@sunandair Yes that duration is just a max you are allowing it to run for, there is also a setting for how long it then has to wait before it can return to hot water heating.  The time of day it can run for can also be restricted by the timer function.

I run mine for max 2 hours overnight and don't allow it to reheat outside of this it typically runs for an hour and that tankful lasts all day on our typical use.

thanks for all the replies. This was very informative and i will be monitoring it from now on with all this knowledge in mind.

I have another question please, not sure if should put it into a new thread: Id like to on occasion be able to blast my heating to get a surge of heat into the floor.  However when I add to my WC curve via the + option on controller or set a constant flow temp, it never reaches it, in fact it does not really get past 30 degrees. Even setting to room temp mode it doesnt seem to go much past 30 degrees. I dont think the system is at its limits because the most im seeing it use it maybe 2kw.

From some basic research it seems to be becuase my delta t restrition that it wont rise higher. Is there a way around this to allow the flow temp to be higher? I dont care about efficiancy being bad. Some images below showing it not rising to set temp:

I could be imagining this but my feeling is that when I do ask it to surge (and it refuses to) it uses loads of electricity anyway and the outdoor unit gets louder than normal even though its not actaully inscreasing the temp past the normal level.

For example: WC curve could be set to ask it to run at 30degrees. it will happily do that and use hardly any energy. then when I ask it to go to 40 degrees everything ramps up (outdoor unit and my power usage goes up) yet its not even getting much past 30 degreees.

To summerise house/system.

8.5kw Split with Mistubishi indoor unit (ehst20d-vm2d)

House is very insualted airtight renovation. Heat looss calc was never done by installer. but iv done a calcualtion on heatpunk retrospectively and it seems to be in the ballpark for a 8.5kw system.

Its all underfloor(180sqm) ~100mm slab. 2 rooms upstairs (total 40sqm) are not heated. Heat travels up enough from downstairs. seems to work ok

I am using WC currently, but I think I will be changing back to set room temp as I think it is ending up cheaper overall even though the COP is worse than in WC mode.

Im not having any issue with it keeping the house warm in either mode and with an average COP of approx 1:5. I understand that low and steady is ideal, however I just want to be able to manually blast the heat now and again. Reason being. . .Every night I charge my batteries and can squeeze around 44kwh from the grid during the 4 hour cheap rate window. Also have an EV so occasionally when not much driving has been done I have some khw spare and Id love on occasion and easily be able to put this into the floor in a short window.

Let me know if you need any more info.

Set temp:

 what actually happened:

You are trying to put in heat to tonnes of concrete in 4 hours that's just not possible. 

The heat pump can't get the flow temp any higher until the return temp increases.

As you said it needs to be low and slow not high and fast a heat pump can't do that.

A heat pump can't do that or won't do that?

I'm not expecting miracles that it's going to get the return temp up that much in a big slab of concrete, but I can't see why it won't increase the flow temp more than 5 degrees more than return temp. I don't care that it becomes very inaficient.

Maybe I'm missing something here. 

It can do provided that doing so doesn't exceed the maximum possible output power.  That's the basic limitation which it can't circumvent.  Your 8.5kW pump can only produce (about) 8.5kW, which only heats water at any given flow rate through a certain number of degrees.  Getting a slab to rise several degrees requires a lot of energy!

@eleusis it can do it but not in 4 hours, you will need to heat slab for potentially days to get it hot enough for the return temp to rise so then the heat pump can raise the flow temp