The hall is heated by the water it evaporates

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 611 tonnes of water, and the heat evaporation takes out of the water — 414 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; eight inverter modules modulate. It is run humidity-led, judged 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. 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 sixteen unmet hours; the dehumidifier draws 166 MWh at a seasonal COP of 3.8, the rooftops burn 146 MWh of gas, and 255 of the pool's 414 MWh come back through the reclaim. Most of that is summer: in winter the cold outside air dries the hall and the dehumidifier runs little, so the latent heat leaves with the ventilation air and a reclaim recovers only what the compressor moves. The head floor itself is close to a wash here — without it 216 MWh came back on 161 MWh of electricity and 102 of gas: holding the head moves heat from the hall to the water, and the rooftops make up the hall.

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

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: three-fifths of the pool's loss here, most of it in summer — in winter the ventilation air does the drying and takes the latent heat with it.

The machines

• Example 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.