Do replacement windows need trickle vents? In England, Part F of the Building Regulations says yes in most cases. New vents must match the old ones, or meet 8000mm² of equivalent area in habitable rooms and kitchens and 4000mm² in bathrooms. This guide explains equivalent area versus slot size, works through a bedroom example with real Astroflame vents, and covers fitting, common mistakes and what HMO landlords should buy.