature: 43/14 (set to 3C) and 43/22 (set to 1.5C) - could there be conflict here?

The control panel also says parameter 43/13 is 4C but I can't find an explanation for this one in the Grant manuals.

43 13 = "Backup heater set point during Frost protection" which I guess means that if you have a backup heater, that's when it will start to help with frost protection?! Do you have one?

Your earlier post implied that maybe there's an issue with the outdoor temp sensor? It's just at the back of the HP at the top, so maybe have a look at that or check the PCB connection? Also I think a hysterisis of 1C is a bit low, and might cause cycling of the frost protection on/off a lot. Do you know why they aren't the default values? Was a note placed on the installation guide?

@mikefl No back-up heater, so can't be that, but thanks for confirming what the parameter is.

43/11 and 43/12 are set low because antifreeze protects the system down to at least -12C. The manual actually says the antifreeze function can be disabled if 'a suitable concentration of glycol is present in the system water.' It can be turned off at the pump PCB but (I presume) it's still switched on here because the protection symbol did come on during the cold snap in December.

I think we probably need to test some different parameter settings, including hysteresis, to see what happens. Temps should drop low enough later to do this.

And, yes, I do wonder about the outdoor sensor. It looks ok from the outside but, as you say, may be an issue with the PCB connection. 

@mikefl The plot thickens. Spoke with a Grant trained engineer who says if the frost protection symbol isn't coming on then neither is the frost protection itself. They've suggested a sensor or PCB issue so this is the next thing to look at with the installer before contacting Grant themselves.

The mysteries of the heat pump... 

There's also a parameter (41 22 I think) which controls how long the system pump runs after the compressor is off. I don't think this can run to hours though.  My money is still on frost protection though. The indicator may be faulty.