Hot tub sanitiser and running cost calculator
A hot tub is a very small volume of very warm water with a lot of people in it, which is why the chemistry is so much more demanding than a pool.
ppm
ppm
%
kW
per kWh
Sanitiser to add
8.04g
- Days until a water change
- 350 Light use, but change quarterly regardless
- In weeks
- 50.0
- Daily heating cost
- 3.36
- Annual heating cost
- 1227.24
- Water changes a year
- 1.0
- Water used per year
- 1565
Show the working
- Water volume = 1500
- Bather uses per week = 10
- Current sanitiser = 1
- Target sanitiser = 4
- Product strength = 56
- Heater power = 3
- Heating hours per day = 4
- Electricity price = 0.28
- increase = max(4 - 1, 0) = 3
- Sanitiser to add = 56 > 0 ? 1500 * 3 / 1000 / (56/100) : 0 = 8.035 71
- Days until a water change = 10 > 0 ? 1500 / 3 / (10/7) : 0 = 350
- In weeks = 350 / 7 = 50
- daily_kwh = 3 * 4 = 12
- Daily heating cost = 12 * 0.28 = 3.36
- Annual heating cost = 3.36 * 365.25 = 1227.24
- Water changes a year = 350 > 0 ? 365.25 / 350 : 0 = 1.043 57
- Water used per year = 1.043 57 * 1500 = 1565.36
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- the water change interval rule of thumb: volume in litres / 3 / average daily bathers, in days
- sanitiser dose = volume x ppm increase / product strength
Never change the water less often than quarterly whatever the arithmetic says. Dissolved solids build up regardless of sanitiser.
A good cover is worth more than any efficiency measure: most of the heat leaves through the surface.