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What Is Limescale and How To Avoid ItUpdated 3 months ago

Limescale develops when the pH of the water is left consistently higher than the pH OK Range of the test strips. The high pH causes any calcium in the water to crystalise, which normally happens within the heater due to the reduced surface area inside the heater. If left unchecked, scale will continue to form and will eventually choke the heater completely.

Limescale can have a number of undesirable effects on your control unit:

Firstly, as the scale continues to form, flow becomes restricted through the heater and the flow switches detect a problem (as seen in the photo above). To avoid damaging the system, the control unit will start to present flow errors such as E1 or F1.

Secondly, because limescale coats the inside of the heater element, output from the heater is reduced. This can lead to increased times to heat the spa water to the set temperature and therefore increased electrical bills. 

You may also come out to your spa and discover the water is much colder than the temperature you had set and doesn't appear to be heating at all, even though the heat icon is illuminated. Because of restricted flow and a scale coated element, the internal temperature of the heater can quickly reach 45C. This causes the heater high limit to activate to avoid causing severe internal damage to the system. The heat icon will remain on but the heater has disengaged itself from the active circuit, meaning no power is going to the heater. The yellow cord rest will need to be pulled to reset the heater's high limit function as seen here. This is only a temporary fix as the high limit will again soon activate because the scale buildup is causing the heater to overheat.

Lastly, because the heater is working so hard and trying to heat through a coat of limescale, it will eventually fail completely. This will result in the RCD tripping every time the heater is engaged.

Limescale is a watercare issue and is therefore not covered under warranty, so it becomes a costly experience that could have been avoided with weekly water testing and balancing. Should you have limescale, you would be responsible for all costs associated with getting the control unit to and from our technical team in Christchurch, technician time of one hour and cost of a replacement heater.

If you have had limescale in the past or are afraid of getting limescale, be sure not to intentionally keep the pH below the OK Range on your test strips. A low pH causes the spa water to become acidic and therefore corrosive as seen below.


 In addition to destroying the heater, this will also damage other components such as the filter pump, cause seals to leak and will damage the spa shell itself as seen in the following image.


If you believe you may have limescale, an easy thing to check is the inlets and outlets on the control unit. These will generally have a chalky white residue all around if limescale is present. 

If you want to go one step further, grab the 3 grey threaded caps that came with the spa and screw into the inlet, outlet and bubbler line (the bubbler cap connects on the outside of the spa). This will save you from losing all of your spa water. Now undo the three grey collars connecting the control unit to the spa. Pull the control unit away from the spa far enough to give yourself enough room to easily access the back flat panel. Unscrew the 5 back panel screws and you will have a good view of the heater. Look at where the metal meets the plastic. If the heater is full of limescale, you will usually see the scale forcing its way out of the heater seam as shown below.

Please note that for customers with a smart control unit, the spa would first need to be drained and then removed from the shell as shown here to access the heater.


If your heater has recently been replaced, you will notice that there is no longer a yellow pull cord. This is because replacement heaters now have a built in reset. If the heater becomes too hot internally (45C) it will disengage itself from the active circuit to avoid damaging the system. The heat icon will remain on, however, and appear to be heating but no power is actually going to the heater. Once the temperature inside the heater cools below 45C, it will re-engage the active and begin heating again as required. This eliminates the need for a manual reset.


For more information regarding water balancing and amount of chemicals to add, please see the attached guide below.

Updated Portable Spas Water Care Guide.pdf.pdf3MB


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