As the desire for a longer swim season grows, pool heaters and heat pumps are becoming more popular.

The units have had significant updates. Modern-day heaters are smaller and designed with angled vent lines. A heater today vs. 10 years ago is half the size with the same Btu output, experts say.

Within the heater category, there has been major sales growth among high-efficiency models. And heat pumps, with their emission-free approach and relative cost savings, have received a big boost in U.S. sales.

Many advancements in heater design and technology have been dictated by the environment. With rising fuel and electricity costs, and greater consumer awareness of eco-related issues, manufacturers have focused on increasing efficiency, automation and functionality. Thus, low-NOx emission heaters are standouts on the market. In addition, solar heating options are more popular.

Following is a basic look at the heating systems on the market and their benefits.

Fossil-Fuel Systems

The workhorses of the industry, these heaters typically run on natural gas or propane. While higher fuel prices have had an impact on this market, the products’ technological innovations have helped improve their efficiency — allowing them to remain an attractive option for consumers. These benefits also influence fossil-fuel heater sales:

Rapid response: Gas heaters are engineered to provide on-demand heating. They can bring the pool or spa up to temperature quickly or maintain the heat, depending on user needs.

Tough action: When sized right, these heaters typically can perform well under extreme weather factors, such as drops in nighttime temperatures and high wind.

Greater efficiency: In recent years, there have been significant improvements in the products’ design — including fan-assisted combustion/exhaust technology, electronic control systems, low NOx emissions and more — that have increased their efficiency. Most offer heater efficiencies in the 80 percent to mid-90 percent range.

Electric Systems

Electric heaters are typically reserved for heating spas or aboveground pools, especially in installations where there is no access to fossil fuels. Most consider them cost prohibitive and an inefficient way to warm larger vessels, but experts say these heaters have unique benefits:

Generous: Electric units transfer all of the heat they generate directly to the vessel, manufacturers say.

Swift heating: Because of their concentrated heat output, electric units quickly

can warm and sustain the temperature of spa water.

User-friendly: Most models are made to be long-lasting, easy to install and compact.

Heat Pumps/Exchangers

Heat pumps have surged in popularity throughout the U.S. While they cost more initially, they have overall lower operating costs and easy installation. Heat pumps offer design improvements and other benefits:

Year-round usability: In areas where temperatures don’t dip below 45 degrees, heat pumps can provide heating all year long. At colder temps, there isn’t enough heat in the air to pass along to the water. These units also are made to automatically maintain the water at the desired temperature, without requiring many adjustments.

Reliability: Heat pumps now carry longer warranties. Plus, newer compressor designs have made these units far more efficient. A typical heat pump now delivers five to seven units of heat for every unit of electricity used, say manufacturers.

Cooling down: Heat pumps, which are basically air conditioners in reverse, derive their heating action from the use of a refrigerant to draw heat out of the air and into the pool water. Because of this design, many units offer a cooling setting that can be used to “refrigerate” the water, lengthening the swim season in high heat areas.

Space savings: While heat pumps are larger, heat exchangers take up little space on the equipment pad.

Solar Systems

Since 1997, the number of sun-warmed pools has soared an average of 13 percent annually, according to the Solar Energy Industries Association. Two types of pool solar systems are in use: electric or thermal, though the latter is the most popular. Obviously, these systems need plenty of sunlight, and the pool should be covered when not in use to aid in heat retention. The following factors have influenced this product segment’s growth:

Cost savings: Solar collectors may cost more initially to buy and install, but operating costs are dramatically lower because solar energy is free.

Looks: Roof-top solar collectors have become sleeker-looking and lower-profile. In some cases, the panels appear seamless instead of segmented. Technological advances also have reduced the number of rooftop collectors needed.

Environment/safety issues: Pilot lights, fuels and corrosive metals are eliminated with solar heating.