HEAVY DUTY filters are more resistant to water system pressure and much bigger in size, so they need rare replacement compared to conventional household filters. Some of them are specially designed to withstand hot water, some are 20 inches high, others are BIG BLUE, i.e. thicker, and some of them are both high and BIG BLUE. They are ideal for softener pre-filters, for cottages where there is plenty of soil in the water, for blocks of flats, detached houses, etc. Don't hesitate to contact us if you need help finding the right filter for you. We will be glad to assist you!