Hayward say they give a lot of thought to the environmental impact of running their products, and indeed they boast some of the best efficiency ratings around. Since they produce all the mechanical necessities for the pool Hayward try to encourage consumers to by full systems. They offer a decent extension to the warranty that makes it worth considering - they call it a bumper to bumper installation.
They sell two main types of heater model - they have pool heat pumps and they have gas heaters. The gas models include those that live above ground and those that go into the ground. They also do electric spa heaters.
The gas powered systems are called the H-series - the above ground models work on an induced draft basis which improves heating efficiency. There are also models called H-Series Low NOx, which as the name suggested reduce the nitrous oxide emissions. These are in ground models that serve for pools and spas.
Of course, using an established and popular company like Hayward has one further advantage - you will probably never have any difficulty laying your hands on spares!