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What Are The Differences Between Hot Tubs and Pools?


There are a number of ways in which a spa is different from a pool. 

Pool and Spa in a Backyard

Size and Portability

Spas are much smaller than pools averaging less than 500 gallons of water.  In addition, most spas are portable in their design and construction, which allows them to be moved to another location.  The size difference between pools and spas seems obvious. What is not so obvious is how this difference can affect the water quality.

Bather Load

As a direct result of the size and gallonage, each bather has a huge impact on a spa compared to a pool.  Four people in a 20,000 gallon pool have 5,000 gallons of water for each bather.  The same four people in a 500 gallon spa have only 125 gallons of water for each bather.

In order to get the same bather load in a 16 x 32 pool, you would have to pack 160 of your closest friends into that pool.  This comparison makes no allowance for the additional “stress” put on the water by the heat.  In fact, for every 30 minutes spent in water heated to 102°, the average person loses about two pints of sweat and body oil.  So add 20 gallons of sweat and body oil to that pool every 30 minutes.  What a nice thought!


Most spas are kept between 100 and 104 degrees.  Hot water is very therapeutic.  It increases the blood circulation to muscles and joints delivering more oxygen to aid in the body’s natural healing process, and removing lactic acid that causes sore aching muscles.  Additionally, the hot water lowers the blood pressure, reducing strain on the heart and other vital organs.   Add to this the near weightlessness your body experiences from being immersed in water, then you can understand why more people are spending more and more time relaxing in a spa.

This hot water is doing more than relieving stress and relaxing muscles.  The heat of the water is also loosening all the oils and cosmetics on skin.  In addition, hot water is an ideal breeding ground for bacteria.  In a pool that’s kept at 84° F, bacteria double in number every 30 minutes.  When you raise the temperature to 104°F, those same bacteria will double in just 14 minutes.

Hotter water has different mineral requirements than cooler water does.  This is especially true of calcium.  The hotter the water gets, the less calcium wants to stay dissolved in water.  The resulting calcium fallout is termed scaling, and this scale can cause considerable problems if proper water balance is not maintained. The use of SpaGuardâ Stain and Scale Control limits the ability of the calcium to come out of solution to form scale.  The heater is the most critical piece of equipment to protect from scaling. Not only will scale formation on the heater reduce the heater’s efficiency, but heavy deposits can cause the heater element or heat exchanger to burn up.  Not a very welcome expense for a customer who bought the spa for relaxation.


Aeration can occur two ways in a spa:

1.        Venturi Aeration through the spa jets.

2.        Aeration from an air blower system.

Aeration through the spa jets occurs when air is mixed with water at the spa jets themselves (either through venturi controls or air blower).  The air displaces the water, causing the water to speed up.  (This is the same principle of physics that makes airplanes fly.)  This fast moving water also creates the pounding action that relieves muscle tension.  So aeration adds to the therapeutic quality of a spa. It can also cause two significant challenges to hot water care:

§  The force of spa water washes more dirt and oil from the skin, causing increased oxidizer/sanitizer consumption.

§  The combined effort of the heat and aeration causes carbon dioxide (CO2) to gas-off (come out of solution) from the water.  CO2 has a very important role in water balance.  Its presence in the water allows for the formation of carbonic acid, which serves to hold the pH down.  When CO2 is lost, the pH of the water has a tendency to rise.  A rising pH in water balance can cause many things to happen.  Some of them are:

  • The sanitizer becomes less active (especially chlorine).
  • The water will turn cloudy.
  • Dissolved metals will come out of solution and stain the surfaces.
  • Scale can form on the surfaces and in the equipment.

Materials for Spas

Most spas are made from different materials than pools.

§  Wood – The origination of the term “Hot Tub” came from these wooden vessels.  Everything from cypress to teak has been used to make hot tubs. Wood is very porous and hard to clean, so it’s rarely used in modern spas.  The tannins in wood form tannic acid when exposed to water.  The curing process for some of these wooden vessels will actually require that the hot tub be filled, then drained after a period of time, to remove the excess tannins from the wood.  The leaching process of tannic acid does cause the pH to drift downward over time.

§  Plaster – Most plaster spas are attached to pools which are also made of plaster.  This relatively porous surface gives microorganisms, dirt and oils a place to hide. Plaster surfaces are more difficult to sanitize and clean than other types of spa surfaces.  Additionally, due to the alkaline nature of the plaster finish, the pH will tend to drift upward.  Calcium is another component of the plaster finish that, given the right conditions, will contribute to the spa’s scale-forming tendency.

§  Thermoplastic- Enduralä/Rovellä/ Acrylic – These materials are vacuum formed into numerous spa sizes and shapes.  The advent of thermoplastics was an important evolution in spa design because it allowed manufacturers to produce units with pre-formed seating for greater bather comfort.  Most of these thermoplastic surfaces are very easy to clean and are more resistant to algae and bacteria growth. Thermoplastic spa interiors have very little effect on the pH or mineral balance of the spa.