How to do big plastic water heater (PET)bottles. There is a 3 in each bottle. 4mm dropper 5 m. This heater uses the greenhouse effect in the bottle. It\'s like the heat of a car in the sun in summer. Because when the circuit stops, the water inside the water pipe will heat more, it is a good idea to use the timer with I. e. 1 minute, 1 minute. This will also protect the pump motor from excessive heating. With a small water pump, the water cycle, the temperature rises. It is best to use a 12 v dc motor pump connected to several solar cells. In order to concentrate more heat in the bottle, you can also try pasting a piece of aluminum foil. The solar water heater is scalable, so you can connect more bottles and bottle groups to heat the pool.