NOTE: The following applies to SPA's with AIR JETS OFF.
See SPA HEAT LOSS for Heating with Air Jets On .

Spa Heat Up Times

The graph to the above shows how fast five sizes of spa heaters with increasing heat output, will take to heat up a spa 20° F assuming a 75°F air temperature. Increase the heat up time by 1% per degree F below 75°F.

The first four heaters shown above are the same size as the three basic Heat Siphon models - 200HP, 375HP and 575HP. The last one is a larger size gas pool heater.

ALL THREE HEAT SIPHON SIZES are more than large enough to maintain all but the largest spa at 102°F (39°C) with no trouble. The size you pick depends on how fast you want to heat up your spa, unless you want to heat with blower supplied air jets on (see SPA HEAT LOSS ).

The 30 Minute Rule

Most spa owners expect their heater to take no more than 15 to 30 minutes to heat their spa to 102°F. By finding your spa volume on the horizontal axis and following it vertically to the intersection of one of the colored lines, then reading the minutes scale, you can see how long it will take that heater to produce a 20°F rise. Here are the spa sizes each model can heat up 20 °F (from 80°F to 100°F ) in 30 minutes:

• 200HP Heat Siphon should be used up to 125 gallon spas
• 375HP for spas up to 300 gallons
• 575HP for spas up to 350 gallons

Should You Heat up on Demand or Maintain Your Spa at 102°F all the time?

If you use your spa more then a few times a week then consider keeping it at temperature all the time. The cost to KEEP a covered spa at 102°F (39°C) for about 8 hours is roughly the same cost as each heat-up.

HEAT SIPHON FAST FACTS -The following formula can be used to estimate heat up time for any size spa using any heater to any desired degree rise: Hours Required = #gallons x 8.34 x #degree rise / heater BTUH output.

What Heat Siphon Models will Heat a Spa?

All models of Heat Siphon are designed to heat either a pool or spa.

Analog Heat Siphon modelsl with the knob turned all the way up to 10, will heat water up to above 104° F before it shuts off.

Digital models have a built in Dual Thermostat so you need only set separate temperatures for the pool and spa once and then just switch modes.

Heat Siphon SAVES 80% vs. Electric Spa Heaters

A normal above ground spa and most in ground spas are 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.

Unless they are designed to allow plumbing in an external spa heater you may not be able to use a Heat Siphon.

The 10 to 12 Kw built in electric resistance heater that comes with many prefabricated spas is equivalent to approx. 36,000 BTUH

HEAT SIPHON's smallest model - the 200HP is 50,000 BTUH or 25% larger BUT it will USE 80% LESS ELECTRICITY

Watch Those Air Jets!!

Air jet heat loss is so much higher than the other modes of spa heat loss, that YOU SHOULD KEEP THEM OFF UNTIL YOUR SPA REACHES THE TEMPERATURE YOU WANT.

(see SPA HEAT LOSS )

Although the trend is moving away from the bubbly blower air jet use in favor of water message jets which require one or more booster jet pumps, many water jet systems inject air into the stream which still increases heat loss.

Most spas use at least an 18,000 to 36,000 BTUH (10 KW) heater which is more than sufficient to heat-up most pre-fabricated spas in less than an hour, there usually NOT big enough to keep up with air jets while heating.

IF YOU HAVE A BLOWER FED AIR JET SYSTEM ON YOUR SPA AND WANT TO MAINTAIN TEMPERATURE WITH THEM ON - you need to size your heater accordingly

(see SPA HEAT LOSS )