Outdoor SPA Heat Loss

By definition ALL SPAS NEED HEATING.

If you have an in-ground concrete spa then the heat losses are identical to a pool (see pool heat loss) .

If you own a spa with a pre-fabricated acrylic , fiberglass or vacuum-formed plastic shell either above or in ground, it will be very well insulated and usually have a built in hard cover - all of which eliminates most heat loss. Since the hard cover is usually opaque it also eliminates most re-radiation heat loss.

Once heated to temperature these spas will only lose heat rapidly when UNCOVERED just like a pool does - (via evaporation, convection and radiation heat loss) unless air jets are on.

HEATING WITH AIR JETS ON

Heat loss when using air jets is significantly greater than any other mode.

In fact, if you want to use a larger blower AND MAINTAIN THE SPA TEMPERATURE WHILE THE AIR JETS ARE ON, the cubic feet per minute (CFM) rating dictates the MINIMUM SIZE SPA HEATER REQUIRED.

The graph shows what percentage of the total heater output is consumed by the blower heat loss based on CFM. If your heater output is more than 90% consumed by the blower, then it will probably not be able to maintain spa temperature over time.

HEAT SIPHON FAST FACTS - A normal above ground spa is usually 25 to 75 sq feet in surface area and 200 to 600 gallons which is about ONE TENTH the size of an average residential swimming pool.

SIZING SPA Heaters

ALL THREE HEAT SIPHON sizes (2.25HP, 3.25HP and 5HP) will be large enough to heat-up just about any size spa to 102°F - IF THE AIR JETS ARE OFF.

The Heat Siphon model (size) you pick will depend mostly on how fast you want your spa at temperature from the time you turn your Heat Siphon on, (see SPA HEAT-UP RATE ).

NOTE: Using blower supplied air jets while heating will cause the temperature to drop if your heater is not large enough - see the sizing discussion to the left on this page.

SPA TYPES

There are basically two types of SPA installations - Spa only and spas with pools:

SEPARATE SPAS :

1. Above Ground -prefab shell
2. In Ground - prefab shell
3. In Ground - concrete

POOL/SPA COMBINATIONS:

1. Spill Over Spas
2. Separate Pool & Spa

Above ground spas usually have a built in electric heater and many are not designed to allow external heater use.

The rest can and usually do allow hook-up to an external heater.