Last week I posed an obscure plumbing problem where the bathroom basin was behaving in a rather peculiar manner. When you wash your hands with warm water then switch to maximum cold, the water coming out of the faucet is hot for some time.

After significant head scratching I managed to establish the root cause of the problem: The “faucet aerator” was to blame.

The faucet aerator is a thing screwed on to the end of the spout; it mixes some air into the stream of water in order to make it all soft and fluffy. The particular faucet aerator fitted had a “water saving” feature, which intentionally limited the flow of water passing through. (Blue mesh in the picture)

This combined with the pressure differential between the hot water and the cold water, meant that when using the mixer to create warm water what was actually happening was this:

1. Hot and cold water rush in to the mixing chamber together.
2. The warm water can’t exit fast enough because the spout is blocked by the “water saver”.
3. The hot water pressure is significantly higher, which overpowers the cold water.
4. Hot water dominates and begins to flow directly into the cold water pipe!
5. As the pipes between the boiler and the basin warm up, the hot water becomes progressively hotter.

Then you desperately attempt to rinse the soap from your hands before they catch fire, but you can’t do it quick enough, at which point you desperately swing the mixer over to maximum cold only to have the scalding hot water that has just backed up into the cold pipe dump out on you, followed by some warm water for a time, and eventually cold water (by which time your hands are already burnt).

Needless to say I quickly did away with the extra “water saving” part of the faucet aerator, and that faucet has been fine ever since!