Sizing a Villa Water Tank and Pump: A Worked Example

Storage volume, pump flow and pump head are three separate calculations that people collapse into one. This example works each of them through for a family villa.

A domestic water system has three questions, and they are answered separately. How much water should be stored, so the house keeps running when supply is interrupted? How much flow must the pump deliver when several outlets run at once? And how much pressure must it generate to push that flow to the highest and furthest outlet? The example below works through all three with illustrative figures, so you can follow the method on your own drawings.

The situation

An illustrative two storey villa houses a family of six. It has a ground level storage tank filled from the municipal supply, a booster pump feeding the house, and the highest outlet is a shower on the first floor. The owner wants to know what storage to build, what pump duty to specify and whether the proposed pump will deliver comfortable pressure at that shower.

Assumptions (illustrative)

• Occupants: 6 people.
• Illustrative daily demand allowance: 250 litres per person per day.
• Required storage: 2 days of demand, as a design decision by the owner.
• Height from the pump to the highest shower outlet: 10 metres.
• Illustrative friction loss through pipework, fittings and the filter: 1.5 bar at design flow.
• Required residual pressure at the shower for comfortable operation: 1.5 bar.

Step 1: Calculate the daily demand

6 people multiplied by 250 litres per person per day gives 1,500 litres per day. This is the figure every other number in the exercise rests on, and it is a design allowance rather than a measurement, so it should be chosen to reflect the household, the garden and whether there is a pool.

If the allowance is wrong, the storage is wrong. A household with heavy garden irrigation or a pool that needs topping up has a much larger demand than the figure used for domestic consumption alone.

Step 2: Size the storage tank

1,500 litres per day multiplied by 2 days gives 3,000 litres. Adding a practical margin of about 20 per cent for irregular use and for the dead volume at the bottom of the tank that the pump cannot draw gives 3,000 multiplied by 1.2, which is 3,600 litres, so a 4,000 litre tank would be the practical selection.

Storage is an autonomy decision. Two days is a choice, not a rule. Doubling it to four days doubles the tank and the space it needs, and also increases the time water sits in storage, which matters for water quality in a hot climate.

Step 3: Work out the simultaneous flow the pump must deliver

Rather than adding every outlet in the house, the design uses a realistic simultaneous demand. Assume two showers and one basin running together, at illustrative flows of 0.2, 0.2 and 0.1 litres per second. The total is 0.5 litres per second, which is 0.5 multiplied by 3,600, giving 1,800 litres per hour, or 1.8 cubic metres per hour.

Flow is driven by how many outlets realistically run at once, not by how many exist. Specifying for every tap open at the same time produces an oversized pump that cycles badly and wears out early.

Step 4: Convert the static height into pressure

A column of water 10 metres high exerts approximately 1 bar, because about 10.2 metres of water equals 1 bar. The 10 metre lift from the pump to the shower therefore needs roughly 10 divided by 10.2, which is about 0.98 bar, say 1.0 bar, just to get the water there with nothing left over.

Height is the unavoidable part of the duty. Any pump that cannot overcome the static lift delivers nothing at the top floor, regardless of how much flow it is rated for at ground level.

Step 5: Add friction and residual pressure to find the total head

Total pressure required is the static lift of 1.0 bar, plus friction losses of 1.5 bar, plus the residual pressure of 1.5 bar needed at the outlet, giving 4.0 bar. Expressed as head, 4.0 bar multiplied by 10.2 metres per bar is approximately 41 metres. So the duty point is about 1.8 cubic metres per hour at about 41 metres head.

This pair of numbers, flow and head together, is what a pump supplier needs. A pump quoted only by power rating tells you nothing about whether it meets this duty.

Step 6: Test the sensitivity of the friction assumption

Suppose the pipework is undersized and friction is 2.5 bar rather than 1.5. The total becomes 1.0 plus 2.5 plus 1.5, which is 5.0 bar, or about 51 metres of head, an increase of roughly 24 per cent in the duty for the same flow. The pump gets larger, noisier and more expensive to run for the life of the building.

Pipe sizing and pump sizing are the same decision viewed from two ends. Money saved on smaller pipes is spent permanently on a bigger pump and higher energy use.

What changes the answer

• Demand allowance: irrigation, a pool or a larger household can increase daily demand substantially.
• Autonomy choice: the number of days of storage is an owner decision that drives tank size and space directly.
• Simultaneity: a villa with many bathrooms used at the same time in the morning needs a higher design flow.
• Pipe sizing: friction loss is highly sensitive to diameter, and undersized pipes raise the pump duty permanently.
• Roof tank or pressurised system: a gravity tank at high level changes the whole calculation and may remove the boosting requirement for some outlets.

What the numbers tell you

• Storage, flow and head are three separate calculations and each needs its own assumption stated.
• A pump is specified by a duty point of flow and head together, never by power alone.
• Static lift is unavoidable, so the top floor sets the minimum pressure the system must produce.
• Undersized pipework is paid for forever through a larger pump and higher running cost.
• Water sitting in storage in high temperatures raises quality and tank material questions that belong in the design.

Limits of this example

Every figure above is an illustrative assumption used to show the method, not a quote, valuation or market rate. Costs, rates, fees and regulations change and must be confirmed with the relevant authority, lender or supplier before you commit.

Domira builds and supervises projects across Saudi Arabia, and this is the level of detail its site teams work at. Send the drawings or the site, and Domira will tell you what the specification has to say before anyone prices it.