How to size a commercial water heater

How to size a commercial water heater

How to size a commercial water heater by Flexiheat UK; industrial water heating sizing;

How to size a commercial water heater by Flexiheat UK

A commercial water heater is required for supplying warm or hot potable water for showers, bathrooms, kitchens, restaurant sinks, washing machines etc and other purposes in a commercial or industrial building, such as schools, hotels, gyms or hospitals. A commercial water heater uses electrical elements or a burner to heat cold water up to the desired hot water temperature which is used when a demand is placed on water heater. Water heaters may use a tank to store the heated water for when it is required. Sizing a commercial water heater is relatively easy process and not a ” dark art ” as some people may think.

They are also used in many industrial water heating applications, when hot water is required for a production process, such as commercial laundries or other uses, as part of manufacturing a product or service.

If you size or select a commercial water heater with more than the capacity required, you’ll end up with higher capital cost and also higher fuel bills than required, on the other hand If you undersize a commercial water heater, then you run the risk of running out of potable hot water, and upsetting your customers in the case of a hotel for example or your production process failing in the event of a hot water process application. Thus, we have written this “How to size a commercial water heater” guide to assist in the correct selection for your water heater requirements.

Commercial water heater definition

A water heater can be defined as a closed tank used to supply potable hot water, which is either heated by electricity, gas (Natural gas or LPG), or any other source, such as solar. The water heater is intended to be supplied with potable water either from a potable water supply, such as directly from the water mains system (these are referred to as unvented hot water system) or via feed tanks when used in an open vent hot water system.

The three vital elements to bear in mind when sizing a water heater for commercial or industrial applications


Water storage tank capacity: The tank or vessel capacity for storing the hot water required. The tank allows a large volume of water to be drawn at flow rates that can exceed the recovery capacity of the water heater- which is useful for peak hot water demands.

The recovery rate: The amount of water in litres per hour, raised at a given water temperature rise and kW input. Also often quoted is the one hour draw capacity of the water heater, which is the maximum hot water production for one hour, normally required at the peak period of the day when the heaviest requirement of hot water will occur.

The power or energy input of the water heater – The amount energy required, normally measured in Kilowatts consumed by the water heater, which is provided by your preferred fuel, either electric, gas or oil. One kW of energy is 3,412 Btu’s, if you use the old imperial system.

These three factors are used together to select the correct water heater size and power rating.

The best method for selecting a commercial or industrial water heater is on the basis of hot water usage. The selection is a combination of these factors of tank size, recovery rate and heat input.

What size water heater do I need calculation for commercial water heating applications

Following is the European standard for hot water demands to DIN 4708, that is commonly used when sizing your commercial water heater or industrial water heater- whichever you may classify your water heater as.


Commercial Building TypeDemand factorBath tubShowerBidetPrivate wash basinPublic wash basinKitchen sinkSlop sinkBar sink
Hotel & Hostels / B&B’s0.550501010158050100
Sport centre / Gym’s12205158040100
School (Day type not boarding)0.81805208040
Commercial Offices15104040

Calculation example for a commercial water heater hot water demand

A Hotel with 100 rooms = Litres of hot water / Hour at 60 °C

  • 100 Bath tub / showers = 5,000
  • 25 Bidets = 250
  • 100 Private wash basins = 1,000
  • 20 Public wash basins = 375
  • 5 Bar sinks = 500
  • 8 Kitchen sinks = 640
  • 5 Slop sinks = 250

This gives us a total hot water demand of 8,015  x the demand factor for a hotel water heating system sizing of 0.5 = 4,007.5 litres per hour continuous at 60 °C

With a commercial hot water requirement / application like this, due to the volume of water required, the only practical solution is a commercial storage water heater, we do a full range of commercial gas water heaters, that we can match to various storage volumes of up to 5,000 litres in a single storage tank ( more when used in series)  and a power input of 300kW. They are also condensing too, thus you’ll have the maximum efficiency for your hot water commercial system.

We also supply commercial water heaters powered by an oil burner, and this range of oil fired water heaters is fuelled by gas oil which is the preferred oil used in commercial and industrial water heating applications.

Or if you require an indirect commercial cylinder, with great heat recovery times, then you need to look at our tank in tank hot water cylinder range or our commercial hot water cylinders

As well as electric powered which we speak in more depth below in this webpage

Hot water demands for apartment blocks with a communal / centralized hot water system

A standard apartment or flat according to DIN 4708 is defined as consisting of 4 rooms, with 3 /4 persons living there, a 150-litre bath (with a fill time of ten minutes) with one wash basin and one kitchen sink.

