Sizing the pipes.
Once the positions of the radiators and pipe runs have been determined, the size of the pipes must be calculated. For this, the pipe runs must be broken down into individual runs between radiators etc. starting furthest from the boiler and working back towards it.
Starting with lounge, the run between its radiator and the next in line (the hall) must carry 4697 BTU. From the limiting factors, it is estimated that 15mm copper pipe can handle 13,620 BTU; so 15mm can easily cope. Then looking at the pipework for the hall and lounge radiators, this has to carry the heat for both radiators (2751 + 4687 = 7,438 BTU), so again 15mm pipe will cope. Further back to the kitchen radiator gives a total of 9,791 BTU, so again 15mm pipe can cope. This is the end of this particular pipe run as the Dining Room radiator is on a separate run. The requirement for the dining room is 6,437 BTU, so once again 15mm copper pipe is suitable.
A quick look at the heat requirement for the first floor, shows a total requirement for 8588 BTU (1858+3321+2266+1143), this is within the capacity of 15mm copper pipe.
The final run is from the boiler to the branches to the front loop downstairs, the lounge and the upstairs circuit, this run handles the full energy load for the house, namely 24,816 BTU (9791+6437+8588), this is well over the capability of 15mm but within the capacity of 22mm.
The hot water take is designed to have an input of 13,000 BTU (for a 2 hour reheat time), this is very near the upper limit for 15mm copper pipe (13,620 BTU), so it would be better to use 22 mm copper pipe.
So, the pipework between the boiler to the branches and the boiler and hot water tank need to be 22mm, all the other pipework can be 15mm. Of course, pipework larger than the required size can be used with no detrimental effect upon the installation, although it will be more expensive, the speed of the water flow and the effect of back pressure will be reduced which can only improve the operation of the system.