POLYPIPE PolyPlumb Straight Coupler, 15mm x 15mm, PolyPlumb Push-Fit Socket x PolyPlumb Push-Fit Socket. PolyPlumb is an integrated flexible plumbing system, incorporating Polybutylene pipes and a complimentary range of push-fit fittings, and can be used in hot and cold water supply, and radiator and floor heating installations. PolyPlumb - the original Polypipe push-fit plumbing system - offers a high degree of reliability, achived through a product design that has been tried and tested over the past 30 years.