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

The average domestic pool / spa combo is set up so that one set of pool equipment can be used 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 and not “heat on demand” like domestic pools. Public spas heated permanently to 36C require the water to be tested every 3 hrs, because they are literally a petri dish of organic compounds – but not everything preceded with the descriptor “organic” is good for you!

The portable acrylic spas people often buy and fit in their backyards are sometimes 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 super high levels of sanitisers / oxidising agents / halides, etc. Often the suppliers “cheekily” market “bromine” sanitisers as a “chlorine free system” which 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 -20C. Over winter the temp would be 10 -12C. The temperature of the bay would be similar – but not as cold during winter as the bay / ocean is quite a heat sink. The water temps indicated are surface temps only with the temperature at full depth in the pool being around 3 or so degree 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 degree 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 26C in summer; Port Douglas 29C. Nobody goes to Hobart for a seaside holiday at 18C 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 using continuously 0.7kw of electricity / hr ($0.08/hr) to run the solar pump to move approximately 150L of water per minute through a labyrinthine complex of capillary tubes (basically 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 (capillary tube / black matt) into the pool water). On a day with direct sun, 28C air temperatures and 24C water temps effectively you will gain a temp increase of about 2 degrees 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 temp gain of 60C (typical temp gain for such a device) uses approx. 2.0kw of electricity for approximately 3 minutes. A simple calculation to show comparative efficiencies is:

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

(2.0 x 1 x 60) / 3 = 40

This is a very simple calculation that basically shows how efficient pool solar heating is (and why we don’t use kettles to heat pool water). But also shows that pool solar heating works so efficiently because the energetic input in bulk comes from the sun and less so from fossil fuels.

Gas Heating

Most domestic spas are heated with gas heaters. This is because gas in Melbourne has always been widely available and cheap, with a simple and proven method by which it can be utilised. The average domestic spa user will want to use the spa on a heat on demand basis requiring the spa water to gain approximately 20C 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 mins 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 when the water temp is lower, heat is transferred in greater volumes; hence the higher temps required for spa water reduces the effectiveness of solar heating to heat spa water.

Heat Pumps

Heat pumps work similarly using thermal temperature differences from the air, electrical power to drive and compress gases and highly thermally conductive metals to add thermal energy to the targeted body – pool water in the case of swimming pool heat pumps. They are relatively efficient because the energy they consume is used to power the process used to collect the heat from the air, leaving the surround air cooler and the water passed through the heat exchanger warmer. My experience with heat pumps is they add a small amount of heat to pool water on a continuous basis – and 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 (hydraulic / sanitation / heating setup) is:

A single set of pool equipment

  • Valving to isolate the pool or the spa, etc. (this valving is controlled by the Jandy Aqualink control system via a smart device – included within quoted price and PDF attached)
  • Pool / spa to be heated through solar pool heating (capillary tube type setup – Sunbather brochure attached)
  • Spa to be heated as an “on demand” gas heating system.

I also believe the following system will work:

  • A single set of pool equipment
  • Valving to isolate the pool or the spa, etc. (this valving is controlled by the Jandy Aqualink control system via a smart device – included within quoted price and PDF attached)
  • Pool / spa to be heated through heat pump.
  • Spa to be heated as an “on demand gas heating system.

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. Nearly every pool builder won’t fit them as a result of the lack of performance that so sharply contrasted with what the sales brochures and sales representatives promised. 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). I am sure my theory is right and the compilation of the numbers correct, as to the units of measurement, etc. I may be little out. These numbers are indicative only.