Is the Built-In Pump Enough? Heat Pump Flow Rate, Available Head & When to Add a Secondary Pump

*The built-in pump must overcome external system resistance at the required design flow.*

Most monobloc air-to-water heat pumps already include a circulation pump. So for many installers the real question is not "which pump should I buy?" but **"can the pump inside the unit move the required flow through my system — and if not, what should change?"**

This guide walks through that check in four steps: work out the design flow rate, estimate the system's resistance, compare it with the unit's available external head, and decide whether the answer is a pipework change, a different control setting, or a secondary pump.

Quick Answer

The built-in pump is usually enough when **the system's total pressure drop at design flow is lower than the unit's available external head at that same flow**. To check it you need three things:

1. **Design flow rate** — from the heat output and the design temperature difference (ΔT).

2. **System pressure drop at that flow** — pipework, fittings, valves, strainer and the worst (index) emitter circuit.

3. **Available external head** — read from the heat pump's pump curve or datasheet at the design flow, after the unit's own internal losses.

If the system resistance exceeds the available head, the usual options are larger pipework, a lower-resistance layout, a reviewed design ΔT, or hydraulic separation with a secondary pump. Each has trade-offs, covered below.

Step 1: Calculate the Design Flow Rate

Flow rate follows directly from heat output and the temperature difference between flow and return:

**Flow (m³/h) ≈ Heat output (kW) × 3.6 ÷ (c × ΔT)**

For water, c ≈ 4.19 kJ/kg·K and density ≈ 1 kg/L, which simplifies to:

**Flow (m³/h) ≈ 0.86 × kW ÷ ΔT**

| Heat output | ΔT 5 K | ΔT 7 K |

|---|---|---|

| 8 kW | ≈ 1.38 m³/h | ≈ 0.98 m³/h |

| 12 kW | ≈ 2.06 m³/h | ≈ 1.47 m³/h |

| 16 kW | ≈ 2.75 m³/h | ≈ 1.97 m³/h |

Two practical notes:

- **Use the heat pump's rated output at the design condition**, not a round number from the brochure. Manufacturers typically state the rated water flow on the datasheet; your calculated figure should be consistent with it.

- **Many air-to-water heat pumps are designed around a ΔT of roughly 5 K.** A wider ΔT reduces flow and pressure drop, but it also lowers the mean emitter temperature at a given flow temperature — so it may push the flow temperature up and reduce efficiency. ΔT is a design decision, not a free lever. (See [Heat Pump Weather Compensation: How to Set the Heating Curve](https://nordthermglobal.com/blog/heat-pump-weather-compensation-how-to-set-the-heating-curve).)

**Glycol systems:** a water–glycol mix has lower specific heat and higher viscosity than water. It generally needs somewhat more flow for the same output and produces a higher pressure drop at that flow. Use the glycol manufacturer's data and the heat pump manufacturer's correction factors rather than water values.

*Schematic only: higher system resistance reduces operating flow on the same pump curve; curves are not product data.*

Step 2: Estimate the System Pressure Drop

Pressure drop (head loss) is what the pump has to overcome to circulate water around a **closed** loop. Add up every component in series along the path that has the highest resistance — the **index circuit**.

Typical items to include:

- **Pipework** — straight lengths, flow and return, using pipe-friction data for the actual pipe material and size.

- **Fittings** — bends, tees, reducers (often handled as equivalent length or a percentage allowance).

- **Strainer / magnetic filter** — easily overlooked, and its drop rises as it fills with debris.

- **Diverter valve** (heating / DHW), zone valves, check valves.

- **Emitter side** — the longest UFH loop plus its manifold, or the radiator circuit with the highest resistance.

- **DHW cylinder coil**, when sizing the hot-water flow path.

Two rules make this easier:

- **Pressure drop rises roughly with the square of flow** (an approximation for typical turbulent flow in heating pipework). 20% more flow means roughly 40–45% more pressure drop. Small errors in flow rate turn into large errors in head.

- **Check pipe velocity, not just pressure drop.** As an illustration, 2.06 m³/h through 22 mm copper (outside diameter; ≈ 20 mm bore assuming a 1 mm wall) gives a velocity of roughly 1.8 m/s, while 28 mm copper (≈ 26 mm bore) gives roughly 1.1 m/s. Actual bore depends on the pipe standard and wall thickness, so use the real internal diameter. The smaller pipe may be noisy and costly in pressure drop; many designers keep domestic heating velocities around 1–1.5 m/s or below. Undersized primary pipework is one of the most common reasons a built-in pump appears "too weak".

Units: 1 m of head ≈ 9.8 kPa ≈ 0.098 bar. Datasheets mix these, so convert before comparing.

Step 3: Read the Available External Head

A heat pump datasheet may show two different pump figures:

- **Pump maximum head** — what the pump itself can produce.

- **Available external head (or available external static pressure)** — what is left for *your* system after the unit's internal losses (plate heat exchanger, internal pipework, flow sensor, sometimes an internal strainer or backup heater).

**Only the available external head is comparable with your system pressure drop.** Reading the pump's own maximum head instead is a frequent mistake and can overstate capacity substantially.

As an illustration of the scale involved: a small circulation pump might list a maximum head of 12 m, yet deliver only around 4 m (≈ 39 kPa) at a flow of 3 m³/h. The maximum head is reached only near zero flow. And that 4 m is still the pump's own figure — once the pump is installed in a heat pump, the unit's internal pressure drop must be subtracted before you know what is left for the system.

