Short definitions of the physical-science terms that underpin plumbing and heating calculations.
Plumbing calculations rest on a handful of physics terms. These plain definitions make the formulas in the worked examples easier to follow. They are general explanations for revision.
- Pressure: force acting over an area, measured in pascals (Pa) or bar. In plumbing it drives flow.
- Head: the vertical height of a column of water above a point; more head means more static pressure.
- Density: mass per unit volume. Water is about 1000 kilograms per cubic metre, so 1 litre is about 1 kilogram.
- Flow rate: the volume of water passing a point per unit time, for example litres per second or litres per minute.
- Velocity: how fast the water moves along a pipe; for a given flow, a narrower pipe means higher velocity.
- Specific heat capacity: the energy needed to raise the temperature of 1 kilogram of a substance by 1 °C; for water it is about 4.2 kJ/kg per °C.
- Conduction, convection, radiation: heat can pass through a material without bulk movement, travel with moving water or air, or cross space as electromagnetic radiation. Conduction occurs in solids and fluids; convection involves fluid movement.
- Thermal expansion: the increase in volume of water (and materials) when heated, which is why heated systems need somewhere for the extra volume to go.
Keep these definitions handy when working through calculations, and confirm any values you rely on against your course material.
Common plumbing phenomena
- Air locks: a pocket of trapped air at a high point in a pipe breaks the continuous column of water, which can stop flow or circulation until the air is vented.
- Capillary action: surface tension and attraction to the surface can draw a wetting liquid through fine gaps or porous material, including sideways or upwards against gravity. Correctly prepared capillary fittings use this effect to draw molten solder into the joint.
- Water hammer: stopping fast-moving water suddenly turns its momentum into a sharp pressure surge, heard as banging pipes; quarter-turn taps and solenoid valves are common triggers.
- Pipe supports: a horizontal run can need closer supports than a vertical run of the same pipe because its weight and contents act across the span, causing sagging. Follow the pipe manufacturer’s spacing, loading and movement requirements.
Electricity and material basics
- Ohm's Law links current, voltage and resistance: current (amps) equals voltage (volts) divided by resistance (ohms). UK domestic mains supply has a nominal voltage of 230V.
- Galvanic corrosion: when two different metals, such as copper and galvanised steel, are connected in a wet system, they can set up a weak electrical cell that speeds up corrosion of the less resistant metal — a reason to keep dissimilar metals separated or use compatible fittings.