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The Solar Hot Tub

If you think about it, heating water with the sun isn’t a new thing so why should a solar hot tub system be any different. You see many houses in the UK with evacuated tubes on the roof, these are used for heating the water. Free showers as long as the sun shines. If the sun doesn’t shine then you might get a free shower anyway, it just might not be hot!

The system design can easily lend itself to heating the water in your hot tub. Can you really get the temperature of your hot tub up to a comfortable bathing temperature? The answer is yes and beyond!

Things I Tried So You Don’t Have To

Here are a few things I initially tried that didn’t produce great results

Black Hose Pipe

Black hose pipe coiled on the roof of the shed. Here’s the issue. Hose pipe isn’t meant for heat and hot water. The temperatures from the sun degrade and soften the pipe very quickly and make the water unable to pass. That’s a fail.

Copper Pipe Sprayed Black

Copper tube sprayed black surrounded in a glass case to prevent heat loss. It cost a lot of money for 50 metres of copper pipe and the bore of the pipe was just too narrow to pass huge amounts of water through. Our hot tub contains about 800 litres of water and a small-bore copper pipe was like drip-feeding a dinosaur. The shed roof struggled under the weight of the copper and glass concoction. Adding more pipe would have seen it crashing through the roof.

So What Did Work?

A purpose built product, actually cheaper than any of the other two I tried.

Here’s a list of what you will need to build your solar hot tub.

How Does it Work?

Here’s a diagram to help you picture how it works.

diagram to show how the solar hot tub system works
solar hot tub

Temperature Probes

2 temperature probes from the temperature differential circuit. One goes to solar mats and the other goes in the hot tub. If the leads aren’t long enough on the probes, you could insert some copper pipe in the return and flow of the water hose pipe and attach the probes to the copper. The idea is that the temperature circuit measures the hot tub temperature and the temperature of the solar mat. The circuit will turn on the pump if the solar mats are warmer than the water in the tub.

probe in the hot tub
probe in the hot tub

Water Pump

The pump is submersible so just dropping it into the hot tub with the hose pipe connected will do the job. Connect the positive and negative to the switched side of the differential temperature circuit.

water pump about to go in the hot tub
Water pump about to go in the hot tub

Temperature Differential Circuit

Connect the plus and the minus from the power supply to the circuit. That’s all the wiring done!

Hose Pipe and Solar Mats

One piece of the hose goes from the water pump to the solar mats. The second piece of the hose goes from the solar mats back to the hot tub. The hose pipe is narrower than the connectors on the solar mats. I first wrapped loads of insulating tape around the hose so it would fit snugly. This worked fine but later I removed this and found a piece of car radiator rubber hose that the hose fits perfectly into and then the radiator hose fits perfectly into the solar mats.

solar heating mats on the roof of the shed
solar heating mats on the roof of the shed

Plug it into the wall socket and away you go. As soon as the sun comes out, the pump starts pumping the water around the mats thus heating the hot tub.

How Well Does it Work?

I live in the UK. On a sunny day here (25 degrees Celsius plus) the two mats would raise the temperature about 2-3 degrees an hour. the temperature rise gets slower as the temperature in the tub gets higher. Once the tub gets to 32 degrees, the final 6 degrees can be slow. Not only is the day cooling down at this point but it takes more of the sun’s energy to heat the water at higher temperatures.

If you have 3 sunny days in a row then you will probably need to turn the solar heating off. There is no fail-safe though it’s unlikely to be able to heat the tub above 42 so we have never worried. We tend to turn the system off if we are going away for a week and we know it will be sunny.

here’s a few photos of the hot tub temperature and the temperature of the water coming back into the tub from the solar mats.

33 degrees C current temperature of the hot tub
Hot tub temperature at 33 degrees C
Thermometer measuring temperature of return flow
Temperature from the mats 37.4

How Much Money Will It Save Me?

Currently, with the energy crisis, I would estimate that heating our hot tub with electricity would cost about £6 a day. The water pump probably takes around 25p to run on a sunny day so I would guess it saves me about £5.75 a day. We generally only go in the tub on the hot days so it suits us. On cloudy days the system can still work by making up the degrees the tub has lost in the night so does still save money on the cloudy days too. It will help to keep the hot tub water higher than its standing temperature so will inevitably save you more money.

As always, if you have any questions then please ask in the comments below. I don’t always include all the detail but feel free to ask.

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