Use this calculator to determine how big your pipes need to be for your heat pump. Note, the calculator gives the minimum size – choose the next available size up.

Power Output

This is the maximum power that you want to get from the heat pump. In Ireland, it’s typically specified as the power you need to maintain a comfortable indoor temperature for your home when it’s -3 °C outside.

Use the heat pump sizing calculator if you don’t know what power output you need.

Temperature Drop (ΔT)

This is how much the water in your central heating system cools as it flows through your radiators or underfloor heating. For example, if the water leaves your heat pump at 45 °C and returns at 40 °C, then your ΔT is 5 °C.

A lower ΔT is generally better for efficiency. Typically, heat pumps run with a ΔT around 5 °C. Check the heat pump datasheet for the maximum allowable ΔT, if applicable.

Flow Speed

This is the speed that water moves through the main circuit of your central heating system. While increasing the flow speed has lots of advantages, there are limits to how far you can push it. See this link for a very detailed discussion of plumbing flow speeds. Or check the table below for the key points:

Flow Speed AboveWhat Could Go Wrong?
1.0 m/sToo noisy for a bedroom
1.5 m/sEasily noticeable noise
2.5 m/sSignificant corrosion

So try to keep flow speeds for residential heat pumps under 1 m/s.

Flow speeds that are too low can also bring problems. Specifically, you should make sure that flow rates are high enough to clear debris that settles in the pipework and allow for the proper operation of check valves. Minimums of 0.5 m/s and 0.75 m/s have been suggested.

Propylene Glycol %

Propylene glycol is a low-toxicity antifreeze that’s often used in heating systems. However, adding propylene glycol to water slightly reduces its heat transfer capacity. Our calculator takes this into account.

Formula for Heat Pump Piping Diameter

Here’s the formula that the calculator is based on:

D = 2000 \sqrt{\frac{P}{\Delta T \times \pi \times s}}

Units are as follows:
D: Diameter (mm)
P: Power (W)
ΔΤ: Temperature Drop through the heating circuit
s: Volumetric heat capacity of heat transfer fluid (J / M3 °C)


The inspiration for this calculator came from a video created Michael Paini and shared by Heat Geek.

The calculator uses thermodynamic data on propylene glycol from engineer’s toolbox.

Appendix 1: Irish/Imperial Pipe Bores

Ireland uses imperial pipe standards. These are based on the internal diameter/bore.

Irish/Imperial Pipe StandardInternal Diameter (Bore)
1/2″12.7 mm
3/4″19.05 mm
1″25.4 mm
1 1/4″31.75 mm
1 1/2″38.1 mm
2″50.8 mm

Appendix 2: British/Metric Pipes

Pipe sizes in Britain are specified by outside diameter. To find the internal diameter, you take the outside diameter and subtract twice the wall thickness. For example, a 22 mm pipe with 0.9 mm walls would have an internal diameter of {22 – 0.9*2} = 20.2 mm.

Use the calculator below to check the internal diameter of your metric copper pipe: