Sorry to hear this. 

To help with the diagnosis can you describe exactly what you are observing when you say 'it doesnt have enough power' and also tell us about the DHW tank and your DHW settings.

Welcome to the forums @tinsel8152.

To help narrow down what might be going on, could you share a bit more detail on your current setup and settings? For example, what temperature are you heating your domestic hot water to, and what are your DHW temperature setpoints?

Irrespective of the current winter temperatures, your heat pump should have no issues getting hot water to 45C without any additional assistance. 

Also, do you know if the backup heater is enabled in the control settings specifically for DHW, or only for space heating? Sometimes the IBH activation logic can be configured separately for DHW and heating circuits.

It would also be useful to know if you’ve checked any error codes or system logs that might indicate why the backup heater isn’t activating.

@editor 

I have set the water to heat to 52C. I have a 300 liter tank. Depending on outside temp, the pump heats the DHW to 45C if it is below -10C and 48C if it is between -5C and -10C. 48C is fine for me but 45C is a little too low for a shower.

I did also check the DIP switch setting that I found in the manual that it is set to be valid to use IBH for DHW.

E: I also checked that there are no errors in the logs.

Can you advise if the tank is a 'heat pump' tank, ie one with a large coil (generally 3 sq m).  Do you by any chance know what flow temperature the heat pump reaches and how long is it set to heat for?

@jamespa 

The tank is specifically this one with 3,1 sq m coil: https://www.osohotwater.com/product/oso-delta-geocoil-dgc/

The pump reaches at least a 55C flow temp. Usually I have it set on a timer for a couple hours a day, but lately i have been testing with DHW set on all the time but the pump just stops heating DHW when it reaches above states temps.

Here is photo from the controller. Temp is 55C but it did go to 56C for a bit too.

Posted by: @tinsel8152

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Here is photo from the controller. Temp is 55C but it did go to 56C for a bit too.

With an LW of 55 you will struggle to get to 52, but you should get to 48 I would have thought.  Does your heat pump allow you to increase the max lwt for dhw?

The other factor to bear in mind is that to heat 300l of water from 10 to 50 takes 2.5 hours with a 6 kW pump, are you heating for long enough?

How far is the cylinder from the heat pump and how well insulated is the route.  Could the water be cooling between heat pump and cylinde

@jamespa Yesterday when I took that photo I got to precisely 48C and then the pump just stopped again even if I disable space heating and only DHW is turned on. I tried later with set temp 60C and there was no difference. As I said earlier depending on outside temp, the pump heats the DHW to 45C if it is below -10C and 48C if it is between -5C and -10C.

I don't heat from 10 degrees. I have the cold water coming to the tank going through a coil in my "buffer tank" for preheating. That way the water going to the tank is closer to the temp I use for space heating, which is 40C if it is -10 outside. 

The tank is right next to the pump so distance should not be an issue here. Insulating could be better still, only the outside connection part is insulated at the moment.

OK, I’m a bit baffled.  Does it switch off after a certain time or when the flow temperature reaches a certain value? Some heat pumps have a 'max DHW heating time' feature to ensure space heating isnt turned off for too long.  Could be that? 

When you say the tank is right next to the pump, is the 'pump' the heat pump- it is the temperature of the flow from the heat pump that I was concerned about when I asked about insulation.

Apart from the above I cant immediately think of an explanation and without further data doubt I will come up with anything.  You may need to log times and flow temperature to get to the bottom of this, unless it’s some simple setting somewhere buried in the installer menu!  Maybe someone else has an idea.  I would say that it feels like the system is 'marginal' (rather than catastrophically faulty) so its probably that something, somewhere, isnt quite getting hot enough for long enough but what exactly is difficult to tell.  If you can somehow turn up the LWT for DHW that may fix it (R32 should be capable of 60-65), but it is also possible that the capacity of the heat pump at -10 isn’t sufficient (do you have the capacity tables/specifications?)

@jamespa 

It switches off based on the temp in the tank. So yesterday when it was -8C outside it switched off when the tank reached 48C. Didn't matter if it was set at 52C or 60C. Flow temp reached 55-56C both times. 

I found in the settings a t_DHWHP_MAX setting(the maximum continuous working period of the heat pump in DHW PRIORITY mode.) that is set at 90 minutes, but it stops much sooner than that.

Posted by: @jamespa

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Posted by: @tinsel8152

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When you say the tank is right next to the pump, is the 'pump' the heat pump- it is the temperature of the flow from the heat pump that I was concerned about when I asked about insulation.

Yeah, it is the heat pump indeed what I mean here. There is only a wall separating them.

I'm thinking it is a setting buried somewhere, that's just hard to figure out with the documentation not telling what all the settings even mean.

Here is the capacity table. Around 9kW heating capacity at -10C i think should be enough.

Posted by: @tinsel8152

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It switches off based on the temp in the tank. So yesterday when it was -8C outside it switched off when the tank reached 48C. Didn't matter if it was set at 52C or 60C. Flow temp reached 55-56C both times

That cant be precisely true if, as you say, the temperature the tank reaches depends on the outside temperature.

According to the table the capacity at -10/60 is only 6k.7W.  That should still be enough!

Posted by: @tinsel8152

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I'm thinking it is a setting buried somewhere, that's just hard to figure out with the documentation not telling what all the settings even mean.

Im inclined to agree.  Possibly some minimum delta between flow and return or between flow and tank temp?

Ive just looked back to your first post and you suggest that the situation has changed (you use the work anymore).  If so then perhaps a sensor has moved?

Measurements and observations, plus going through the menu, is the only way to track this down I fear.  Sorry I couldn't be more helpful.