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Once the Space Heating requirement is calculated we then need to calculate the requirement to provide heating for Domestic Hot Water. Above this was assumed to be 2kW or 3kW (the DHW requirements for a typical UK household). However, to create a more accurate measurement we need to add in the amount of energy required to heat the water in the hot water tank whilst not disrupting the hot water for the central heating system (note that this does not apply for combi-boilers which have no hot water tank needs).

The first consideration is the size of the hot water tank. Obviously larger houses will have larger tanks. When selecting the size of the hot water tank, a simple rule of thumb is that for a typical domestic household, you should allow between 35 and 45 litres for every occupant. Obviously, if you have a young household, or use the bath more than shower, you should allow 45 litres per occupant. If you are older and use showers, you should allow 35 litres of capacity. However, it must be noted that personal habits also play a big part in total hot water use. Two households of the same size can use completely different amounts of hot water, with one of them being as much as twice as the other! In determining how much hot water you require you should consider the following:

  • A bath uses 100 litres of hot water (@ 40 degrees Celsius)
  • Showers use 35 litres of hot water (@ 40 degrees Celsius)
  • Kitchen sinks use 0.2 litres of hot water per second at 60 degrees
  • Hand basins use 0.15 litres of hot water per second at 40 degrees
  • Cleaning requires 3 litres of hot water per person per day
  • Cooking requires 2 litres of hot water per person per day
  • Dishwashing requires 20 litres of hot water
  • Washing machines require 30 litres of hot water

The following average consumption values can be used as a general rule (hot water requirements per person per day):

Low Consumption                           =             20 – 30 litres

Average Consumption                   =             30 – 50 litres

High Consumption                           =             50 – 70 litres

Let us assume that a 4 person household has decided they have Average Consumption values of 50 litres of hot water per person per day. They also say that they use the dishwasher and washing machine twice a week and the temperature of the hot water needs to be 45 degrees Celsius. Therefore we can calculate that their Hot Water Requirement is:

DHW Requirement (DHWr) = 4 persons x 50 litres (@ 45 degrees) + 16 litres (@ 45 degrees)

DHW Requirement (DHWr) = 216 litres (@ 45 degrees) per day

This means your water tank should be able to hold at least 216 litres.

From this, we can use the following equation to get the Hot Water Power Requirement (DHWpr):

DHWpr = DHWr x C x T


C = the specific heat capacity of water = 1.16 Wh/kgK

T = the temperature difference between hot and cold water (K)

We always assume the cold water to be supplied 4 degrees Celsius and we want the temperature of the water to reach 45 degrees Celsius. Therefore, the power requirement is:

DHWpr = 216 x 1.16 x (45-4)

DHWpr = 10272Wh per day

DHWpr = 10.3kWh per day

However, note that in the UK the water temperature is required to be 60 degrees Celsius. In this case, for the homeowner to get the 45 degrees they require, they will mix 60 degrees hot water at the tap with some cold water. This means that a smaller amount of hot water at 60 degrees is required, albeit it will require the same energy content for the smaller amount of water to reach 60 degrees as it does for the larger amount of water at 45 degrees. In this case the following formula can be used:

V2 = [ (T1 – Tc) / (T2 – Tc) ] x V1


T1 = old temperature

T2 = new temperature

Tc = cold water temperature

V2 = [ (45 – 4) / (60 – 4)] x 216 = 158 litres
You can have a water tank designed however you want it to be. They can be made to measure, with the required heating and volume specifications set to your needs. In the above example you would want a hot water tank in the region of 150 to 200 litres. The more standard water tanks (so called “off the shelf”) are Single Coil Tank’s (we will get onto other types of tanks later on in the document). This is the more traditional system installed in UK homes where renewable energy systems are not used. The water in the tank is heated by an internal coil which is fed by the boiler. The tank is fed from a cold water storage cistern, contained in the loft, with a capacity equal to or greater than it.

A conventional system has two water tanks in the loft:

1. A large cold water storage tank: draws cold water from the mains to refill and itself feeds the hot water tank.

2. A smaller feed and expansion tank: this acts as a feed and expansion vessel for the boiler circuit and space heating system.

The boiler heats the water directly for the space heating requirements and passes this through the hot water tank coil (also known as a heat exchanger), which then heats the water in the tank for DHW use. The water from the boiler is then passed around the central heating system and finally back into the boiler. The water tank stores the hot water for DHW use and is refilled by the cistern in the loft (which gets water from the mains). The Feed and Expansion tank feeds the boiler system, which in itself feeds the radiators or under-floor heating system for space heating requirements. Thus the DHW and space heating water are kept separate, avoiding problems of scaling or corrosion.

heating-a-house-5 This type of system is called a “vented system”, whereby the feed pipe is open to atmospheric pressure (i.e. the tank in the roof uses gravity to create the pressure in the water system). The pressure created isn’t as good as it could be and sometimes pumps are needed to increase the water pressure.

In 1989 “un-vented systems” were introduced and these operate purely from the mains water. The principles of the system are the same, although as the system is fed from the mains, there is no need for a cold water storage tank in the loft. The water pressure tends to be a lot higher, and because of this there are many safety features built in to the system.

Back to calculating the heating requirement for properties DHW needs: Let us assume you have a 120 litre hot water tank. We then calculate how much energy it would take to heat the water in the tank as if it had been emptied and needed a complete heating.

We know that UK mains water is provided at 4 degrees Celsius and that we have to heat the water to at least 60 degrees Celsius (this is a requirement by UK Health and Safety to kill off any legionnaire’s bacteria). Thus we need to heat the water by at least 56 degrees in the tank.

We also know that 1 litre of water requires 1.16 watts of energy to heat it by 1 degree Celsius in an hour. Thus, the total amount of energy required to heat the tank would be:

(120 * 1.16 * 56) = 7,600 watts hours = 7.6 kWh

Thus, we would need a boiler off at least 8kWp (excluding any space heating requirements).

Next we will start to look at the actual boiler system itself and how it provides space heating and DHW heating.

The importance of thermal insulation:

Without going into calculations, it can be estimated through mathematical formulae that not having insulation on piping can create losses of up to 500Wh per annum. In the above example, this equates to (500/7600) 6.5% of total annual output. Similarly, if the water tank is not properly insulated this can create thermal losses of another 500Wh, giving a total of 13% annual losses just because of poor insulation on the water tank and piping!


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