In the UK, the most common outside tap thread is 3/4" BSP, and some older taps may be 1/2" BSP. To tell which you have, measure across the thread: about 26.4 mm is 3/4" and about 21 mm is 1/2".
How to measure your outside tap
Take off any connector so the thread is bare. Hold a ruler across the end of the tap where the connector screws on, and include the whole thread in the measurement. Then match it to Hozelock’s chart:
- 20.99 mm across: 1/2" BSP
- 26.44 mm across: 3/4" BSP
- 33.25 mm across: 1" BSP
The BSP size doesn’t match what you measure: a 3/4" BSP thread measures just over an inch (26.44 mm) across. Hozelock also writes 1/2" and 3/4" BSP as G1/2 and G3/4. For how BSP differs from the US NPT pipe thread, and when a joint takes a washer or tape, see BSP vs NPT.
Tap connectors for 1/2" and 3/4" taps
A tap connector screws onto the tap and gives the hose a click-on fitting. Hozelock’s 2175 tap connector has a 3/4" BSP thread and comes with a 1/2" BSP adaptor for older taps, so the one part fits either size. Its 2158 connector is for 1" BSP taps.
The connector on the hose clicks onto the tap connector. How the two halves fit together, and which brands mix, is on garden hose quick connects.
Backflow: keeping hose water out of the mains
Backflow is water running back from a hose into the mains supply. The Water Supply (Water Fittings) Regulations 1999, Schedule 2, paragraph 15, require every water system to contain an adequate device or devices for preventing backflow. Water Regs UK says byelaws apply in Scotland instead.
Water Regs UK treats a hose on an outside tap as a high contamination risk, because the end of a hose can lie in a bucket, a puddle, a drain or a pond. It says every hose union tap (a tap made for a hose) needs backflow protection, and that the local water company decides whether anything less than fluid category 5 protection is acceptable. Check with your water company which device your tap needs.
Turn off the tap’s isolating valve, or the stopcock, before you change a tap or a tap connector.