Mains-fed garden hoses are the easiest way to top up, but UK water hardness and the hose itself can cause problems. Here is how to do it properly.
Yes, almost every UK hot tub is filled and topped up directly from a mains-fed garden hose. Two things to be aware of: the hardness of your local water, and the state of the hose itself. A 20-pound hose-end pre-filter solves most of what can go wrong.
An old rubber hose that has sat in the sun for a season can leach plasticisers and pick up biofilm inside the bore. The first water out of it should never go into your tub. Run it for 30 seconds onto the lawn first.
A new food-grade hose (often sold as a caravan hose) is worth keeping just for tub top-ups. Twenty pounds, lasts years, never used for the lawn. The water tastes and smells noticeably cleaner.
South-east England has very hard water. A refill there can introduce calcium hardness above 400 ppm before you have added a single chemical. Some areas of Scotland and the North West have very soft water, which is its own problem because it strips calcium from the heater element.
A hose-end pre-filter (a small carbon and resin canister that screws onto the hose) takes out chlorine, metals, and a chunk of the hardness. They last about three refills and cost roughly 15 to 20 pounds. They pay for themselves in chemicals saved on the first fill.
Top up through the filter housing, not the open shell. The mains water gets dispersed and filtered as it enters, rather than landing as a cold puddle on the bottom of the tub.
Dose for the volume you added, not for the whole tub. A 50-litre top-up does not need a full sanitiser dose, just a quick check and a small adjustment.
Retest after an hour with the jets running. The new water needs to mix before any reading is reliable.
No. Rainwater carries leaves, insects, and atmospheric pollution, and it is not chlorinated. The cost saving is not worth the chemistry chaos.
Mixing hot from the kitchen tap can shave hours off a refill in winter, but never run a domestic hot supply through a garden hose. The hose plasticisers come out faster at temperature. Use a clean bucket.