March 25th, 2021 by Sarah Miles

Hot tubs need to be hot – the clue is in the name after all. The average temperature you would normally expect in a hot tub is between 37°C to 39°C with most reaching a maximum of 40°C. If your hot tub is seemingly working ok but the water is just not getting warm enough, or is gradually losing heat, there could be a number of reasons. Some are simple to fix and others a little more complex.

Here is a rundown of the common causes of heat loss in a hot tub. Work through them and hopefully, you can resolve without having to call an engineer out.

Issues with the cover

The biggest area you’re likely to lose heat from in a hot tub is the surface of the water, especially if the air is much colder than the water. Obviously, there’s not much you can do about this while you’re using the hot tub, but an inefficient cover could be causing it to lose heat when you think it’s safely tucked up. If the cover is not sufficiently insulated, is ill-fitting, has rips or sags, or just doesn’t fit tightly enough, you could be losing heat this way. If you can see steam escaping from anywhere, this is a major indication you have a problem. It could even be that the cover is fine, but has simply not been replaced properly. Repair or replace the cover to resolve. You could also use a floating spa blanket to retain more heat.

Outside temperature is too low

Many people love to use their hot tub all year round, which is fine, but bear in mind that cold winter temperatures can make it a struggle for some spas to get up to full heat. This tends to apply more to portable spas, spas that lack sufficient insulation, or lower-cost hot tubs that have less powerful heater elements. If this is what you have, and you don’t fancy upgrading, you may be able to improve things by insulating the hot tub underneath or around the sides, using a floating spa blanket, or buying a more efficient cover.

Water level too low

Your water level doesn’t stay the same once you’ve filled your hot tub. Water is mainly lost through evaporation, but it can also be lost as people get in and out of the tub. If the water level gets too low it can cause the hot tub skimmer to start sucking in air instead of water, which will make the heater cut out as a safety precaution. Try increasing the water level and see if this works.

Dirty filters

If your filters are dirty and clogged up it can restrict the water flow, which is a common cause of heat loss. Clean filters are an essential part of good hot tub maintenance anyway so give them a thorough clean with a spa filter cleaner and see whether that makes a difference.

Blocked pipes

In the same way that dirty filters restrict water flow, so do blocked pipes and lines. A system flush will run through the whole unit, clearing any blockages, dirt, bacteria, and general gunk from the pipes and allow them to run freely. Again, all part of hot tub maintenance best practice so never a bad idea and should always be done whenever you change the water.

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The spa hasn’t had long enough yet

If you have just refilled your spa from scratch, you may just need a little more patience as bringing it up to the desired temperature can take a while. You may need to run the heater continuously for up to two days to achieve this. Wait until it’s at the right temperature and then set the timer to run for 4-8 hours per day, as needed to maintain it.

Heater not running for long enough

To maintain the temperature, the hot tub heater is designed to run for an appropriate period of time every day; usually around 4-8 hours (as mentioned above). Various factors can affect how long you will need this to run for including the volume of water and the outside temperature. If your hot tub is not retaining heat sufficiently, you may just need to adjust the timer to run for longer each day.

The heating element has scaled

Limescale development is common in hot tubs and can happen because you are in a hard water area or your water is too alkaline. Limescale will affect the heating element, and this will compromise its performance. Cloudy water can be an indicator of scaling so if this is also an issue, this could be the cause. Use a limescale remover to get rid of it. In future, keep a closer eye on pH levels to ensure the water doesn’t become out of balance again.

Next steps

Hopefully, at least one of the above steps has resolved your issue. However, if you have checked and tried all of them, then it may be that there is some kind of technical fault such as an issue with the thermostat or heater, and this will almost certainly require the attention of a hot tub engineer.