There arent many of these hitachi pumps around here so you may not get an answer.

I had a not dissimilar fault with my Vaillant albeit I did get a (pretty useless) fault code.  Diagnosis was made easier by a function called 'live view' which is essentially a stream of consciousness display telling you what the heat pump is currently doing and a wat of viewing current values of certain sensors.  If yours has something similar it might be worth watching it while you trigger a DHW cycle, or create the conditions that should trigger a DHW cycle.

I had the Hitachi Yutaki 7kw split ASHP system.  As James said, look for the live view settings in the controller might give you a clue as to what is going on. I did find it helpful in understanding what was going on.

Hi @jamespa , @trebor12345 ,

Thank you both for the reply.

Can you clarify what you mean by "live view settings"? On the System Controller itself, I already have a live sensor readout screen (TDHW, SDHW, TSUP, TRET, S1, SSUP, etc.) which I've been watching closely - that's how I know SDHW stays at 0 even when TDHW is well below the DHW setpoint. If you mean something different/more detailed than that, I'd love to know where to find it.

@trebor12345 - is the "Refrigerant Cycle" screen you posted (TGas, Freq, Td, Curr, Pd, Te, Twi, TLiq, Evi, Evo etc.) what you meant by live view? Is that on the outdoor unit's own display, or accessed through the System Controller / a service tool? I hadn't seen that screen before - happy to look at it during a stall if it's accessible to me, but not sure yet how to read it or what would indicate a problem versus normal operation.

For context, the core symptom is: DHW setpoint (DSET) is 45C, TDHW sits at 19-20C, but SDHW (DHW demand) reads 0 - the controller isn't registering any call for DHW heat at all, despite being far below the reheat threshold (setpoint - differential = 40C). No fault code is ever shown (FAUL = 0).

What we've ruled out / tested so far:
- P26 (Summer Switch-Off) was blocking all heating in summer - fixed, set to OFF
- P30 (No-Load Function) was disabled, causing pointless continuous cold-water circulation with zero demand - fixed, set to 1
- P27 (Max DHW Loading Time) as a timeout cause - ruled out by directly measuring recovery speed: a full 19C to 45C recovery consistently completes in under 1.5 hours (confirmed via power consumption monitoring), so it's not remotely close to the 2-4hr window we've set for P27
- Fast vs slow TDHW drop rate - deliberately tested this by (a) taking a real bath (fast large draw) and (b) manually cooling the DHW sensor directly with a cold pack for an instant drop - in every one of these tests, SDHW responded correctly and the heat pump started reheating immediately. So a fast drop does NOT appear to reproduce the issue.
- The only reliable fix once it happens is a full power-cycle of the System Controller, which immediately clears it and DHW reheating starts within minutes

So at this point it seems to be a genuinely intermittent state - something silently blocks DHW demand recognition, without triggering any documented protection function (P27, frost protection, tariff/timer block, etc.) and without any fault code, and it doesn't seem tied to how fast or slow TDHW drops. It clears itself with a power-cycle, which is at least a usable workaround, but I'd like to understand the actual cause if possible.

Any pointers to where a deeper "live view"/diagnostic screen exists (System Controller menu, outdoor unit display, or a service tool) would be really appreciated - happy to go capture readings from it next time this happens.

Thanks again for taking the time to help.

Sent you a private email with contact details for direct support, they will be able to help.

Thank you @trebor12345