Central heating Pipe Sizing.

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 correct procedure of choosing pipe sizes is as follows.

  1. Flow rates: First of all, the flow required by each radiator is calculated, using the following equation: Required flow rate = Required radiator output / (specific heat capacity of water* temperature drop across the radiator). Most manufacturers quote the output of their radiators with a temperature differential of 11C So using this, the equation simply becomes: Flow rate (kg/s) = Radiator output (watts)/ 46000
  2. Flow resistance: The resistance of piping and fittings between each junction and the radiator must be calculated. The fittings and bends are most conveniently dealt with, by adding their equivalent length of straight pipe (a). to the actual length of the pipe(b). Typical equivalent lengths are given in table 4. As the calculation of resistance of the pipe is reasonably complex, Table 5 gives the values for different flow rates and pipe sizes. This is then multiplied by the calculated pipe length a+b to give a resistance for each section of the system. Usually, where a boilers resistance is significant, it will be quoted in the manufacturers specifications. The resistance of pressed steel radiators is normally so low it can be ignored.
  3. Index Circuit: The circuit with the greatest total resistance (the " index " circuit) is then identified. It is usually that to the most remote radiator, but sometimes to one of the larger radiators. This total resistance must include the resistance of every component in the circuit, including the boiler, pump isolating valves and so on.
  4. Pump pressure: the pressure available from the pump is then obtained from the manufacturer's performance curves. These come in the box with the pump see figure 26. For this purpose, the total flow to be handled by the pump must be known. It will include the flow already calculated for all the radiators plus the flow through the bypass.
    The flow through the hot water cylinder in a fully controlled system is often ignored, as when the cylinder is taking a lot of heat, the flow to the radiators is often inadequate. However, it is recommended that the flow through the cylinder is taken into account at all times.
    Accurate estimation of the flow rate through a bypass is difficult, and assumed figures of 10kg/s for a 15mm bypass and 20kg/s for a 22mm by pass may be used. Boiler manufacturers sometimes quote the minimum bypass flow rate the boiler requires. If so, the larger of these two figures should be used.
  5. Pressure Comparison: If the pressure available from the pump at the required flow rate exceeds the resistance of the index circuit, the pipes are adequately sized. If there is sufficient margin, the pump may be set at a reduced rating. Alternatively, smaller diameter pipes may be used. An examination of the calculations will reveal where these may be used to the best advantage.

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.

Towel rails and airing cupboards


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.

Pump over run

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.

Table 4: resistance to the flow of fittings, etc as equivalent lengths of straight tube in meters
Type of fitting etc Nominal Pipe size * (mm)
8 10 12 15 22 28
Straight valve 0.11 0.15 0.20 0.30 0.40 0.60
Angled valve 1.00 1.50 1.80 2.00 4.30 6.0
Capillary elbow 0.16 0.21 0.28 0.37 0.60 0.83
Compression elbow 0.24 0.33 0.42 0.60 1.00 1.30
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
Manifold connection 0.60 1.00 1.20 n/a n/a n/a
Minimum radius (machine) bend 0.12 0.16 0.20 0.26 0.41 0.58
Sweeping bend 0.06 0.08 0.10 0.13 0.21 0.26
* Copper tube to BS 2871 part 1 table x
Table 5a: Maximum heat flow rates through pipes.
Nominal Pipe size Internal Area (m2) Water flow rate (litres/sec)

Maximum theoretical heat flow rate. (Ignoring resistance)

(Kwatts )

8mm 0.0000363168 .054475217 2.506
10mm 0.0000608212 .091231851 4.197
15mm .000149571 .0224356839 10.320
22mm .000339795 .509691992 23.446
28mm .000564104 .846156565 38.923
Table 5b: Resistance to flow in pipes (in meter head of water per meter length)
Flow rate (kg/s) Heat flow (watts) 8 10 12 15 22 28
.005 230 .007 .002 .001 - - -
.010 460 .022 .006 .003 .001 - -
.015 690 .044 .013 .005 .002 - -
.020 920 .073 .022 .008 .003 - -
.025 1150 .111 .032 .012 .004 .001 -
.030 1380 .151 .044 .017 .006 .001 -
.035 1610 .197 .058 .020 .007 .001 -
.040 1840 .243 .072 .025 .009 .001 -
.045 2050 .300 .089 .032 .011 .002 .001


2300 .362 .018 .039 .014 .002 .001
.06     .148 .055 .018 .003 .001
.07   .192 .072 .024 .004 .001
.080   .247 .092 .030 .005 .001
.090     .110 .037 .006 .002
0.100   .134 .044 .007 .002
0.120   .183 .061 .009 .003
0.140     .080 .013 .004
0.160     .102 .016 .005
0.180     .125 .019 .006
0.200     .152 .023 .007
0.250       .034 .010
.30       .047 .014
.350 1610     .062 .018
.40       .079 .023
.50         .034
.60         .047
.70         .062