@Renewables345, this is beyond my own knowledge of Samsung cascade controls, but a few things jump out.

First, how large is the property, what was the calculated design heat loss and what led to the decision to cascade two 12kW units rather than install a single machine? I’m not suggesting the design is necessarily wrong, but 24kW is a substantial amount of heat pump capacity and the reasoning may help others understand what the system is supposed to do.

The 80l/min reading doesn’t make much sense to me if the flow sensors aren’t actually connected. My understanding is that each 12kW unit should have its own sensor, and Samsung quotes a normal range of 7–58l/min per unit, so I’d be wary of treating the figure on the controller as a genuine measurement. It may be combining something, showing a default value or simply displaying nonsense because it isn’t receiving the information it expects. The lack of an E911 fault wouldn’t necessarily convince me that the flow is fine, but I stand to be corrected.

On the short sensor cable, I’d ask Samsung to confirm exactly how they want the cable extended, or whether the sensor or controller needs to be repositioned.

The fact that the system stops in both heating and HW modes makes me wonder whether the problem is somewhere common to both like the cascade configuration, loss of demand, pump-control arrangement, temperature sensing or the controller deciding it has reached a thermo time to turn off condition. 

My very tentative view is that I would resolve the flow sensor connection before chasing more obscure faults. At the moment, one of the controller’s important inputs appears to be missing, while the value being displayed does not seem easy to reconcile with that.

@editor The property is over 350m2, and an old stone building. Heat loss is was just over 22kW at design. 

The 80l/min is standard for if the units don't have a flow sensor connected, as there is no signal to tell it otherwise. We have installed the flow sensor, but the cable only reach 1.5m in comparison to the thermistor cables, which are closer to 5m. I was hoping for some wisdom from the group, Samsung have been slower to respond, and haven't really given much advice.

The DHW temp is only reading at 29c and we put on a heating call just to test, both and the same issue, seems like it's short cycling, but without an obvious cause.

I agree with you on the flow sensors, but I would have expected the E911 fault code, which wasn't present.

Posted by: @renewables345

↑

The 80l/min is standard for if the units don't have a flow sensor connected, as there is no signal to tell it otherwise. We have installed the flow sensor, but the cable only reach 1.5m in comparison to the thermistor cables, which are closer to 5m. I was hoping for some wisdom from the group, Samsung have been slower to respond, and haven't really given much advice.

I think the logic of this is that the flow sensor would normally be located almost adjacent to the unit (most manufacturers include it within the unit!)

Posted by: @renewables345

↑

I agree with you on the flow sensors, but I would have expected the E911 fault code, which wasn't present.

Flow sensor not present and low flow rate are not necessarily the same!

@mars is spot on in all respects imho.  The only things I can add are:

You could try it in single unit mode first, before attempting a cascade (if the plumbing suits obviously).  Less to mis-configure and diagnose

I dont know how Samsung cascade works, but it may just be that one unit views the other as a 'backup heater'.  Check

If you can get it working then please post a system diagram and the key config parameters, you may get more suggestions 

But first of all connect up the flow sensors!

@jamespa It's a funny system, but was out only option left, without a significant install cost.

It didn't make sense to me initially with the flow sensors, but speaking to Samsung, they also weren't sure. It just seemed strange it didn't kick out straight away.

I've been sent some extension bits, and should be able to get it all sorted today. I'll send through the diagram later!