The Importance of Proper Pipe Sizing in Heating Systems

Many people underestimate how crucial proper pipe sizing is for the efficiency and functionality of heating systems. Undersizing pipes can put unnecessary strain on circulation pumps and prevent the correct amount of energy from being delivered to the property. This not only increases wear and tear on the pump but can also lead to boiler or heat pump cycling, reduced system efficiency, and a decline in comfort for the occupants.

One key point to understand is that even a small increase in pipe diameter can significantly improve flow capacity. For example, a 2-inch pipe, though it sounds like only double the size of a 1-inch pipe, actually has a 288% higher flow capacity. In other words, one 2-inch pipe can carry almost as much water as four 1-inch pipes, which highlights the importance of getting the pipe size right.

Another crucial factor to consider is the temperature differential in your heating system and its effect on pipe-carrying capacity. Heat pump systems, for instance, are often designed with a temperature differential of 5°C between the flow and return water, modern condensing gas boilers typically operate with a 20°C differential whereas electric boilers will typically run at a 15°C differential. This means that heat pumps require up to four times the amount of water to deliver the same amount of energy as boilers. Using the earlier example, a heat pump would need a 2-inch pipe to supply the same amount of energy that a 1-inch pipe could deliver from a boiler. To illustrate further, the flow rate of a 10 kW heat pump operating at a 5°C temperature differential is equivalent to that of a 40 kW condensing boiler with a 20°C differential.

It’s important to note that the temperature differential, not the water temperature, determines the carrying capacity of the system. For example, a boiler operating at 80°C flow and 70°C return would require the same pipe size as a boiler running at 50°C flow and 40°C return, as both have a 10°C temperature differential.

When sizing pipes, the goal is to ensure the water velocity is appropriate. If the pipe is too small, the water velocity will be too high, leading to noise, increased friction, and excessive load on the circulation pump, making it difficult to deliver adequate water. On the other hand, if the pipe is too large, the water velocity will be too low, which can cause sludge buildup and make it harder to remove trapped air pockets. The recommended water velocity is just under 1 m/s, with a maximum of 1.5 m/s and a minimum of 0.5 m/s.

Taking these principles into account, the tables below provide the capacities of common pipe sizes. We’ve highlighted the minimum (0.5 m/s), recommended (1 m/s), and maximum (1.5 m/s) capacities for pipes ranging from ½-inch to 4-inch diameters, across temperature differentials (ΔT) from 5°C to 20°C.

By understanding the relationship between pipe size, temperature differentials, and water flow, you can optimise your heating system for improved efficiency and comfort. Operating the system at too high a flow rate will make the boiler operate at a higher output for longer, the sooner the boiler starts to modulate, the lower the energy consumption and running costs. Balancing the flow across individual radiators based on the radiator output to maintain the chosen temperature differential between flow and return is therefore imperative in operating an efficient heating system.