Gas Heater vs Heat Pump - Your first question might be one of cost. One alternative to a gas heater is a heat pump which runs on electricity and uses the outside air to heat your pool or spa. The data shown at left is based on a gas heater and comparable heat pump in the upper range of capabililty. A gas heater will heat your pool in the range of five to seven times faster than a heat pump and it costs half as much as a comparable heat pump. The main issue, however, is operational cost. Depending on energy costs, a heat pump can save you up to 75% over propane gas and 40% over natural gas. If you are in the northern states, you must be aware that heat pumps are only effective when the outside temperature is above 40 degrees. Below that temperature, the heat pump switches over to electric heating coils (resistive heating) which costs more than either gas.
Propane vs Natural Gas - Gas heaters are manufactured to burn either of two gases: propane (LP) and natural gas (NG). Many home owners have access to NG to heat their homes, so NG being available, they would also use it to heat their pools. If you don't have NG piped to your house, you will have to buy a large LP tank and have it trucked to your house periodically. Subject to fluctuating prices, LP gas operating costs can be two and a half times the cost of NG.
Low Nox - Parts of Texas and California have emission standards for heaters. Low Nox heaters are designed to meet the Nox Emission standards set by the California South Coast Quality Air Management Commission for 2001 and the Texas Natural Resource Conservation Commission Code. Texas requirement is for commercial only, not residential.
Durability - For longer heater life look for a cupro nickel heat exchanger and tubing vs the standard copper units. Cupro nickel holds up better in fluctuating pH levels and salt systems. Also many manufacturers are going to polymer/resin headers for superior flow performance and erosion resistance from high flow conditions. ASME requires cast iron headers for commercial applications.
Size - The required size of a heater depends on the size of your pool or spa and how fast you want to heat up your pool. In the northern states you will probably want the larger heaters to compensate for the colder temperatures. Above ground pools use heaters from 55K to 130K BTUs. In ground pools and spa general run from 100K to 400K BTU. For an average 15' x 30' pool (15,000 gallons), a 300K BTU heater would raise your pool temperature 2 degrees per hour; a 400K BTU heater, 2.7 degrees per hour. For a 20' x 30' pool (20,000 gallons) the temperature rise for 300K and 400K heaters would be 1.5 and 2 degrees respectively. By contrast these same heaters would raise the temperature of an average size spa 30 degrees in 35 and 20 minutes respectively.
Millivolt vs Electronic Ignition - Millivolt is the term used for a standing pilot light which stays light all the time. The other option is an electronic ignition which lights the burners with an intermittent spark like today's gas grills. This is a more efficient way to control your heaters gas usage and avoids gas leaks if the pilot goes out. The picture shows the position of the ignitor used in an electronic ignition system.
Force air - Some heaters, notable Hayward, have forced draft system which constantly move air through the combustion chamber at a precise flow rate. This eliminates all outside weather variables that can affect the heating performance, such as high wind conditions. Other manufacturers, like Raypak, eliminate these conditions thru their built-in wind resistant design.
Controls - Heater controls generally vary by the size of the heaters. The smallest heaters may just have a single knob connected to a thermostat to control the heater. The larger heaters might have smart microprocessor with LCD displays and diagnostic read-outs to set and continuously monitor operating status of the heater. Picture shows the control panel for a Sta-Rite heater.
Thermal efficiency - Most heater have an efficiency rating of between 79-83%. This rating is related to the heat transfer from the burners through the heat exchangers to the water. For example, a 300,000 BTU heater with an efficiency rating of 83% actually transfers 249,000 BTUs when heating water. The Jandy Hi-E2 heater has one of the highest thermal efficiency rating at 95%.
Solar Blankets - Regardless of which heater you ultimately choose, invest in a solar blanket to cover your pool when it's not in use. It will keep your added heat in the pool and save you 40 to 50% in operating costs. Click here for Solar Blankets,