The heat energy that water can hold is related to its mass, as with specific heat of building materials mentioned in, and strictly speaking we should refer to its mass in kilograms.
However, as 1 litre of water has a mass of 1 kg at normal temperature and is only slightly lighter at circulating temperature, and as pump performance is always given using volume flow, it will make things easier to work in litres throughout.
The metric unit of energy is the joule but, unlike the old imperial British Thermal Unit, it is so small that most people avoid it. However, I mention it here to explain an odd number that we will need. It takes 4187 joules to raise the temperature of 1 litre of water by 1 °C; to raise the temperature by 10 °C would therefore need 41 870 joules. When the litre of water cools by 10 °C it gives up 41 870 joules.
A watt is defined as a rate of energy transfer of 1 joule per second so if the litre of water gives up its 41 870 joules in only one second, it is transferring heat at a rate of 41 870 watts.
From this you can see that if we divide the heating requirement in watts by 41 870, it will tell us how many litres must flow every second in our heating circuit. If the heat losses of your house amount to 167 480 watts, then dividing by 41 870 produces a requirement of 4 litres per second (4l/s). This flow rate must be supplied through pipework and the next thing is to find out what size
will be needed.
A 15mm diameter tube 1 m long can contain 0.145 litres of water. If 4l/s are required, we need the water contained in 27.6 metres to flow every second; this means the water will be travelling at a speed of 27.6 metres per second or in more vivid terms 62 mph, which is a bit reckless around the house! In practice it is found that a velocity of 1 metre per second is as fast as you can go if vibration and noise are to be within acceptable limits.
As one metre of tube holds 0.145 litres, and we cannot exceed a velocity of 1 m/s then 0.145 l/s is all we can push through this size of tube. If 1 l/s cooling 10°C provides 41 780 watts, then 0.145 l/s will provide 6058 watts, which is the maximum heat load of 15 mm tube. Table 6.1 gives rounded limits for the other sizes of small-bore tube.
I said earlier that a pump’s performance can be measured by reference to the water flow rate and the height to which it can be lifted. The pump’s ability to lift water has many applications but in a heating system no lifting is necessary, since the circuit is already full of water held up by static pressure. The pump is needed to create a pressure differential to get the water moving and to overcome friction.
As water travels along a tube, friction exists between it and the tube walls. If there is twice the length of tube, there will also be twice the friction. If the same pump mentioned earlier, which can deliver 2 litres per second with a head of 3m, were to pump along a length of pipe and at the end, 2 l/s could be lifted to a height of only 2 m then the pump has lost 1 mofits head overcoming the friction in the pipe.
Put another way, this length of pipe, for a flow rate of 2 l/s, produces a head loss of 1 m. If the flow increases, the friction will increase. Table 6.2 shows the head loss, in milimetres, per metre run of pipework, for different sizes of tube at varying flow rates. As we use a standard 10 °C temperature drop, the heat carried is always proportional to the flow rate so I have shown the heat load in watts.