Low-flow fixtures and water pressure
Applies to: Assumes fixtures fed either from a mains-pressure supply or from a stored tank at gravity head.
Last worked on 2015-03-30
Pressure and flow are different quantities, and most disappointment with a low-flow fixture comes from swapping one when the problem was the other.
Pressure, flow and the difference
Pressure is the force available at the outlet; flow is the volume that actually arrives per unit time. A supply can have plenty of pressure and poor flow if it is restricted somewhere along its length, and it can have adequate flow at low pressure if the pipe is generous and the run is short.
A shower that feels weak may be short of either. A restricted fixture on a low-pressure gravity system feels much worse than the same fixture on a mains-pressure system, because the fixture's restriction and the system's low head compound.
How restricting fixtures actually work
Aerators mix air into the stream so that a smaller volume of water covers the same area and feels similar at the hand. Flow restrictors simply limit volume. Pressure-compensating devices vary their aperture so the flow stays roughly constant as supply pressure changes.
The first two behave very differently on a low-pressure system: an aerator may barely function, while a plain restrictor makes an already weak flow weaker. Pressure-compensating types are the ones designed for a variable supply.
Check the supply before changing the fixture
Time how long a known container takes to fill at the outlet in question, and compare it with the same measurement at a fixture close to the incoming supply. A large difference between them points at the run rather than the fitting.
Blocked aerators, part-closed fixture stops, a throttled gate valve, and years of scale in a length of small-bore pipe all produce the same complaint and none of them are solved by a new showerhead.
Terms on fixture packaging
- Aerated
- Air is drawn into the stream. Feels fuller at a given volume, but needs reasonable pressure to entrain the air.
- Laminar
- A smooth non-aerated stream. Loses less heat and splashes less; feels different rather than weaker.
- Flow restricted
- A fixed aperture limiting volume. Behaves badly on low-pressure supplies.
- Pressure compensating
- The aperture varies with supply pressure to hold flow roughly constant. The type suited to variable or gravity supply.
Measure first. A five-minute container test at two outlets identifies whether you have a fixture problem or a supply problem, and the two lead to entirely different work.