How much you save per degree, the temperatures that genuinely cost you nothing, and the comfort threshold most UK owners settle on.
Yes, running cooler costs less. The honest savings between 40C and 38C are small. The savings between 38C and 35C are noticeable. Below 32C, the tub starts to feel less inviting, especially in winter.
On a typical UK 1,200-litre tub with a good cover, every degree drop in set temperature saves roughly 4 to 6% of the running cost. Over a winter month at 60 pounds of running cost, that is around 3 pounds per degree.
The savings compound. Going from 39C to 36C saves around 12 to 18 pounds a month in winter, which is a meaningful chunk.
38C is the standard set point. It feels properly warm in winter and is comfortable for adults in summer.
36C suits households with children or older bathers, and is the temperature most physiotherapists recommend for circulation.
34C is summer-only territory. It feels lukewarm in February but pleasant on a warm August evening when the air is already 22C.
Most modern controllers have an economy or sleep mode. It usually drops the set point by 4 to 6C between filter cycles and reheats only when needed. The savings are real but only if you genuinely use the tub less than twice a week.
For a tub used several times a week, economy mode actually costs more in many cases because each reheat overshoots and the heater works harder. Stick to a constant set temperature if you bathe regularly.
Slightly. Cooler water needs less sanitiser because organic activity is slower, but the change is small. Test as usual and adjust if your free chlorine is climbing.
Above 40C is uncomfortable for most adults and unsafe for sessions longer than 10 minutes. Most UK manufacturers cap the set point at 40C for that reason.