Direct fired natural gas or propane heaters are the most common type of heater found at pool and spa equipment pads.
This type of heater incorporates a finned copper tube heat exchanger that is enclosed in a combustion chamber. The water is heated as it passes through this copper tube due to heated air which is warmed by burning fuel.
The heater has three sections that include the ignition assembly with gas valve and burner tray, the main control circuit section, and the heat exchanger.
Fuel is burned in the inside the combustion chamber in the burner tray. The gas valve, which supplies gas to the pilot and main burners, is in the ignition assembly.
A gas regulator, either separate, or built into the gas valve, controls the amount of gas to the burners. Gas is supplied to the burners and in the presence of air, ignited.
There are three main types of ignition: standing pilot, spark ignition, and hot surface ignition.
Standing pilot, or Millivolt ignition runs with the pilot lit at all times. The flame is converted to a small amount of electricity (millivolts) to run the control circuit.
A thermocouple converts heat into direct current. The current is needed to use the control circuit which allows electricity to pass on to the main gas valve to fire up the unit.
With spark ignition, the pilot is lit only when the heater is in use. Power (120 or 240 volts) is brought to the heater and connected to a transformer that reduces the current to 24 volts to provide the electricity to an intermittent ignition device (IID) to spark the pilot on. The IID simultaneously sends current to the gas valve to open the pilot gas line. When heat is required, the pilot provides an ignition source for the main burners.
A hot surface igniter is found on newer heaters, and considered the safest as it eliminates the pilot altogether. Like a light bulb filament, as electricity passes through the igniter, it glows red hot.
Whentheproperoperatingtemperature is achieved, the gas valve is opened which allows gas to be ignited by the red-hot element of the hot surface igniter.
When that flame is sensed by the flame sensor, the hot surface igniter is shut down.
Sizing
Gas heaters are sized from about 50,000 to 5,000,000 Btu and are great for both continuous heating and heating only when needed. Typical residential heaters are about 399,000 Btu’s. If a heater does not appear to be sized correctly, it should be replaced.
Sizing a gas heater requires a simple calculation. First, determine the desired temperature rise based on the ideal water temperature and the air temperature for the coldest month of pool use. In Northern California, for example, low winter temperatures average about 50 °F. For those wishing to swim in water of about 80 °F, the temperature rise would be 30 °F.
Next, calculate the pool’s surface area.
Multiply the surface area times the temperature rise times 12*.
For example, if the temperature rise is 30 °F and the pool is 300 square feet, the required Btu’s needed are 108,000.
*This formula is based on a 1 ° F temperature rise per hour and an average 3.5 mile per hour wind speed.
It important to remember that gas heaters are not 100 percent efficient. Most are about 70 to 80 percent efficient, so it is important to account for this.
In this example, 108,000 Btu’s are needed. However, for a heater that is 75 percent efficient, the actual needed Btu’s are 144,000.
Experts advise that heaters continue to lose their efficiency as they age and they lose efficiency for a number of reasons.
One issue is a build-up of chemicals on the heat exchanger tubes that reduces water flow.
Another issue is when condensation drips onto the heater ’s burners, interfering with the flame pattern and reducing combustion.
Over time, low gas pressure will also cause damage to internal parts in the heater, ultimately blocking the heat exchanger.
Finally, if the heater has not been properly ventilated, a soot layer can form. This will insulate the heat exchanger, and heat does not transfer to the water as well.
For these reasons as well as the fact that new heaters begin with higher efficiencies (90%) as well as reduced emissions, it is recommended to upgrade old heaters: newer heaters reduce energy consumption and lower operating costs.
