The boiler and central heating pump must be able to provide adequate output, flow rate, and circulating pressure to feed the underfloor heating system.

When assessing the boiler output, it is recommended that an allowance of 15% be added to the output of the live floor area, to allow for downward heat loss through the floor, and non-effective pipework loss. It is recommended that a flow and return temperature difference of 15ºC should be used in the calculation to determine the flow rate.

Example continued

Pipe length 10.2 x 6.55 = 67m
Boiler load 950 x 1.15 = 1092.5W = 1.093kW
Design flow 1.093 = 0.018kg/sec
4.186x15
Total pipe length:
Length of pipe in screed	67m
Length of circuit to link to C.H system	5m
	72m
Head loss @ 0.018kg/sec	3960Pa
Pipe 72 x 55 =	543Pa
Valve	4503Pa
Therefore at the underfloor heating branch off the central heating system, the circulating pump must be capable of supplying 0.018kg/sec at 4.503kPa.