Hot Tub Heaters

Hot Tub Heater Selection and Sizing

Each manufacturer makes different claims about the efficiency and effectiveness of its hot tub heaters, but all estimates depend on a hot tub that is covered and generally isolated from severe weather elements, such as wind, snow, or extremely cold temperatures.

Gas heaters are rated by the output of heat as expressed in British thermal units (Btu's). Gas heaters for hot tubs range from 50,000 to 200,000 Btu. Electric heaters are rated by the kilowatts consumed and, therefore, the Btu's produced. Here is a comparison of energy use and output of the most common models:

• 1.5-kilowatt (1500-watt) heater = 5119 Btu
• 5.5-kilowatt (5500-watt) heater = 18,750 Btu
• 11.5-kilowatt (11,500-watt) heater = 37,500 Btu

The following chart compares typical electric-fuelled and gas-fuelled heaters in terms of performance:

Temperature Increase (per Hour) in Degrees Fahrenheit

As you can see, electric heaters are not practical in larger hot tubs / spas. Another way to calculate the thermal output of an electric heater is with this formula:

Heater input (kilowatts) x 410 % hot tub total gallons = degree (Fahrenheit) increase per hour

Using this formula, we assume a 500-gallon hot tub and a 5.5-kilowatt heater. The calculation looks like this:

5.5 (kilowatts) X 410 = 2255
2255 % 500 gallons = 4.5°F increase per hour

Cost of Operation

The cost of operating a heater is simple to figure out if you know what your customer pays for 1 therm of gas or 1 kilowatt of electricity.

A therm the unit of measurement on your gas bill is 100,000 Btu per hour of heat. My last gas bill showed I pay about 50 cents per therm. The hot tubs heater model tells you how many Btu's per hour your heater uses. Divide that number by 100,000 to find out how many therms per hour that will be. Next, determine how many hours of operation are needed to bring the temperature up to the desired level. Let's try an example with a 200,000-Btu heater operating for 2 hours each day:

200,000 Btu % 100,000 = 2 therms per hour
2 therms x 2 hours = 4 therms per day
2 therms X 50 cents = $1 per day

Of course this calculation assumes the heater is operating for the entire 2-hour period. If you reach the desired temperature in less time, the heater will shut off and will restart only as needed to maintain that temperature.

How much does it cost to keep a standing pilot burning in a millivolt heater? Pilots use between 1200 and 1800 Btu per hour, so the cost calculation is the same as the formula already described. By the way, the temperature of that little flame is over 1100°F, so when you remove a pilot assembly for repair, don't grab one that has recently been lighted.

Electricity is sold by the kilowatt-hour (1000 kilowatts consumed per hour) which equals 3412 Btu. I pay 15 cents per kilowatt-hour, so if I have a 5.5-kilowatt heater, the calculation looks like this:

5.5 kilowatt X 15 cents per kilowatt-hour = 82.5 cents per hour

Once again, the actual daily cost will depend on the desired temperature and starting temperature of the water.

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