Choosing a pipe size is not too much of a consideration for some installers. There are many who will simply use long lengths of 8mm or 10mm throughout, without regard to Radiator requirements. As it is both easier and cheaper to do so.
Unless you are installing a low end budget system, it is very important to ensure that the installer actually knows what he is doing.
Is he fitting pipe work of adequate size?
Central heating pipes should be sized so that there is always sufficient water flow to enable each radiator to deliver it's rated output. This involves ensuring that the level of resistance of the circuit in the central heating system which offers the greatest resistance to the water flow, does not exceed the pressure available from the pump. Also, to avoid noise, the velocity of the water in the pipe should not exceed 1.5m/s, these are the principal limiting factors in pipe sizing. Ignoring the lower velocity due to friction at the pipe walls. (Volume of water = Pipe area* water velocity) the absolute maximum outputs of various pipe sizes are given in table 5a. In actual practice these outputs will be much lower due to resistance of the pipe and the additional resistance due to fittings, bends and valves and the fact that the pump will have to share it's output with the rest of the circuit.
From table 5 below. The maximum flow rate through an 8mm pipe gives a resistance of .362m per meter length. Divide this into the standard pump pressure of 5 meters head, gives a maximum theoretical pipe length of just under 14 meters, that's both to and from the radiator. The maximum pump pressure will actually never be achieved in practice, since the pump has to provide flow to all of the circuits. For this reason, if using 8mm pipe, the maximum length to and from the radiator from the main Pipes should not really exceed 6m. ie the radiator must be within 3m of the main pipes. If you allow for the pipe rising from under the floor to the radiator you will see that the actual distance is reduced to less than 2.5m. and as you can see from the tables, the biggest radiator an 8mm pipe can supply would be less than 2.5 Kilowatts.
The actual flow rate to each radiator will not automatically be identical to the design calculations. It is usual for the smallest radiators and those closest to the pump to have excessive flow rates, resulting in reduced flow to the largest or furthest away radiators. So subsequent balancing of the system, using the lock shield valves must be carried out to ensure the flow to each radiator is within the design criteria. Pipes to the cylinder should be sized such that during the summer months, when heating is not required, the entire output from the boiler can be transferred to the cylinder at the design temperature drop. This normally means 22mm pipe work to the cylinder. This will maximise the efficiency of the system, as well as maximising the heat up rate of the hot water. As alluded to above, the size of heat emitter which can be supplied through micro bore pipe work is limited by the heat carrying capacity of the relatively small water flow rates. these are restricted by both the pipe resistance and the velocity limitation of 1.5m/s, mentioned above. Therefore a higher temperature drop through the emitters is sometimes adopted (the absolute maximum recommended is 16C although this means a slightly larger radiator.
Some boilers require a bypass to be fitted to prevent local boiling, which gives rise to a continuous singing noise, known as "kettling" so called as it sounds like a kettle coming to the boil. indeed it is the same thing causing the noise, as very small bubbles of steam are produced. A by-pass, adjusted to ensure sufficient flow rate through the boiler, will help to prevent this phenomenon. Manufacturers normally give information if a boiler requires a by pass and / or a pump over run. A by pass should be:
A lock shield valve should be fitted in the bypass, which should be connected between the main flow and return immediately after the pump but before any control valves. By passes should also be fitted to systems with thermostatic valves on all the radiators, to ensure a minimum flow rate when all the valves are satisfied and therefore shut off.
A further consideration is that of residual heat left in the boiler after shut down. Most modern boilers have a very low water content, and the energy in the boiler heat exchanger can cause boiling of the water in the heat exchanger if the flow were to stop at the same time as the burner was shut down. This can cause excessive noise, even banging of the system pipe work as ejection of the steam produced occurs up the open vent, or through the pressure relief valve in a sealed system. Obviously a fault condition. Boilers with this characteristic will normally incorporate a pump overrun device, which, in conjunction with a bypass allows the pump to continue to operate after the burner has shut down. That is until the heat exchanger temperature cools to a predetermined figure. If a pump overrun is required, the manufacturer will state this in the installation instructions. These instructions will also give details of where the by pass is required and the additional electrical wiring which will be necessary . This will include a permanent electrical connection to the boiler, uninterrupted by time or temperature controls. The pump power supply will also need to be taken from the boiler. It is very important to adhere to these instructions, as failure to do so will result in potentially dangerous situations.
|Type of fitting etc||Nominal Pipe size * (mm)|
|Square tee piece||0.27||0.37||0.49||1.00||1.6||2|
|Swept tee piece||0.22||.029||0.38||0.60||0.75||1|
|Minimum radius (machine) bend||0.12||0.16||0.20||0.26||0.41||0.58|
|* Copper tube to BS 2871 part 1 table x|
|Nominal Pipe size||Internal Area (m2)||Water flow rate (litres/sec)||
Maximum theoretical heat flow rate. (Ignoring resistance)
|Flow rate (kg/s)||Heat flow (watts)||8||10||12||15||22||28|