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What sort of flow rate should be expected from the hot and cold taps on a bathroom sink?

The flat is a single story high, the plumbing is what I'd describe as UK traditional, the mains water flows into a header tank at ceiling level in the airing cupboard, below that there's a hot water tank with an electric immersion heater. The bathroom cold water comes from the header tank, and the hot water comes from the hot water tank.

The overall 'head' of water is only about 1 meter between the level of the taps and the water level in the header tank. There is probably about 5 meters of 15mm pipe between the tanks and the taps (including lots of twists and turns)

I'm used to houses where the header tank is in the attic and the sink fills in seconds.

I know that an overall head of 1 meter isn't much, but what sort of flow rate (in terms of litres/minute) is reasonable to expect? I'm trying to work out if there's anything actually wrong with the plumbing, or if I'm just not used to how the plumbing works in a flat.

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  • Flow rate is usually set by water pressure and the size of pipes. Something like 3 to 5 gallons(20L) per minute is considered okay.
    – crip659
    Commented Nov 14 at 13:28
  • If your water system is pressurized from the water mains, then the head of these tanks would be irrelevant. But it is possible that your tanks are not pressurized from the mains and if that is the case then the head on these tanks would determine the flow rate. A 1 m head would give a very low pressure of about 0.1 atm so about 1.5 psi. This would give a very low flow from a standard faucet. The way a system like this would work would be the mains pressure stops at an automatic fill valve which keeps the tank at a set fill level but the top of the tank is at ambient pressure (like a toilet).
    – Jim Stewart
    Commented Nov 18 at 14:50
  • Is this property in a village, a town or a city in the UK? Which one? In my locality Dallas TX USA, the entire plumbing of the house, including the water heater tank, is at the pressure of the incoming water main (about 50 psi - 70 psi, except for the tanks or cisterns of the toilets). In some US localities the mains pressure is 100 psi or higher and each residence has a pressure reducer to lower the pressure in the house plumbing.
    – Jim Stewart
    Commented Nov 18 at 15:13
  • @Jim-Stewart - The mains water pressure is actually irrelevant, since both the hot and cold water come from an (open) tank in the airing cupboard. The fill-rate INTO the tank is almost certainly related to mains pressure, but the flow rate OUT of the tank isn't.
    – ConanTheGerbil
    Commented Nov 19 at 9:35
  • Thanks for confirming that what I assumed was only a possibility is reality. What are the bathing arrangements in the flat: a shower or a tub only?
    – Jim Stewart
    Commented Nov 19 at 11:17

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Fixtures (i.e. faucets, showers, etc.) have flow rates specified by their manufacturers as gallons per minute at a specified pressure. Standards and/or codes may also require a certain flow rate, however, that is usually a maximum (to limit water usage). These vary by location.

This document from the World Plumbing Council mentions the following as typical maximum for the UK:

  • lavatory: 1.6 gpm or 6 L/min
  • shower: 2.6 gpm or 10 L/min
  • sink faucet: 2.1 gpm or 8 L/min
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