Solar Hot Tubs - Make your hot tub energy efficient for 2020

With the increased efficiency of solar thermal panels, heating hot tubs has become more and more popular using solar. The basic principle of a solar hot tub is to use the solar collectors during the day to heat the tub. In the evening you may find you will still need an auxiliary heater. We always recommend using keeping your existing heater and running the solar on a secondary loop.

If you are in a warmer climate or using your hot tub in non-freezing conditions, we often recommend Active "Direct" Open Loop systems when customers are looking for a simple or low-cost solution. In many cases an Active "Direct" Open Loop system can be installed using existing hot water stores and cylinders, most of the existing pipework remains the same and these systems integrate nicely with existing boilers. It is best used with a solar PV panel and a DC 12V solar circulating pump.

If you intend to heat your hot tub during freezing conditions via solar then you would need a system that uses heat pipe collectors with antifreeze mixture and a heat exchanger that integrates with the existing hot tub circulation system. For the solar loop a DC pump and PV panel can also be used. Alternatively, the hot tub could be used as a secondary zone for domestic hot water heating or solar space heating. In this way the SDS (Solar Distribution System) will control the heat of the hot tub as well as hot water. A dual zone system allows for maximum use of solar all year round.

Hot Tub Solar Thermal System Design

Solar Thermal water heating systems capture the sun's energy in the form of heat which is transferred to hot water cylinders. Replacing the need to burn fuels such as gas, coal or wood. Depending on the size of your hot tub, your daily hot water requirements, your current hot water system, your budget and where you are located there are a few ways you can put a system together. Below we outline some of the system designs that you could implement

Active "Direct" Open Loop

In an active Direct open Loop System, the water you use in your bath and taps would be the same water that at one time would have been heated by the sun in your solar panel (or other collector).

Colder water is taken from your hot water cylinder and pumped (the active part) to the solar collector to be heated. Hot water is then returned to the cylinder. In the UK to protect from freezing we install a controller that will shut off the supply to the collector once the outside temperature drops to a certain level.

We often recommend Active "Direct" Open Loop systems when customers are looking for a simple or low-cost solution. In many cases an Active "Direct" Open Loop system can be installed using existing hot water stores and cylinders, most of the existing pipework remains the same and these systems integrate nicely with existing boilers and connects easily to your hot tub.

Active Drain-Back System (with Water)

With an active Drain Back system using water the temperature of the water in the cylinder is monitored, once the temperature drops below a certain level water is pumped up to the Solar Collectors to be warmed. When the water is not being used it drains back into the system to avoid freezing. Drain Back systems are also popular due to their ease to install and simple design.

Active "Indirect" Closed Loop (With Antifreeze/Glycol)

Active "Indirect" systems use a dedicated loop to transfer a fluid such as Glycol between the Solar Collector and the hot water storage cylinder. The fluid is heated up in the collector, this heat is then transferred to the water in the cylinder via a heat exchanger (often a heating coil similar to a heating element in a kettle).

The second coil in this kind of system is used to transfer heat from other heating methods such as boilers and immersions when the Solar Collectors aren't being used.

Active "Indirect" Closed Loop systems are very popular in the UK but slightly more expensive then the direct systems as a twin coil cylinder will need to be installed if you don't already have one and there is more pipework.

Passive Thermal Siphon

This is where a water storage tank is placed higher than the solar collectors to allow the heat captured by the collectors to rise into the tank and warm the water. This is not a very efficient system but as it doesn't expend any energy it may be useful in some situations.

Batch Heaters

This is where the storage tank or cylinder is the collector, the water is warmed by the sun as the cylinder warms up. With this type of system thought needs to be given about what happens to the stored heat at night.

Solar Thermal Equipment

  • Solar Thermal Collectors: - Also called Solar Panels, can be flat plate or evacuated tube.

  • Hot Water Cylinder: - Often but not always a twin coil cylinder, with heat from the collector heating the water in the tank for use in the hot water system.

  • Solar Thermal Controller: Used to control temperatures and the pump.

  • Solar Thermal Pump: Used to circulate fluid between the collector and the pump.

  • Mounting Kit: Used for installing onto roofs and other suitable surfaces.

  • Pipework: To connect the components to existing hot water system.

Choosing a Solar Thermal Collector

Solar Thermal Collectors form the head end or energy capturing end of a Solar Thermal water heating system. There are broadly two types of collector, Flat Plate Collectors and Evacuated Tube Collectors.

When evaluating the two different technologies the three main things to consider are:

  • The available space on the roof or other mounting area

  • The location of the installation in relation to the sun

  • When the hot water demands are likely to be in terms of the time of year

What is a Flat Plate Solar Collectors?

