What is the best way of heating a pool and spa?

The average pool/spa combo designed so that they can both be run through one set of pool equipment. There are a variety of ways of setting this up for heating a pool and spa.

You can use one set of pool equipment to run both the pool and spa. Valving is used to distribute the water and isolate the pool or the spa as required (depending on usage). Commercial pools and spas use designated equipment to run either the pool or spa. This means there is twice the amount of equipment required – a considerable added expense. Also, commercial spas have onerous water testing and sanitation requirements because they are permanently heated.  Domestic pools do not have to be “heated on demand” like this. Public spas are permanently heated to 36°C and therefore require the water to be tested every 3 hrs. The heat causes them to become literal petri dish of organic compounds (and not the ‘good’ organic compounds)!

People often buy portable acrylic spas for their backyards which can be permanently heated by a low capacity electric resistance type heater. Such a method is reasonably effective because the polymer construction material the shell is built from, the thick foam filled covers, the air gap surrounding the outer spa shell and the deceptively small water volume make them energetically quite efficient. As to the sanitising of the water, these types of spas run high levels of sanitisers/oxidising agents/halides, etc. Often the suppliers “cheekily” market “bromine” sanitisers as a “chlorine free system.” This is deceptive marketing as Bromine is familial in its makeup and functionality with chlorine based systems.

Understanding Ambient Water Temperature

Ambient temperatures at the beach

Ambient water temperatures in Melbourne are little known and as a result poorly understood. During peak summer, standing pool water – unheated – would be in the realms of 18°-20°C. Over winter the temp would be 10°-12°C. The temperature of the bay would be similar – but not as cold during winter as the ocean is a heat sink. The water temperatures indicated are surface temperatures only with the full pool depth temperature being around 3°C cooler.

If you think the standing temp of water at bay temperatures isn’t too bad – as you will swim in the bay over summer without too much discomfort, remember people swim in the bay mostly on those 30°C plus days when cooling off is a great option. Domestic pool usage is about cooling off but also about relaxing – and “chilled water temps” aren’t that relaxing. As a further descriptor people idealise ocean holidays on the Sunshine Coast – water temp 26°C in summer; Port Douglas 29°C. Nobody goes to Hobart for a seaside holiday at 18°C in summer.

Solar Heating

By far and away the most commonly used method of heating a pool is solar heating – using capillary tube. This works by continuously using 0.7kw of electricity / hr ($0.08/hr) to run the solar pump. This then moves approximately 150L of water per minute through a labyrinthine complex of capillary tubes. To put it simply, this is a system whereby the surface area of the pool water is increased and heat/thermal energy from the sun is transferred through the solar collector into the pool water.

On a day with direct sun, 28°C air temperatures and 24°C water temperatures, you will  gain a temperature increase of about 2° as the water re-enters the pool after travelling through the solar. This doesn’t sound like much but remember the water volume of 150L / min is huge. A domestic kettle heating water with a typical temperature gain of 60°C uses approximately 2.0kw of electricity for 3 minutes. A simple calculation to show comparative efficiencies is:

(0.7 kw x 150L x 2°) / 1 min = 210 relative efficiency (kWLC/min)

(2.0 kw x 1L x 60°) / 3 min = 40 (kWLC/min)

This is a simple calculation that shows how efficient solar heating is (and why we don’t use kettles to heat them)! But also shows that it works so efficiently because the bulk of the energy input comes from the sun.

Gas Heating

Most domestic spas use gas heaters as the primary heating system. This is because gas in Melbourne has been widely available, cheap, and is easily to utilised through proven method. The average domestic spa user will want to use the spa on a heat on demand basis requiring the spa water to gain approximately 20°C in a given volume of approximately 1500L of water. A 400Mj/hr gas heater will heat the spa water in approximately 40 minutes which seems about as long as most customers want to wait around to heat their spa water.

Energy input required to heat the spa water not accounting for “losses” = 4.2J x 1,500,000 x 20 = 126MJ.

In 40 minutes of operation, the gas heater theoretically produces / transfers 0.66 x 400 = 264MJ of thermal energy to pool water – a relative efficiency of around 50%.

The rate thermal energy is transferred between systems eg. spa water and the solar collector tube is proportional to the temp difference:

The greater the thermal gradient, the greater the volume of heat transferred; the smaller the gradient the smaller the volume of heat transferred.

So, the heat is transferred in greater volumes when the water temperature is lower; hence the higher temps required for spa water reduces the effectiveness of solar heating to heat spa water.

Heat Pumps

Heat pumps work similarly, by using thermal temperature differences from the air, electrical power to drive, compress gases and highly thermally conductive metals to add thermal energy to the targeted body. In this case, that body is the pool water. They are relatively efficient because the energy they consume is used to power the process used to collect the heat from the air. This leaves the surrounding air cooler and the water passed through the heat exchanger warmer. My experience with heat pumps is they only add a small amount of heat to pool water on a continuous basis. They are not anywhere near as effective as claimed.

What is the most efficient setup for heating a pool and spa?

heating a pool and spa

As per the reasoning above, I believe the best way of setting up the client’s pool/spa combo is:

A single set of pool equipment

  • Valving to isolate the pool or the spa, etc. (A control system controls this valving via a smart device. This is included within quoted price)
  • Solar heating to heat the pool/spa (capillary tube type setup – please see Sunbather brochure attached)
  • An “on demand” gas heating system to heat the spa

I also believe the following system will work:

  • A single set of pool equipment
  • Valving to isolate the pool or the spa, etc. (A control system controls this valving via a smart device. This is included within quoted price)
  • Heat pump to heat the pool/spa
  • An “on demand” gas heating system to heat the spa

Good luck with your decision.

PSNearly every pool builder I know tried using heat pumps about 7 – 10 years ago when the technology became widely available. As a result, most pool builders won’t fit them due to lack of performance, contrasting the promises from sales brochures and representatives. I will fit and supply whatever the client chooses to use. I just won’t warranty the performance of the units.

PPSIt’s been a long time since I had to work through calculations (however rudimentary they may be). Although I believe my theory is right and the compilation of the numbers correct,  some measurements may be little out. These numbers are indicative only.