How much pressure should a house tap have?

Typically between about 100 and 400 kPa. Too little gives a weak flow; too much wears fittings.

Why is hot water stored at about 60 °C?

It stops harmful germs (such as Legionella) from growing. A mixing valve then lowers the temperature at the tap to avoid scalds.

Why add a check valve?

It lets water flow one way only, so dirty water cannot flow backward into the clean supply.

Water and Hot-Water Supply Systems in Buildings

Water comes from a source (river, lake, reservoir or well), is cleaned at a treatment plant and sent through mains to buildings. A raised tank or pump gives the pressure: p = ρgh, about 9.81 kPa per metre of height. Pipes are sized so the water speed stays below about 2 m/s. A water heater warms the water for taps, using E = m × c × ΔT.

Water resources and water supply

Only a small part of the world's water is fresh water we can use. It comes from surface water (rivers, lakes, reservoirs) and groundwater (wells and bore wells). Rain fills them again through the water cycle, but we can use water faster than it is refilled, so saving water matters.

From source to tap

Intake: water is taken from the source.
Treatment: screening, settling of dirt (sedimentation), filtering and killing germs (chlorination or other methods) make it safe to drink.
Storage and pumping: clean water goes to storage reservoirs and towers.
Distribution: underground mains and branch pipes carry water to buildings.
In the building: service pipe, meter, valves, tanks, taps and heaters.

Water quality is tested often. Leaks in pipes waste a lot, so pipes are checked and repaired.

Equipment for water and hot-water supply

Cold-water supply methods

Direct supply: the street main pressure feeds the taps. Simple, but depends on the main.
Overhead tank (gravity): the main fills a roof tank; gravity feeds the taps. Steady pressure, but the top floors get low pressure.
Pressure (booster) pump: a pump with a pressure tank keeps pressure steady, even in tall buildings.

Equipment

Pipes: copper, stainless steel, PVC, CPVC, PPR or PEX; each has its own temperature limit.
Valves (gate, ball, check, pressure-reducing), water meter, strainer and float valve in tanks.
Pumps and pressure tanks.

Hot-water supply

Storage heater (geyser/tank): heats a tank slowly; cold water enters at the bottom, hot water leaves from the top. It has a thermostat and a safety (pressure-relief) valve.
Instant heater: heats the water as it flows; no tank.
Energy can come from electricity, gas, solar collectors or a heat pump.
Insulate hot pipes. Store at about 60 °C to prevent germs and mix to about 45 to 50 °C at the tap to avoid scalds.

Design of water and hot-water supply

1. Estimate the demand

Daily use per person is about 135 to 200 litres in many cities. Daily demand = people × litres per person. Tank volume is usually one day of demand or a little more.

2. Pressure

Water pressure from height is p = ρ × g × h. With ρ = 1000 kg/m³ and g = 9.81 m/s², every metre of height gives 9.81 kPa. A tap needs a minimum pressure (often 30 to 100 kPa) and not too much (below about 400 kPa, or pipes and taps wear).

3. Pipe size

Flow Q = area × speed, so v = Q ÷ A. Keep speed below about 1.5 to 2 m/s in building pipes. Faster water makes noise, wears pipe and causes pressure shocks (water hammer). Longer pipes and bends also lose pressure by friction.

4. Hot-water energy

E = m × c × ΔT, with c = 4.186 kJ/kg·K. 1 kWh = 3600 kJ. Heater time = energy ÷ power.

5. Safe layout

Keep hot and cold pipes apart, never join drinking-water pipes to drainage or dirty sources, add check valves to stop back-flow and drain valves for maintenance.

Key formulas and definitions

p = ρ × g × h (≈ 9.81 kPa per metre of height)
Q = A × v, so v = Q ÷ A
A = π d² ÷ 4
E = m × c × ΔT (c = 4.186 kJ/kg·K); 1 kWh = 3600 kJ
Daily demand = number of people × litres per person

Worked examples

1. A tap is 20 m below the water level in a tank. Find the pressure at the tap.

p = ρgh = 1000 × 9.81 × 20 = 196 200 Pa ≈ 196 kPa.

2. What tank height above the tap gives 100 kPa?

h = p ÷ (ρg) = 100 000 ÷ (1000 × 9.81) = 10.2 m.

3. Water flows at 0.3 L/s through a pipe of inside diameter 20 mm. Find the speed.

Q = 0.0003 m³/s. A = π × (0.01)² = 3.14 × 10⁻⁴ m². v = 0.0003 ÷ 0.000314 = 0.96 m/s. Good.

4. A family of 5 uses 135 L per person per day. Find the daily demand and a tank size.

Demand = 5 × 135 = 675 L/day. A 700 to 1000 L tank covers one day or more.

5. How much energy heats 100 L of water from 20 °C to 60 °C? How long does a 2 kW heater take?

m = 100 kg. E = 100 × 4.186 × 40 = 16 744 kJ = 16 744 ÷ 3600 = 4.65 kWh. Time = 4.65 ÷ 2 = 2.3 hours.

Common mistakes

Thinking the pressure depends on the size of the tank. It depends on the height (depth) of the water only.
Mixing mm and m in the area formula. Convert 20 mm to 0.02 m first.
Using a very thin pipe to save cost. Water speeds up, makes noise and wears the pipe.
Setting the geyser very hot and no mixing at the tap. Water above about 50 °C can scald.

Practice quiz

Practice: answer these yourself

Type or choose your answer, then press Check. Use a hint if you are stuck; the full solution appears after you answer.

Water and Hot-Water Supply Installations

🎬 Step-by-step story

🤔 Common doubts, cleared

Is all the water in the world usable?

Why does the water go up to a tank on a tower?

Does a bigger tank give more pressure?

Why does hot water come out of the top of the heater?

Why does a narrow pipe make the water faster?

What is the best pipe size?