"Jump on in! The water feels great!"
Pool heaters are necessary if you have a spa and are usually necessary to heat a pool. Ingenious methods for heating pools have been developed, and if you are a "do-it-yourselfer" you have probably browsed Youtube and Google and seen American ingenuity at its best. But for most folks getting a new pool we typically install a Pentair natural gas pool heater. These are high quality units designed to work well with pool pumps and filters to take the chill off pool water when needed. The good news is that one pool heater is usually sufficient for both the pool and the spa. Pool heaters are pricier components so it's important to chose a good brand that can be maintained and repaired with parts that are easily available.
You'll also want to consider the size of the pool you intend to heat. The larger the pool, the more energy the system spends to heat it up. It typically takes a day and a half to heat up an average sized pool. Never-the-less, nothing can beat the feeling of stepping into a pool that feels good to the touch.