Specifying a cold plunge tub with a chiller for an Indian summer: the six numbers that matter

Start with litres, not with the tub

A chiller does not cool a tub; it cools water. The first number on any order is therefore the volume the chiller must hold at 3-5 °C, in litres, with the bathers in it. A tub that is sold as 500 litres holds about 360 litres of water with two adults in it (each displaces about 70 litres), and a tub that is sold as 800 holds about 660 with the same two. Our rule of thumb, used across the wooden and free-standing tubs, is that a 1 HP unit holds up to 260 litres, a 2 HP unit up to 570 and a 4 HP unit up to 1,200. Above 1,200 litres you are no longer buying a tub chiller; you are buying a pool plant.

Pull-down and hold are different jobs

Pull-down is the first fill, taking the water from where it starts to 4 °C. Hold is the daily job: removing the heat that arrives through the shell, from the sun and from the bathers. Pull-down is an energy problem and the table shows its size; the energy to cool one litre by one kelvin is 4.186 kJ, or about 1.163 Wh.

Water drawn from a rooftop tank in a hot month may start well above 30 °C (we have no measured figure to quote), which is why the second column exists. A chiller whose useful cooling is a few kilowatts at those conditions needs hours, not minutes, to pull the larger tubs down. Plan the first fill for the evening before the first session.

The design ambient is a sentence on the order

Cooling capacity falls as the air around the condenser gets hotter, and as the water gets colder. A datasheet that states a capacity at a mild ambient and a warm water temperature describes a different day from the one you care about. Write the design conditions into the BOQ: a 45 °C ambient at the unit, 4 °C leaving water, and the capacity or the litres held at those conditions. Our own quotes state the ambient they were sized for, and we ask other suppliers to do the same. See the chiller page.

Where the chiller lives

• In shade and with air. A closed cupboard recirculates its own exhaust and loses capacity; a niche with a louvre at each end does not.
• Close to the tub. Every extra metre of hose adds heat gain and pump head. A few metres is normal; long runs are drawn as an exception with insulation noted.
• With a service side. One metre of clear space for the technician, and a floor drain for the condensate.
• Out of the guest's eye. On a terrace, behind a screen wall or in a plant room, never in the photograph.

Power: single phase, three phase and the generator

Ask for the running and starting current of the unit, the supply it needs and the protective device your electrician should fit. Small units are normally single phase and larger ones three phase; the datasheet decides, and so does what your building provides. If the chiller must run during a power cut, the generator set has to carry the starting current, which is several times the running current. That is a line on the electrical drawing, not a surprise in the monsoon.

Water: hardness, filtration and mosquitoes

Borewell water can be hard, and scale on the heat exchanger costs capacity year after year. Test total dissolved solids and hardness before you order, and ask whether the exchanger is titanium or 316 stainless and what the cleaning interval is. Keep a lid on the tub and circulate the water through a filter and a UV stage, because still water in a warm garden is a mosquito nursery.

Eight lines to paste into the BOQ

1. Water volume to be held, in litres, with the bathers in the tub.
2. Design conditions: leaving-water temperature 4 °C, ambient at the unit 45 °C, source-water temperature for the first fill.
3. Net cooling capacity at those conditions, in kW, and the first pull-down time.
4. Electrical data: supply, running current, starting current, protective device.
5. Free-air requirement and the permitted recirculation, with the niche or plant-room dimensions.
6. Sound pressure at 1 m and at the nearest boundary.
7. Water-side material, flow rate, pump head and the filtration and UV stages.
8. Commissioning: pull-down log, a hold test at 4 °C and a signed temperature test.

A worked example: a 420 litre oval tub in a Delhi May

Take the oval timber tub: 420 litres, one adult with room to move. A 2 HP unit is the right size, since 1 HP stops at 260 litres and 2 HP is rated to 570. Assume a 30 °C source for the first fill: about 12.7 kWh has to be taken out of the water; assume a 35 °C rooftop tank and it is about 15.1 kWh. Divide by the unit's net cooling at 4 °C water and 45 °C air to get the time: a unit that nets 1 kW at those conditions needs about 13 hours for the first, one that nets 2 kW about 6. Those two kilowatt figures are illustrations, not our datasheet; the point is the division, and that the number to demand from every supplier is the net cooling at your design conditions.

When two quotes for the same tub differ by a third