Marlow is a 900 m² public pool hall under a six-metre ceiling with a 25 m, six-lane pool — 325 m² of water at 28 °C — held at 28–30 °C air and a 60 % ceiling, the Handbook's rule that a hall is kept a degree or two above its water so the water evaporates less and the bathers are not cold. The pool is a moisture source in the room: its surface evaporates by the natatorium relation, more when the air is dry and nothing once the air is saturated at the water's temperature, with the Handbook's activity factor for the bathers and the splashing falling to the unoccupied 0.5 when the doors close. On London's year that is 550 tonnes of water, and the heat evaporation takes out of the water — 373 MWh — is what the pool heater has to put back.

The machine is a heat-pump dehumidifier that breathes the hall's own air: an eight-row coil takes the water out, a reclaim condenser on the pool water takes the heat the pool lost back into it first — the building hands it the pool's own loss, hour by hour — and the condenser in the air stream reheats with the rest; seven inverter modules of the catalog's ZP49KHE scroll modulate. It is run humidity-led, judged, modulated and cycled on the hall's ceiling alone every hour, because a machine that reheats with its own condenser can never meet a cooling requirement and the ordinary rule left it either idle or flat out (and until 5 September the unit's over-delivery was still read off the dry bulb, so it never unloaded — the coffee dryer's case found it). Two gas rooftops behind it bring the outside air, heat the hall in winter and take the summer surplus. The unit holds a 38 °C condensing floor to favour the reclaim, the way a real pool unit does — by flooding its condenser, since the condenser sits in the supply stream and cannot give up air. On London the hall holds 28–30 °C with no hour over its ceiling and three unmet hours; the dehumidifier draws 116 MWh at a seasonal COP of 5.8, the rooftops burn 397 MWh of gas, and 353 of the pool's 373 MWh come back through the reclaim. (Until 4 September the unit ran an invented map that read a COP of 3.8 at a pool hall's warm evaporating temperature; on the maker's map the compressor wastes far less heat into the hall, so the rooftops carry more of the winter on gas — 146 MWh before.) It comes back in every month alike, 27 to 31 MWh: unloading to its ceiling rather than cycling, the unit runs at part load all year with its head held, so the reclaim sees the compressor in winter as in summer. The head floor is not a wash on this map — without it 196 MWh came back on 102 MWh at the unit and 223 MWh of gas: holding the head moves 157 MWh from the hall to the water, and the rooftops make up the hall on gas, 174 MWh of it for the 157 moved. It stays declared because it is what the machine does; whether a pool unit should is a question the figures now put.

What it cost to learn. Four wirings were measured before this one: drying the whole stream and reheating with gas held the ceiling for 12.4 MWh of gas in one summer week; face-and-bypass around the coil dried nothing at night, since dehumidifying without net cooling needs reheat; the inline condenser under the temperature rule cooked the hall to 40 °C; and the first humidity-led unit's reheat never reached the hall at all — the model credits a room only with air moving the way it asked, which is right for a fan and wrong for a dehumidifier, and a reclaim that changed nothing in the rooftop's gas to the kilowatt-hour is what said so. The last of them was the head floor: a condenser inside the supply stream cannot hold its head by cutting air, so the model floods it instead, which is what the machine does.

What it reads on London

Evaporation	552 t of water a year; 373 MWh taken from the pool, of which 353 MWh (95 %) come back through the reclaim condenser
Dehumidifier	669 MWh at the coil on 116 MWh of electricity — seasonal COP 5.8, holding a 38 °C head; the rest of its condenser heat reheats the hall
Rooftops	318 MWh of heating on 397 MWh of gas between two; 0.3 MWh of summer cooling each — the winter is theirs now that the unit unloads to its ceiling instead of running flat out
The head floor	353 against 196 MWh reclaimed, for 14 MWh more electricity and 174 more of gas — without the floor the condenser heat lands in the hall and the rooftops burn 223 MWh instead of 397; the floor moves it into the water, and on this map that is not a wash
Hall	28.0–30.0 °C, RH 0.58 mean / 0.63 max, 0 hours over the 60 % ceiling, 3 unmet hours
What it cost to learn	whole-stream drying with gas reheat: 12.4 MWh of gas in a summer week — why pool units reheat with their compressors

What it taught

• A pool is a moisture source that follows the room's own humidity; a fixed latent gain would evaporate the same into saturated air.
• A dehumidifier is run on humidity, not temperature: reheating with its own condenser, it can never meet a cooling call, and judged on one it does nothing or everything.
• Dehumidifying without net cooling needs reheat, and where the reheat comes from is the whole economy of the machine: gas, twelve megawatt-hours a week; the compressor, free.
• A reclaim condenser is worth what the compressor moves: nineteen-twentieths of the pool's loss here, in winter as in summer, because a unit that unloads to its ceiling runs all year and its head floor sends the heat to the water.

The machines

• Pool-hall dehumidifier, 5 m³/s — open in the builder
• Example packaged rooftop with gas furnace (two of them) — open in the builder

Sources

• ASHRAE Handbook — HVAC Applications, Places of Assembly: the natatorium evaporation relation and activity factors
• ASHRAE 62.1 Table 6-1, swimming pools and decks: 2.4 L/s·m²
• PVGIS TMY, London

Readings of the shipped model on the building's own PVGIS typical meteorological year; engineering estimates from catalog data and published correlations. The building runs on every plan, including Free, a few times a day.