Context

ADR 0068 shipped boost as a write of register 0x0000. On the one heater reported so far (issue #198) that write is acknowledged with the value 00 in every mode, while acknowledgements of writes the heater keeps carry the written value. The heater ignores it, and the boost switch does nothing.

The same heater answers a status read of functional parameters 10, 15 and 16 (FPI1 00, FPI2 bitmap 43) with result code 0 and the main parameter in place, as 05 00 <MP> 00 43 00 00 00 0C 4A 00 (issue #198, corpus atlantic_calypso_probe_status_fp_high):

FP15 000C is the operating-mode capability word: bit 2 absence (away) and bit 3 relaunch (boost) are set.
FP16 4A00 is the current operating-mode word: relaunch in bits 8–9 and absence in bits 10–11, each 0 keep, 1 on, 2 off, so 4A00 reads "boost off, away off". Bit 14 is set; its meaning is not known.

The default status read (FP1–FP7) gets result code 5, "parameter unknown", because the heater holds none of those parameters. Registers 0x0000 and 0x0005 change with a boost or holiday started on the display: 01 during a boost, and in 0x0005 the holiday days still to run (04 four days before the end, 02 two days before) or FF for a holiday set with no end date (corpus atlantic_calypso_exchange_register_read). They are durations, not on/off flags. During a holiday the status main parameter reads 00 00 (corpus atlantic_calypso_probe_status_mp_fp).

Decision

Boost and away are written in FP16 with one EXECUTE: main parameter D400 (keep the target) and FP16 4900 (boost on, away off), 4A00 (both off) or 4600 (away on, boost off). No other value is sent: there is no "both on", and the "keep" code is not used untested. Bit 14 stays set as the heater reports it.
A duration goes first. Boost on writes register 0x0000 = 1 (a duration of 1; the unit is not known) and away on writes register 0x0005 = 255 (until turned off) before the FP16 EXECUTE. Both acknowledgements are logged; an ignored duration write does not stop the sequence. Off sends FP16 4A00 only, so ending one also ends the other.
The state is FP16. The water heater poll's status read asks FPI1 00 / FPI2 43, exactly the read the heater has answered: it returns the target, FP15 and FP16 in one exchange, plus FP10, which no decision uses and the decoder skips. Boost shows on while FP16 bits 8–9 are 1, away while bits 10–11 are 1. After each write the hub reads this status at once; the 45 s heating poll follows.
Registers 0x0000 and 0x0005 are durations. The poll keeps reading them; they are logged and never decide a state.
Away is part of the water heater entity, offered always (entity features are fixed at startup). Boost stays the opt-in switch. Once FP15 has been read, a write whose capability bit is clear is refused and logged.
No target while away. The heater reports a main parameter of 0000 then, which is no target: the entity shows the target as unknown, and a target set while away is logged, not sent.
Both are experimental until a heater confirms them on hardware.

Consequences

Boost and away from Home Assistant work on a heater that honours its operating-mode word; the poll costs no extra exchange.
The whole poll (target, then the registers) rests on a read the heater has answered in three modes; FP10 costs two bytes per poll.
FP16 has only been seen at rest. If it does not track a running boost, the shown state stays off while the heater boosts; a later record would then take the state from the durations.
Ending a boost also ends a holiday and the other way round. Home Assistant's away mode has no end date, so away from Home Assistant runs until turned off; a holiday set on the display with an end date still ends on its own, and the entity follows.
A heater that ignores the duration writes still gets the FP16 command; the logged acknowledgements show which part it honours.

Considered options

Register writes only (0x0000 = 1, 0x0005 = 255): the boost write is ignored on hardware.
Registers as the state: they carry durations, and a duration can outlive or precede the mode it belongs to.
Read-modify-write of FP16: needs an extra read before each write and sends combinations no heater has been seen to accept.
FPI2 03 (FP15 and FP16 only): two bytes shorter, but never seen answered; a refused status read would stop every reading of the poll, not only boost and away.
The old FP1–FP7 read for the target plus a second read for FP15 and FP16: keeps the target path unchanged, but costs a second exchange per poll.