A Flat Plate Solar Collectors can be described most simply as a heat absorbent material mounted in a sunny spot. The collector absorbs heat from the sun and then transfers that heat into fluid that is circulating around the system. Where a traditional boiler would burn gas, oil or coal to heat the water you would use the sun's heat, not needing to burn anything.

What are Evacuated Tube Collectors?

An Evacuated tube collector look a bit like fluorescent strip lights, although they work quite differently. An evacuated tube system consists of individual tubes with fluid circulating through them absorbing the sun's heat. From there the system is much the same as the Flat Plate system mentioned above.

Flat Plate Vs Evacuated Tube

A flat plate system is generally slightly cheaper as they usually have less engineering costs associated with their manufacturer. Yet in ideal conditions, measuring peak performance a flat plate system could compete or even beat the performance of an equivalent evacuated tube system.

An evacuated tube system is likely to perform slightly better in lower light environments so you may find in a like for like comparison with an Evacuated Tube system, the tubes would squeeze a bit more heat out of the available sun in Spring/Autumn or on duller days.

Both have their uses, and both work effectively in the UK, we don't believe that one technology is better than the other. The right collector to choose will be the one that looks best on your property and is big enough to provide for all of your hot water requirements.

Cost Vs Performance

When evaluating collectors it's always useful to think about cost Vs mounting area Vs performance, for example if you had a large south facing garden or roof at the optimum pitch and a big flat plate collector this system is likely to deliver equal performance or more likely better performance when compared to a smaller, more expensive evacuated tube system.


If you had only a small amount of available space on a wall, roof or balcony or less the perfect conditions, you may find the curved nature of the tubes makes up for lack of space by always offering a larger surface area to the sun, you may find you get more energy per sqm from Evacuated Tubes in less than ideal conditions.


Both Flat Plate and Evacuated Tube systems require very little maintenance. There are few moving parts and the inner workings are well protected from the elements. If something happens to the collector, it's easy to swap out whole collectors and individual tubes.

Both systems will perform better when clean and free of dirt/dust so if dirty and where feasible its worth washing them both in much the same way as you would clean a window using just water.

Most Solar Collectors have been designed to expel water and dirt effectively on their own so if you can't get to them and don't notice a drop-in performance, they are likely to be ok.

Choosing a Solar Hot Water Cylinder

The Solar Hot Water Cylinder is arguably the most important and influential part of a Solar Thermal hot water system design it determines:

  • How much hot water will be available for use in the property

  • How big or how many solar collectors it will take to heat all of the water

  • What type of solar collector to use?

  • The Solar Thermal Design to implement

If you get the size and type of cylinder for the installation right, then usually the other elements of the system will fit nicely into place.

Single Coil Cylinders:

Single Coil Cylinders can be used with Solar Thermal installations, if you already have a water heating system with a single coil cylinder then you do not neccessarily need to replace it. When using a single coil cylinder a 'direct' heating method is used, this is where the hot water from the collector warms the water in the cylinder directly whilst returning colder water to the collector for re-heating.

Twin Coil Cylinders:

Having a twin coil cylinder installed is like having a combined dedicated solar cylinder and a normal cylinder at the same time. Fluid in a closed system circulates from the Solar Collector to the Cylinder through a coil running through the cylinder. Colder water is returned to the collector to be heated. The second coil is connected to a traditional boiler or other heating source. A Twin Coil cylinder is essentially getting hot water from 2 independent sources. When looking at twin coil cylinders, long thin ones are generally better.

Investing in a twin coil cylinder is great for those who:

  • Need to replace or upgrade their existing cylinder anyway

  • Want water to be heated from more than one source simultaneously

Solar Thermal Benefits

  • Long Warranties & system life

  • Few planning issues

  • Quick to install

  • Simple Maintenance

  • Some can be roof integrated

Does solar thermal really work in the UK?

Contrary to popular belief the amount of solar radiation received across the UK makes our country ideally suited to delivering a highly efficient solar water heating system.

The solar heating map highlights the medium to high intensity of sunlight received across much of the UK. For the areas which are exposed to lower levels of sunshine, solar companies have specifically developed a range to utilize both direct and indirect sunlight. The map below gives you an idea of your total exposure.

Wood-fired water heater

Looking for another way to heat your hot tub? Wood-fired water heaters circulates water using the principal of thermosiphon (the pumping action created by rising hot water), eliminating the need for a circulating pump or electricity. This unique feature opens up a whole new range of possibilities for alternative hot tubs. Now you can have a basic soaking tub without pumps, chemicals, or high maintenance. Most wood-fired water heaters can be retrofitted to your existing hot tub or put together a low-cost soaking tub, using a wooden tub or stock tank.

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South Lanarkshire



Telephone: 0800 112 3886




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