Check out our tool selection guide or chat with a drain brain! Get quotes from our selected suppliers by only filling in form! Keeps drains clean and deodorised during extended periods of none use. Boiler feed units are sized based on the steaming rate of the boiler vs system lag remember, we’re focusing on the small amount of water between the boiler’s water line and its low-water cutoff—not the total water in the boiler. Effective and environmentally friendly! Gas central.