It also specifies, within the standard, that the hot water storage capacity and the performance of the heat exchanger in terms of efficiency are both significant factors in determining the number of apartments a water heater or heaters can serve. A general guide is as follows  –

Number of apartmentscontinuous  requirement (litres per hour)
Domestic Hot Water Temperature60 °C45 °C


Commercial electric water heater sizing

You size a commercial electric water heater, on the exact same principles as a gas or oil powered version, just the fact that you require electricity as the fuel source or power for the water heater. So use the above data to establish your hot water requirement, then look at our webpage commercial electric water heaters, for the available sizes and power outputs than we can supply. Just make sure you have the Amps available to power the water heater.

Commercial tankless water heater sizing

Sizing a commercial tankless water heater is a different case, as a commercial tankless water heater heats up water on demand rather than storing warm water in a storage tank for later use. This means they are generally more efficient as they only ever heat as much water as you need and are smaller in size than storage water heaters.

But they have a down side as heating water on demand requires using a larger burner in the case of a gas or oil-fired commercial water heater or electrical heating elements than a commercial storage water heater would use, which means that the energy demand while heating the water can be very high. So, you have to ensure you have the power available to satisfy this requirement.

The most common commercial tankless water heater is gas powered, as gas ( either natural gas or LPG gas ) is great at producing a lot of energy via a relatively small supply pipe.

You size commercial tankless water heaters via the number of hot water outlets they have to satisfy, and to what water temperature, we go more into this on this dedicated webpage – gas multipoint water heaters, that are used in commercial tankless hot water systems.


Industrial water heater sizing / calculation

Often industrial water heaters, are sized in a slightly different method, as normally, there are different water temperature rises, specifically required for the process in question
For industrial water heating processes, there are normally 3 elements to sizing your water heater.

These are –

1. The volume of hot water that you require – How many litres of hot / warm water do you require?
2. The temperature rise that you want that water to go through e.g., from the incoming water temperature of 10 degrees C to 70 degrees C etc
3. Lastly your time requirements – how quickly do you want that water to heat up in, or how many litres you require of water per hour your process requires.

When you have established these factors, then you can use the following calculation to establish the kw rating required for you water heater
To heat a specific volume of water by a given temperature rise in one hour.

Volume of water in litres x 4.2 x the temperature rise in degrees centigrade divided by 3600

(just for note – 4 being a factor and 3600 being a given constant)

Thus, for a 1,000 litre water heater, the calculation would be as follows –

1000 litres of water, to be heated from say 10°C to 70°C, giving a temperature rise of 60 °C would result in-

1,000 x 4.2 x 60 / 3600 = 70 kW per hour

This means that the 1000 litres of water would be heated in one hour by 70 kW of applied heat, for the temperature rise of 60°C, whether its by gas, oil or electric

We can use this result to conclude other factors – e.g., to heat the same water volume of 1,000 litres in half the time (i.e., 30 minutes) we would need twice the heating power i.e. 140 kW, as 70 x 2 = 140  kW.

Should the process require a 2-hour period recovery period for the industrial water heater, then we can lower the power input to 35 kW
When sizing an industrial water heater and for a commercial water heater, these are the actual power input figures into the water heater, so you need to ensure that the heater has a nett input of this figure, but its also prudent to add a small percentage for standing heat losses, if you are using a storage water heater.

Alternatively, if you have a specific water temperature rise, and you know the kW input rating of the unit, then you can calculate the water flow rate produced, then use the flowing formula –

m (kg/s) = P (kW) / the specific heat of water (Kj / Kg ºC) X ΔT of the water in Degrees C

Whereby –
m = mass flow rate (kg/s) this is litres per second as one litre of water weighs one kilogram
P= Power in kW
Specific heat of water = 4.187 Kj/Kg ºC
ΔT = temperature increase in C

An example
A water heater, with thermal power input of 70 kW, with a temperature rise of 60 degrees C then it would be –
m = (70kW) / ((4.187 kJ/kg ºC) (60ºC))
thus – 70 / 4.187 x 60
70/251.22 = 0.2786 kg/s or 0.2786 Litres a second – times this by 60 seconds to get per minute then by 60 minutes to get the volume per hour – thus 1,003 Litres per hour for a 60 Degree C water temperature rise.

Hopefully this sizing / calculation guide for commercial and industrial water heating applications has provided the information and formulas to help you size the correct water heater for your application. Should you require any more information on our commercial and industrial water heaters, please don’t hesitate to contact us at Flexiheat UK