Choosing a heater for your pool
Swimming pools and spas gain heat in three ways. The first is natural sunlight, absorbed directly by the water. About 90% of the sunlight that reaches the surface of a pool is absorbed. Time of year, shade availability, pool location, and layout affect the amount of sunlight that can be absorbed. Secondly, an indirect energy source in sunlight that has been absorbed by the pool deck and transferred by conduction to the water through the pool wall structure. The third source of heat gain is by fossil fuel, electric, heat pump, heat exchangers or solar heating systems.

Gas Heaters

The fuel in a typical gas heater system is natural gas or propane. Natural gas is lighter than air. If the heater burner tray is flooded with gas but not ignited, the natural gas will escape. Natural gas has an additive, giving it an odor that can be detected.

Propane gas is heavier than air and also has an additive to give it an odor. If for some reason the propane fails to ignite, it will remain on the bottom of the heater. Should the concentrated propane ignite suddenly, a violent explosion will occur. The force of this energy release will blowout through the open front panel of the heater, possibly causing serious injury or death to anyone standing close to the heater. The pool and spa operator should always follow the manufacturer's troubleshooting instructions for ignition problems.

There are two methods of ignition for a gas fired heater: millivolt and electric pilot. Millivolt ignited heaters have a pilot light that is constantly burning. The heat of the pilot is converted to a small amount of electricity by a thermocouple. With the proper switches, the thermocouple creates a control circuit. Electricity passes through the circuit when it is closed, opening the main gas valve. The gas floods into the burner and is ignited by the pilot light. State or local codes may ban millivolt ignition. The temperature in the burner chamber can approach 1,100 OF (593.33 °C).

Electronic ignition heaters produce an electronic spark when they are turned on. This spark ignites the pilot, which then ignites the gas in the burner in the same way as in the millivolt ignited heater. The control circuit is usually 25 volts AC.

Electric immersion element heaters are often used for spas. They have a high cost of operation and a slow heat rise or recovery time. An electric coil is immersed in water that flows through the unit.

Gas Heater sizing

The main concern in properly sizing the heating system is to understand what the desired water temperature is and the amount of time it will take to achieve that temperature under normal conditions. A heater that is too small will have a long heat rise time. A heater that is too large will increase the initial installation cost.

The process of selecting the type and size of heater will differ as to whether the heater is for a pool or a spa operation. The main consideration for a pool is the amount of heat loss through the surface. For a spa, the main consideration is the amount of time to reach operating temperature. Another concern is whether the heat will be used to maintain a certain temperature or will be used intermittently. For maintenance heating, surface area is important. For intermittent heating, water volume is important.

Other decision factors include wind, altitude, and how the pool is shaded. For every 1,000 feet (300 meters) above sea level, the heater output requirements increase by 4%.

When sizing a heater, multiply the number of gallons by 8.33 (pounds per gallon) and by the temperature rise:

BTUs = Gallons x 8.33 x °F temperature rise

The answer is the number of BTUs required to heat the pool or spa initially but is not a good representation of the maintenance heating needs.

Here's an example using a 6-degree temperature rise in a 40,000 gallon pool. You want to raise the temperature of your 40,000 gallon pool from 74 °F to 80 °F. How many BTUs will this require?

Example 1:

BTUs = Gallons x 8.33 x OF temp. rise

BTUs = 40,000 gallons x 8.33 x 6

BTUs = 1,999,200

 This number can either be divided by the desired heat-up time to give you the required heater output, or it can be divided by the heater capacity to give you the heat-up time a given model will provide.

The amount of heat required to maintain a pool at a specified temperature is usually much less than the initial heat up requirements. Determining site conditions such as air/water temperature difference, wind conditions and shade levels will be necessary to calculate maintenance heating requirements.

In worst case situations unblanketed pools can lose as much as 50% of total pool temperature rise. Blanketed pools reduce these losses to the 15% to 25% range.

Example 2:

Using a 10 degree temperature rise in a 40,000 gallon pool. You want to raise the 40,000 gallon pool from 72 degrees to 82 degrees and maintain it at that 82 degree temperature. Your pool uses a bubble blanket for heat retention.

BTUs = Gallons x 8.33 x degrees of temperature rise x.20 (20% of daily loss factor)

BTUs = 40,000 x 8.33 x 10 x .20 BTUs = 666,400.

 Once you know how many BTUs per hour the heater provides, you can calculate the number of therms provided. Divide the BTUs used per hour by 100,000 to calculate the therms per hour consumed. Multiply this number by the hours operated per day to get the daily consumption of therms. The cost per therm is shown on most gas bills.

Metric Cost of Operation Example

You have a 5,300 liter spa and wish to raise the temperature by 4.5 degrees Celsius. How many kilojoules are needed?

5300 x 4.18 x 4.5 = 99,693 kilojoules

There are 3,600 kilojoules per kilowatt hour Daily kilowatts required to raise the temperature are:

99,693 + 3600 = 27.7 daily kilowatts

If your heater output is 5 kilowatts, then your heater will need to run for:

27.7 - 5 = 554 hours

Multiply the 27.7 kilowatts by the cost per kilowatt to know the final cost.

Heater Installation

The American National Standards Institute offers guidelines: ANSI Standard 2223.1. This standard sets clearance requirements based on the external temperatures of heaters, which may vary from manufacturer to manufacturer.

All heaters must be installed at least five feet from the inside wall of a spa unless the heater is separated from the spa by a fence, wall or other permanent barrier.

The heater must be installed on a level, noncombustible base such as brick or concrete. If concrete blocks are used as a base, they must be aligned so the cells are all pointing in the same direction; the end should be left open. When such hollow masonry is used, the pad must be at least four inches high and covered with at least a 24-gauge piece of sheet metal.

If the heater is placed in an area exposed to high winds, the unit either must be installed at least three feet from the nearest wall or a wind block must be constructed to help minimize the wind effect.

The heater should be installed downstream of the pump and filter and upstream of any automatic chlorinating, brominating, or ozone disinfecting equipment.

If the circulation system is run with a timer, the heater should be equipped with a separate low- voltage switch that deactivates the heater before the pump is turned off. This useful circuit is known as the heater's fireman switch. On a millivolt heater, the length of wire between the heater and the timer should not exceed 30 feet. Resistance on longer runs will reduce the millivolts to a level that will not support reliable operation of the gas valve.

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