Why You Can't Pipe a Heat Pump the Same Way You'd Pipe a Gas Boiler

A common mistake in residential installations is when installers used to working with gas boilers instinctively apply the same pipe sizing to heat pumps, reasoning: “if a 24 kW gas boiler ran fine on 3/4-inch pipe, a 16 kW heat pump will be more than fine too.” As MEP designers, we can say clearly: that’s not true — the hydraulic sizing of the two types of equipment is fundamentally different.

The physics behind it: the difference in temperature differential (ΔT)

The volume of water (mass flow) that needs to circulate through a heating system is determined by the equipment’s operating temperature differential. A gas boiler runs at a high temperature with a large ΔT (20 K), while a heat pump operates at a low temperature with a distinctly small ΔT (5 K).

Let’s look at an exact numerical example: for a 24 kW gas boiler (ΔT = 20 K) the required water flow is about 1.0 m³/h, while for a 16 kW heat pump (ΔT = 5 K) the required flow is about 2.7 m³/h. In other words, the lower-output heat pump needs nearly three times the volumetric flow (more water) than the larger boiler.

The system faults caused by undersized piping

If you constrain the heat pump to the familiar 3/4-inch pipe, you’ll run into the following serious problems:

1. Critical flow velocity: velocity reaches 2–3 m/s, causing noisy operation (whooshing, clicking) and internal pipe erosion.
2. Massive pressure loss: the built-in circulation pump becomes overloaded and can’t deliver the water volume the unit critically needs.
3. High-pressure cutout (fault code): because the water can’t carry heat away from the condenser fast enough, the unit overheats and shuts down on a safety trip.
4. Degraded COP: the compressor is forced to run at a higher pressure and temperature, which drastically hurts efficiency and drives up the electricity bill.
5. Defrost faults: in winter operation, the unit can’t find enough thermal energy to defrost the outdoor coil, resulting in unstable operation.

The correct engineering design

A heat pump isn’t a “small boiler.” For stable, economical operation, the following design rules must be followed: a larger pipe diameter (instead of the usual 3/4-inch (22×1.5 mm) pipe, a 16 kW heating output calls for DN25–DN32 internal diameter — e.g. 28 mm or 35 mm steel pipe), plus hydraulic separation (a buffer tank or hydraulic separator is mandatory so the unit’s mass flow is decoupled from the instantaneous control state of the heating circuits).