Can I fill my hot tub with harvested rainwater?

From time to time, we get asked about filling a hot tub or swim spa using harvested rain water or filling from other untreated water sources, like bore holes or abstracted water from water courses. Sometimes this is as an alternative way to fill a spa during a hose pipe ban, on other occasions it’s about utilising existing accessible water supplies or running a hot tub somewhere that’s off grid.

While completely possible and potentially a clever way round a hose pipe ban, or a way to utilise free or nearby resources, using untreated water to fill a spa isn’t ideal. Before you pump outdoor rainwater anywhere near your spa shell, there are several strict water treatment and filtration steps worth following to protect both your equipment and your health.

Filtering out physical debris

Roof runoff and garden water butts are full of invisible contaminants. If you do go ahead with a rainwater fill, you should ideally run the water through a dedicated pre-filter before it even touches your spa. At an absolute minimum, attach a Darlly PureStream+ inline hose connector filter to your pump setup. This will trap organic matter, sediment, heavy metals, and physical debris before they enter the system.

Boosting your filtration cycles

Once the spa is full, do not hop straight in. You need to give your hot tub’s filtration system plenty of time to clear the new water. Go into your spa’s digital control panel and increase the filtration cycle times so the water moves constantly for the first few days.

Because rainwater carries a high organic load, your spa filters are going to work overtime. You will need to remove and clean them regularly during this initial setup period. Simply rinsing them with a hose isn’t enough here; you must use a dedicated filter cleaning solution to properly break down oils, biofilms, and fine debris that untreated water introduces.

The vital importance of shock dosing

Most importantly, you must heavily shock dose your tub to sanitise the water—this step is absolutely non-negotiable. Rainwater sitting stagnant in outdoor water butts, often at warm summer temperatures, creates a perfect breeding ground for dangerous bacteria and algae. A heavy shock dose of chlorine or bromine is the only way to completely kill off these invisible pathogens and ensure the water is hygienically safe for bathers. Once shock dosed and settled, you can then dose normally as you would with any hot tub and monitor the water chemestry and balance with quality test strips.

Be sure it’s safe with a lab test

Because untreated (not mains) water chemistry can be unpredictable, and storing water without sanitisers and water movement adds additional risks, we recommend getting a professional opinion before you bathe. We offer a dedicated laboratory water testing service. We suggest doing both a microbiological test and a specific test for legionella to give you a fuller picture of your water quality. The microbiological analysis tests for coliforms, escherichia coli, pseudomonas aeruginosa and an aerobic plate count at 37 degrees. For more information about water testing please get in touch.

The final verdict: At your own risk

In summary, while filling your hot tub or swim spa with harvested rainwater or other untreated (not mains) water is technically possible, it is not recommended. Using another water supply requires extensive extra steps, additional filtration accessories, heavy chemical treatment, and enhanced monitoring to keep the water hygienically safe. Ultimately, opting for a rainwater fill is something you take at own risk, so weigh the extra maintenance time and effort carefully against standard water management alternatives.