The main from the heat source has to carry enough hot water to deliver the heat your radiators need. We work out that water, then pick the smallest pipe that carries it without the water running too fast. The worked example below uses the six example radiators on a combi boiler in 15 mm copper.
Step 1 - The Heat the Main Carries
We add up the radiator outputs you enter. What else goes through the main depends on the heat source:
- Combi boiler: the radiators only. Hot water is made inside the boiler.
- Boiler and cylinder: the radiators and the cylinder coil at the same time, as on an S-plan or Y-plan. We take the coil as 12 kW.
- Heat pump: the radiators or the cylinder in turn, through a diverter valve, so the main carries the larger of the two. The heat pump heats the cylinder at its full output, so we use the output you enter, or the radiators' total if you leave it blank.
Example: 1,500 + 1,000 + 800 + 1,200 + 1,000 + 600 W = 6.1 kW on a combi.
Step 2 - The Water That Heat Needs
Water gives up its heat as it cools across the radiators. The less it cools, the more water has to flow to deliver the same heat:
Flow (kg/s) = Heat (kW) ÷ (4.19 × temperature drop)
We take a boiler as running at 70°C flow with a 20°C drop, and a heat pump at 45°C flow with a 5°C drop. The flow is turned into litres using the density of water at the average of the flow and return temperatures.
Example: 6.1 ÷ (4.19 × 20) = 0.0728 kg/s. At 60°C water weighs 983 kg/m³, so that is 0.074 l/s, or 4.4 l/min.
Step 3 - Choosing the Size
The water's speed is the flow divided by the area inside the pipe:
Speed (m/s) = Flow (m³/s) ÷ bore area (m²)
The main is the smallest size, from 15 mm up, that keeps the water under 1 m/s. Faster water is noisy and wears the pipe, and it takes a bigger pump to push it round. The bore is worked out from each size's typical wall thickness.
Example: 15 mm copper has a 13.6 mm bore, an area of 145 mm². 0.074 l/s through it runs at 0.51 m/s, under 1 m/s, so the main is 15 mm.
Step 4 - What Each Size Carries
The table under the result runs Steps 2 and 3 backwards: the most water each size carries at 1 m/s, and the heat that water delivers at your heat source's temperature drop.
Heat (kW) = bore area × 1 m/s × density × 4.19 × temperature drop
Example: 15 mm copper at 1 m/s carries 0.145 l/s, which at a 20°C drop is 12.0 kW. At a heat pump's 5°C drop the same pipe carries about a quarter of that.
Where These Figures Come From
| Value | Used for | Basis |
|---|
| 4.19 kJ/kg·K | Heat carried by water | Specific heat capacity of water, close enough across heating temperatures |
| Water density | Kilograms to litres | Published values for water, interpolated at the average water temperature |
| 70°C / 20°C drop | Boilers | A common design basis for condensing boilers, which need a cool return to condense |
| 45°C / 5°C drop | Heat pumps | Typical heat pump design temperatures; many are designed at 5 to 7°C |
| 1.0 m/s | Largest water speed | A limit in common use for domestic heating mains to keep them quiet; not a figure set by a standard |
| 12 kW | Cylinder coil on a boiler | A typical domestic cylinder coil rating; check your cylinder's |
| 15 mm | Smallest main | The smallest size normally run from a heat source |
| Bores | Water speed | Copper from the half hard wall thicknesses in BS EN 1057; plastic from typical PB and PEX wall thicknesses, which vary by maker |
Referencing a standard describes where a value comes from. It does not mean this tool is certified against it, or that its output is a design to that standard.
What It Doesn't Allow For
To stay quick, this calculator sizes one thing, the main from the heat source, on one test, the water's speed. It does not:
- Allow for pipe length, bends, fittings or pressure drop. A long main can need a bigger size than speed alone gives, so that the pump can push the water round.
- Size the circuits, branches or tails to each radiator, or say where the pipe can step down.
- Check the pump, or a pump built into the boiler or heat pump, can deliver the flow.
- Work out the water in the system, the expansion vessel, or balancing.
- Size underfloor heating, manifolds or microbore systems.
- Check the radiator outputs. They are the ones you type in. A radiator's catalogue rating at ΔT50 is far more than it gives at heat pump temperatures.
- Use your heat source's own design temperatures or cylinder coil rating, where they differ from those above. Leave out a heat pump's output and the main is sized on the radiators alone, which can be a size too small if the heat pump is bigger.
Treat the result as a guide to the main's size, not a heating design.
Before you install anything from this:
- Check each radiator's output against the manufacturer's data at your actual flow and return temperatures.
- Check the heat source's design temperatures, its pump's available head and any minimum flow, and the cylinder coil rating, in the manufacturers' instructions.
- Have a qualified heating engineer design the whole system, including pipe runs, pump and balancing, before ordering or installing.
If the method described here does not suit your installation, or you have any doubt about a figure, do not use these results as the basis for specifying or installing heating pipework. Responsibility for the design, and for checking these figures, rests with the person specifying the system. ProWorks provides this tool as a free aid, gives no warranty as to the accuracy or fitness for purpose of its output, and accepts no liability for any loss, cost or damage arising from its use.