Hard-Water Playbook for Oxfordshire: Keep Your Hot Tub or Swim Spa Clear and Scale-Free

Much of Oxfordshire sits over chalk and limestone aquifers. As rainfall filters through those rocks it picks up calcium and magnesium, which is why our tap water is typically hard to very hard. For hot-tub and swim-spa owners, that means a higher risk of limescale, cloudy water, and shortened life for heaters, pumps, and cartridges unless you manage it proactively.^{1, 2}

Step 1 - Check your local water hardness

You can look up an indicative figure by postcode via your water supplier (most of the county is served by Thames Water; some border areas fall under Severn Trent). For a precise reading at home, use a hardness test strip on the cold tap you will fill from.

• Thames Water: Check by postcode³
• If you are near the county boundary, check your supplier's page too (e.g., Severn Trent "Check my water quality").⁴

How to read the number: UK hardness is commonly reported as mg/L (ppm) as CaCO₃. The Drinking Water Inspectorate classifies: Soft ≤100; Moderately hard 101-200; Hard 201-275; Very hard ≥276 mg/L as CaCO₃.²

Step 2 - Pick the right targets for your system

Targets differ slightly depending on whether you have a Hot Spring FreshWater Salt System  (hot tubs) or a standard chlorine/bromine setup. Many swim spas use standard chlorine or bromine; some use manufacturer-specific salt-chlorine systems. Always follow your model's manual.

A) FreshWater® Salt System (Hot Spring hot tubs)

Hot Spring's Salt System documentation is clear: to protect the cartridge and ensure effective operation, aim for calcium hardness ~50 ppm (and keep it between 25-75 ppm during use). Typical chemistry set-points otherwise remain familiar.

• Calcium hardness (CH): target ~50 ppm (operating 25-75 ppm)
• pH: 7.2-7.8
• Total alkalinity (TA): 40-120 ppm
• Free chlorine: 1-5 ppm
• Salt: ~1,750 ppm

Why lower CH? Scale forms readily on salt-system components. Starting low (~50 ppm) helps maximise cartridge life.^{5, 6, 7}

Note: The ~50 ppm CH target is specific to the Hot Spring FreshWater Salt System. Do not assume the same CH target for other brands' salt-chlorine systems without checking their manual.

B) Standard chlorine/bromine systems (hot tubs and swim spas)

For conventional setups, owner manuals typically recommend:

• Calcium hardness (CH): 75-150 ppm
• pH: 7.2-7.8
• Total alkalinity (TA): 40-120 ppm
• Free chlorine: 1-3 ppm

This gives enough calcium to protect metal parts and finishes without driving heavy scale. Swim spas generally follow the same ranges unless your manufacturer states otherwise.^{8, 9}

Step 3 - Soften on fill (Oxfordshire tips)

With hard source water, reduce calcium before you bring the spa into service:

• Portable hose-end softener during fill - ideal if your postcode is consistently "hard/very hard". This is especially useful for swim spas due to their larger volume.
• Calcium-removal pillow (e.g., Vanishing Act®) placed in the tub after fill - Hot Spring specifies this when CH is above target; use the XL pillow or repeat cycles for very hard water or larger swim-spa volumes.^{5, 10}

Important: The UK trade body BISHTA advises not doing a complete fresh fill directly from a domestic ion-exchange softener for standard spas, because some calcium is beneficial for equipment. Follow the manufacturer's guidance for salt systems (lower CH is a documented exception achieved with the approved methods above). This caution applies to both hot tubs and swim spas.¹¹

Step 4 - Balance and sanitise

Once you have hit your CH target:

1. Set pH 7.2-7.8 and TA 40-120 ppm (adjust alkalinity first, then pH).
2. Establish sanitiser: free chlorine 1-3 ppm for standard systems or 1-5 ppm for the Hot Spring Salt System.
3. If you are using the Hot Spring Salt System, insert the titanium cartridge last and set the output on the control panel.⁶

Step 5 - Keep scale from coming back

• Stay in range: High pH + high CH = scale on heaters, heat exchangers, jets, and the shell. Test weekly.
• Use a scale inhibitor if your area tests "hard/very hard". It binds minerals to reduce precipitation (you still need to keep pH/TA in range).
• Rinse filters weekly and deep-clean regularly; minerals caught in media can redeposit. Swim-spa filters are larger - plan adequate drying time after cleaning.
• Wipe the waterline to remove early deposits before they bake on.

Troubleshooting: what if you see scale?

1. Bring pH back to 7.2-7.6 and re-test CH/TA.
2. Use spa-safe cleaners on affected surfaces (avoid strong household acids/abrasives).
3. For Hot Spring Salt System owners, confirm CH is still in the 25-75 ppm band; if it has drifted up, run another calcium-removal cycle before resuming normal use.⁶

Targets at a glance

Why our water is hard (the short version)

The Thames Basin's principal aquifers include the Chalk and Oolitic limestones, which supply a large share of the region's public water. As groundwater flows through these rocks it dissolves minerals that later show up as "hardness" at your tap - entirely safe to drink, but troublesome for hot tubs and swim spas if unmanaged.^{1, 2}

References

1. British Geological Survey (Thames Basin aquifers) - "There are three principal aquifers within the Thames Basin: the Chalk, the Oolitic limestones of the Jurassic and the Lower Greensand." bgs.ac.uk
2. Drinking Water Inspectorate (Water hardness) - Classification table for mg/L (ppm) as CaCO₃ (Soft ≤100; Moderately hard 101-200; Hard 201-275; Very hard ≥276). dwi.gov.uk
3. Thames Water (Water quality checker) - Postcode tool showing hardness and other parameters. thameswater.co.uk
4. Severn Trent (Check my water quality) - Postcode tool for areas near the county boundary. stwater.co.uk
5. Hot Spring (FreshWater® Salt System Start-Up Guide) - "Your target calcium level is 50 ppm ... use Vanishing Act® / portable softener if over 75 ppm." PDF
6. Hot Spring (FreshWater® Salt System Owner's Manual) - "Calcium hardness must be between 25-75 ppm; a 50 ppm starting target is recommended." PDF
7. Hot Spring (Highlife® Owner's Manual) - Notes lower CH requirement when using the Salt System. PDF
8. Hot Spring (Limelight® Owner's Manual, 2025) - For standard care: CH 75-150 ppm; for Salt System: 25-75 ppm. PDF
9. Freeflow® Spas Owner's Manual - Standard care ranges including CH 75-150 ppm. PDF
10. Hot Spring FAQ: Reducing calcium hardness - Use of Vanishing Act® and guidance for high-CH fills. hotspring.com
11. BISHTA factsheet: Hot tubs and hard water - "Industry advice is to never use water from a water softener to do a complete fresh fill of your hot tub or swim spa." bishta.co.uk

This article is informational and not a substitute for your owner's manual. Always follow the specific guidance for your hot tub or swim spa model.