Pressure, Temperature & Flow: Reading bar, degC and L/min on Boiler and Water Datasheets

Open any boiler brochure, water datasheet or gas certificate and three more units arrive at once. The mains is quoted in bar, the heating runs at a flow temperature in degrees Celsius, the hot tap is rated in L/min, and the boiler's output is back in kW. They describe a single system: water under pressure, moving at a rate, carrying heat. Once you can read all three, you can tell whether the mains will run a shower, whether a combi will keep up with two taps, and whether the screed was poured warm enough to cure.

The three units at a glance

A useful way to keep the first and last apart: pressure is the push, flow is the result. High pressure with a pinched pipe still gives a weak trickle, and a wide pipe at low pressure can still flood out. The two are related but not the same number, which is the single most common confusion on a plumbing quote.

bar: pressure in the pipe

Pressure is how hard water or gas is pushed along a pipe, measured in bar. One bar is roughly the pressure of the atmosphere at sea level, so the figures you meet are small whole numbers rather than the hundreds you see on electrical or structural specs.

UK mains cold water usually arrives at 3-4 bar of static pressure, which is plenty to run taps, a shower and a combi boiler directly off the mains. Your water company has to deliver at least 1 bar at the boundary stop tap by law, so if you measure below that you have grounds to complain. Inside the house the figure can drop further through old narrow pipework or when a neighbour draws at the same time.

Low pressure is why an upstairs shower can dribble while the kitchen tap runs fine. If the house is fed from a cold tank in the loft rather than straight off the mains, pressure at any outlet comes only from the height of water above it: about 0.1 bar/m of vertical drop. A shower head three metres below the tank sees barely 0.3 bar, which is why gravity-fed showers so often need a pump while a mains-pressure shower does not.

Pressure shows up as a test as well as a supply figure. New water and heating pipework is pressurised on a plumber's test rig before it is hidden in walls or floors, and underfloor heating loops are held at 5 to 6 bar before and after the screed is poured, so a leak shows up while it can still be reached. Gas arrives at a far lower pressure than water, measured in millibar at the meter, and the engineer checks it holds before signing the supply off.

degC: temperature

Temperature in degrees Celsius, written degC, sets how the heating runs and how safely wet materials cure. It is the one unit on this page you already use every day, so the only new part is where it shows up on a build.

A modern condensing boiler is most efficient when it runs a low flow temperature, because the cooler the water returning to it, the more heat it recovers from the flue gases. Building regulations now cap the design flow temperature of a new system at 55degC under Part L, which is why a new boiler and larger radiators are often specified together: cooler water needs more radiator surface to heat the room.

Temperature also governs the wet trades. Concrete and screed cure by a chemical reaction that slows in the cold and stops near freezing, so most groundworkers will not pour below about 3 to 5degC and protect a fresh pour from frost overnight. Screed dries fastest at a steady 20degC, the temperature its drying-time figures assume; a cold, damp room can double the wait before a floor can be tiled. The drying maths behind that wait is set out on the screeding task.

L/min: flow rate

Flow rate is how fast water is actually delivered, in litres per minute (L/min). It is the figure you feel in a shower or a filling bath, and it is the one most people mean when they say "pressure" even though the two are different. Pressure is the push behind the water; flow is how much gets through once the pipe, the fittings and the open tap have had their say.

This matters most for a combi boiler, which heats water on demand instead of storing it. A combi can only warm the water as fast as it flows through, so its hot-water output is quoted directly in L/min at a given temperature rise. Ask for two outlets at once, a shower and the kitchen tap, and the flow splits between them, which is why a combi that feels strong alone can go weak when the dishwasher fills.

Getting that measurement is a job for your plumber during first fix plumbing: they run a tap into a measuring jug for a timed minute, or use a flow cup on the mains, and read the litres directly. A weak result early gives you time to upsize the incoming main or choose a different system before the walls close.

kW: heating output

Output is back in kilowatts (kW), the same power unit you meet on every appliance. A domestic combi typically runs at 24-35 kW, but read the two numbers on the datasheet carefully: the headline figure is usually the hot-water output, and the central-heating output is often lower, so a 30kW combi may put only 24kW into the radiators.

Output, flow and temperature are the same fact stated three ways. The kW a boiler delivers is set by how much water it heats (L/min) and by how far it lifts the temperature (degC). Push more litres per minute through, or ask for a bigger temperature rise, and you need more kilowatts to do it. For how watts, kilowatts and the rest of the electrical notation work, see volts, amps and watts explained.

How the three connect

Read in order, the units describe one chain. Pressure (bar) is the push that gets water moving. Flow (L/min) is how much actually arrives once the pipework has throttled it. Temperature (degC) is how hot it is when it gets there, and the product of flow and temperature rise is the heat delivered, back in kW.

The practical version is short. Weak bar or a narrow pipe gives poor L/min, poor flow starves a combi of the water it needs to make kW, and a flow temperature set too high wastes a condensing boiler's efficiency. Each unit checks a different link in the same system, which is why a plumber asks for all three before sizing anything.

Why the small print matters

These units are where a heating quote quietly over- or under-promises. A combi sized on its headline kW can still fail on the day if the mains cannot deliver the L/min to match. A shower specced for mains pressure will disappoint on a gravity tank that only musters a fraction of a bar. A new system run at the old 70degC flow temperature breaks the Part L design and leaves rooms cold. Read the pressure, the flow and the temperature on the datasheet, and check each against the supply you actually have before you sign the boiler off.

For the wider set of symbols you meet across a build, from W/mK on insulation to kN on structural calculations, see the units and symbols decoder.