Hot Water Cylinder Sizes

The sizing of hot water cylinders is one of the most critical parts in designing efficient hot water systems to ensure they meet the demand.  

Hot Water Cylinders - Domestic Size Guide UK and Ireland.Over specifying your hot water cylinders will use more energy to heat hot water which simply isn’t required. However, designing cylinders to be too small will inevitably result in a lack of available hot water.

Whilst every project will have its own specific needs, the starting point is to understand what the potential hot water demand is likely to be over the course of a day and at peak periods.

So as a rule of thumb you will use:

  • 70-80 litres of hot water to run a bath
  • 45 litres of hot water for a 5 minute shower
  • 50-100 litres of hot water for a full load in the washing machine12-20 litres of hot water per cycle in the dishwasher
  • 0.2 litres of hot water per second when you run the kitchen taps
  • 0.15 litres of hot water per second when you run the sink taps

However calculating the optimum size for hot water cylinders isn’t a case of multiplying the above by the number of people in the property.

The main consideration is understanding the peak demands and ensuring you have the right combination of cylinder capacity and re-heat times.

Domestic Hot Water Cylinders Sizing

Specifying the size of domestic hot water cylinders is a little easier to determine as the demand tends to be more predictable.

However, we would recommend sizing the cylinder depending on the number of bedrooms, bathrooms and showers. This should provide a cylinder which suits the property and provides a degree of flexibility and future proofing.

Indirect hot water cylinders powered by central heating boilers tend to require a smaller capacity due to the fact that a boiler has more power available than a standard 3kW immersion heater. The larger the boiler the quicker the reheat time.

Our general guideline for the sizing of domestic hot water cylinders is therefore as follows:


Indirect Hot Water Cylinders

Direct Hot Water Cylinders

1 Bedroom + Bath/Shower 75/120 120/150
2 Bedroom + Bath/Shower 150 180
3 Bedroom + 2 Baths/Shower 180 210
4 Bedroom + 2 Baths/Shower 210/250 250/300
Larger Properties 300+ 300+


Thermal Store Hot Water Cylinders Sizing

The above information is designed to provide a general rule of thumb when it comes to the sizing of hot water cylinders. However thermal stores require a different approach, particularly when there are multiple fuel sources and the thermal store is being used for other purposes such as central and underfloor heating.

As this is a bit more in depth, we have created a dedicated page to provide full details. To find out more, check out our Guide to Thermal Store Cylinder Sizes.

For more advice on Hot Water Cylinder Sizes, call our UK Technical Sales Team now on 01592 611123 or get in touch through our contact page.

You may also be interested in viewing our web pages on:

McDonald Engineers: Hot water cylinders in a variety of sizes for distribution across the UK and Ireland.