How Often Should You Drain and Refill a Hot Tub?

Most UK hot tub owners know they should change the water from time to time. Far fewer know when. Leave it too long and the water becomes harder to balance, filters clog sooner and the spa starts to smell “off” even when the sanitiser reading looks fine. Our guide to why a hot tub smells helps you tell old water from other causes. Change it too often and you waste water, chemicals and heating time for no real gain.

Mid-October is a sensible moment to decide. Cooler nights mean more soaks, autumn leaves load the filter, and fresh water before winter makes the months ahead much easier. This guide sets out a practical UK schedule, the warning signs that mean you should drain sooner, and what to do when you refill.

The simple answer for a home hot tub

For most home hot tubs, plan a full drain and refill every 4 to 6 months, depending on how much you use it. A tub used for a few short soaks a week sits towards 6 months while the water keeps behaving. A busy household, regular guests or a spa used most days sits towards 4 months.

Your situation	When to change the water
New owner, first fill	Often the first change at 2 to 3 months, while everyone is using the spa a lot. Usage then settles down and you move to the normal schedule.
Normal family use	Every 4 to 6 months
Light use	Towards 6 months, if the water still tests steady and smells fresh
Heavy use or frequent guests	Towards 4 months, or sooner if the warning signs below appear
Portable and inflatable spas	The smaller volume gets dirty faster, so watch the warning signs closely rather than waiting for the calendar

Why the water goes “tired”

Every soak adds skin cells, sweat, cosmetics and detergent residues from swimwear. Rain splash, pollen and garden debris join them. Filters catch a lot of that load, but dissolved waste builds up in the water itself. Over time you need more chemical to hold the same reading, foam appears more easily, and the spa never quite smells fresh.

That is different from a short-term problem such as cloudy water after a party, which often clears with shock, filtration and a filter clean. When the water has been in the spa for months and every correction fights you, a full change is the honest fix. Our guide to cloudy, foamy or green hot tub water helps you tell the difference.

What your pH is telling you

With our chemicals, the pH tends not to be disturbed. You make one initial adjustment for your tap water after filling, and from then on it is normally pretty stable. Keep these readings in range:

Test	Target
Total alkalinity	80 to 120 ppm
pH	7.2 to 7.6
Free chlorine	1.0 to 1.5 ppm

That makes pH a useful early warning. When it starts drifting up and down, that is a good sign there is too much detergent in the water, and a full water change is a good idea. Rinsing off before you get in, and not wearing swimwear that was washed in strong detergent, slows this down. If the water has started to irritate your skin or eyes, our guide to itchy skin and sore eyes after a hot tub helps you find the cause.

Signs it is time to drain now

Do not wait for the calendar if any of these show up:

• The pH keeps drifting up and down after you have settled it
• The water stays dull or cloudy after you have balanced pH, shocked and cleaned the filter
• Foam keeps returning even when you are careful with lotions and detergents
• There is a persistent musty or “chemical” smell that sanitiser alone does not clear
• You need more sanitiser than usual just to hold a normal reading
• Scale films or chalky deposits are building on the waterline in a hard-water area

If several of those appear together, drain. Fresh water is cheaper than chasing a lost cause with bottle after bottle from the essential chemicals range.

Autumn into winter: what UK owners should do

If your last full change was in spring or early summer, you are due around now. Summer sun cream leaves a heavy organic load, and leaves and damper weather add debris just as evening soaks increase. Drain, clean the shell, service the filter, refill and balance before the first hard frosts.

Keep the spa filled and powered through winter so freeze protection can run. Emptying for the whole season is only for special cases, as explained in Should I empty my hot tub in winter?

How to drain and refill without drama

Switch the spa off at the isolator before you open the drain. Attach a hose to the drain valve, route it to a suitable drain, and empty the spa fully. Wipe the shell, waterline and pillows while it is empty. Our step-by-step on how to use the drain valve shows the locking cap so you do not unscrew the whole fitting by mistake.

While the spa is empty, rinse or deep-clean the filter with Canadian Spa Hot Tub Filter Cleaner (1 litre, £20), and replace the cartridge if it is due. Hard-shell and swim spa filters last 6 to 12 months. For Glacier filters, replace every 6 months for best performance, or up to 12 months for economy. Portable spa filters need changing every 4 to 6 weeks. See when to replace your hot tub filter and the Glacier Antimicrobial 50 sq ft Filter (£44.99) if your model takes that cartridge.

Refill with a hose through the filter housing where possible, so air is less likely to trap in the pump. Fill to the level shown in your owner’s manual, power up, bleed any airlock if the jets do not move water, then heat.

Balance the new water before you soak

Fresh tap water is not ready for bathing. Test total alkalinity and pH, make that one initial adjustment into the ranges above, then set your sanitiser. Once the water is balanced, it should hold steady, so you are testing to confirm rather than correcting every week. Shock after the first heat-up with Non-Chlorine Spa Shock (1 kg, £25) so leftovers from the fill and clean-down are oxidised. Keep the cover off briefly after shocking, then replace it and let filtration run.

In hard-water parts of the UK, dissolved calcium climbs with every top-up between full changes. A stain and scale inhibitor helps between drains, but it does not replace a proper water change when dissolved solids have built up.

Quick checklist

• Diary a full drain and refill every 4 to 6 months, towards 4 for heavy use and towards 6 for light use
• New owner? Expect the first change at 2 to 3 months, then move to the normal schedule
• Drain now if the pH keeps drifting, the water will not clear, foam returns or the smell lingers
• If your last change was in spring or early summer, do an autumn change before winter soaks ramp up, and keep the spa filled and powered in cold weather
• Clean or replace the filter while empty, then refill, make the one initial pH and alkalinity adjustment, shock and heat

You shouldn't use a hot tub or sauna while pregnant. For any medical condition, check with your GP before soaking. The NHS is the right place for health advice; this article is about water care only.

If you are unsure whether the water can be saved or needs changing, call Canadian Spa Company on 01293 824094 and we will talk you through what you are seeing.