The A4s is a universal air pressure switch for the boiler and water heater, which pressure is 39 Pa for ON, and 60Pa for OFF. Unlike the traditional pressure switch, it can reduce temperature error and control it in a small range. Meanwhile, the A4S can work well in extreme humidity or extremely cold environments, which is also better than the previous generation. So you can use it with satisfaction in the device. Besides, the AS4 series is not only high precision but also sealed structure in a positive/ negative chamber, which is dustproof and waterproof. For more pressure & flow products information, welcome to visit our product page.