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Yutaki M (RHUE-3AVHN) + XEK23232A System Controller — DHW reheat randomly fails to start, no fault code, fixed only by power-cycling controller

Air Source Heat Pumps (ASHPs)

Last Post by rgabor 18 hours ago

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rgabor

(@rgabor)

New Member Member

Joined: 18 hours ago

Posts: 1

Topic starter 06/09/2026 5:12 pm [#3154]

Hi all,

Hoping someone here has hit this before, because I've run out of things to check against the manual.

Setup:

Heat pump: Hitachi Yutaki M, RHUE-3AVHN, mono-valent (CONF = 1)
System Controller: XEK23232A
DHW via diverting valve (P1 = 1) — relay on the controller's DHW output closes an external motorized valve to isolate radiators during DHW loading
All original Hitachi parts (heat pump, controller, room controller, RF unit, sensors), bought ~2 months ago as an unused/uninstalled unit. Self-installed (DIY).
No timer input, no DHW electric heater fitted.

Current parameters: CONF 1, P1 1, P2 0, P3 5, P4 2.0, P5 15, P6 55, P7 1.0, P10 45, P11 5, P12 10, P21 20, P22 2, P23 20, P24 0, P25 0, P26 OFF, P27 4.0, P30 1, P31 3, P34 45

DHW works fine for a while (heats to 45°C, reheats correctly when it drops a few degrees), then randomly just... stops trying. TDHW will be sitting way below the P10‑P11 (40°C) reheat threshold — I've seen it stuck at 19°C — with SDHW = 0, relay de-energized, heat pump in standby, and FAUL = 0 the entire time. No fault, no error, it just doesn't register any DHW demand at all.

The only fix I've found is a full power-cycle of the System Controller — the moment it comes back up, it immediately starts heating the tank correctly (confirmed via power consumption monitoring). It's not slow to respond either — I measured a 19°C → 34°C rise in 21 minutes after a restart, so a full recovery clearly finishes well under an hour. That rules out the "Maximum DHW Loading Time" (P27) safety timeout as the cause, since there's no way a normal reheat cycle is running anywhere near that long.

What I've already ruled out / fixed along the way:

P26 (Summer Switch-Off) was blocking all heating in summer — fixed, set to OFF.
P30 (No-Load Function) was disabled, causing pointless cold-water circulation with zero demand — fixed, set to 1.
Wiring/relay confirmed correct (on the DHW output, not the heating output).
No parameter changes between a successful reheat cycle and the next failed one — it just silently stopped registering demand.

Question:

Has anyone seen the System Controller silently stop registering DHW demand (SDHW stuck at 0) despite TDHW being clearly below the reheat threshold, with no fault code, that only clears on a power-cycle? Is there some internal state/timer not covered in the SMGB0066 service manual that could cause this? Any pointers on how to monitor or reset it without a full restart?

Thanks in advance!

This topic was modified 1 hour ago by Mars

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