Flow Through Heaters are ideal for heating up your spa water. They're tube-style heaters with an internal heating element. Water is heated as it flows through the heater, hence the name. The heaters range in length from 8" to 19." However, the most common flow through heater length is 15". The heaters also come in different diameters. They vary between 1.5", 2", and 2.5". Manufacturers we carry range from Gecko, to Jacuzzi, to Balboa and many more! We also carry heater elements for those heaters. Check out our Flow Through Heater collection at PoguePoolSpa.com! Feel free to give us a call at 1-866-739-5658 if you have any questions regarding any of our products.