Read the curve at your design flow, not at the curve's maximum. The operating point is where the system curve crosses the pump curve. If that point sits at a lower flow than your design flow, the system will run with a reduced flow — and therefore a wider ΔT than intended.

Also check how the pump is controlled. Depending on the model, the built-in pump may run at a fixed speed, on a selectable speed step, or under variable-speed control (for example targeting a set ΔT or flow). The available head figure you read should match the control mode the system will actually use.

Step 4: Respect the Minimum Flow Rate

This is the part generic pump-sizing guides rarely mention, and it matters specifically for heat pumps.

Most heat pumps specify a **minimum water flow** for safe operation. Below it, a flow switch or flow sensor may stop the unit to protect the heat exchanger. Low flow can also cause:

- Unstable operation and short cycling.

- Difficult or incomplete **defrost**, since defrost draws heat from the water circuit.

- Protection trips when most zone valves or thermostatic valves close at the same time.

Practical ways to protect minimum flow include keeping some circuits permanently open, using an automatic differential-pressure bypass, or hydraulic separation that keeps the heat pump's primary flow independent of the heating side. Which is appropriate depends on the system.

Note that **minimum flow and minimum system water volume are two separate checks**. Minimum flow is about how much water passes through the heat pump at any moment; minimum volume is about how much water is available to limit short cycling and support defrost. Adding water volume alone does not guarantee flow through the unit. (See [Do You Need a Buffer Tank with an Air-to-Water Heat Pump?](https://nordthermglobal.com/blog/do-you-need-a-buffer-tank-with-an-air-to-water-heat-pump).)

When Do You Need a Secondary Pump?

A secondary pump is usually considered when:

- The distribution system's resistance clearly exceeds the available external head, and pipework changes are not practical.

- The system has several zones or emitter types with their own pumping needs (for example, UFH and radiators at different flow temperatures — see Can a Heat Pump Run Radiators and Underfloor Heating Together?).

- The heat pump's flow must stay stable while the heating side varies widely.

**Do not add a second pump in series or in parallel with the built-in one without checking the combined pump curves, the control strategy and the heat pump manufacturer's requirements.** Without that check, two pumps on the same circuit may work against each other, push the heat pump's flow outside its permitted range, or cause erratic flow readings. Where the primary and heating-side flows need to be controlled independently, a **hydraulic separator** — a 4-port buffer tank or low-loss header — is often the preferred solution, so each pump serves its own circuit.

Hydraulic separation has a cost too. If the secondary circuit draws more flow than the primary supplies, cooler return water can mix into the flow, lowering the emitter supply temperature — which may lead to a higher heat pump flow temperature and lower efficiency. Primary and secondary flows should be balanced as part of the design, not left to chance. (See Buffer Tank vs DHW Tank: What's the Difference in a Heat Pump System?.)

*Hydraulic separation allows independent primary and secondary flow control.*

Common Sizing Mistakes

- **Comparing system resistance with the pump's maximum head** instead of the unit's available external head.

- **Adding static height to the pump head.** In a sealed, closed heating loop the water going up is balanced by the water coming down; the pump overcomes friction, not building height. Static height matters for system pressure and expansion vessel settings — not for pump head.

- **Forgetting the strainer, filter and diverter valve** in the index circuit.

- **Undersized primary pipework**, then blaming the pump.

- **Oversizing the pump** — a bigger pump raises velocity, noise and electricity use, and may push flow beyond what the heat exchanger or emitters are designed for.

- **Ignoring minimum flow** when every zone valve can close.

- **Using water data for a glycol system.**

- **Adding a second pump without checking combined pump curves, controls and manufacturer requirements.**

Worked Example: 12 kW Heat Pump, ΔT 5 K

*All pressure-drop values below are illustrative, to show the method. Use manufacturer data for your actual components and the actual unit's pump curve.*

**1. Design flow**

0.86 × 12 ÷ 5 ≈ **2.06 m³/h** (≈ 34 L/min)

**2. Index circuit pressure drop (illustrative)**

Each component's pressure drop must be calculated at the flow that actually passes through it. The primary pipework carries the total design flow (≈ 2.06 m³/h), while the longest UFH loop carries only its share of that flow as distributed at the manifold. Do not apply the total flow to an individual UFH circuit.

| Component | Pressure drop |

|---|---|

| Primary pipework, flow + return, incl. fittings allowance | 15 kPa |

| Magnetic filter / strainer (clean) | 5 kPa |

| Diverter valve | 3 kPa |

| UFH manifold + longest loop | 20 kPa |

| **Total** | **≈ 43 kPa (≈ 4.4 m)** |

**3. Available external head at 2.06 m³/h (hypothetical)**

Read this from the actual unit's pump curve. To show both outcomes, assume two hypothetical datasheets:

| | Unit A (hypothetical) | Unit B (hypothetical) |

|---|---|---|

| Available external head at 2.06 m³/h | 60 kPa | 35 kPa |

| System pressure drop | ≈ 43 kPa | ≈ 43 kPa |

| Result | ≈ 17 kPa margin | ≈ 8 kPa short |

**4. Compare**

- **Unit A:** the built-in pump is likely sufficient. Whether a ~17 kPa margin is enough for filter fouling and real-world deviations should be checked against the manufacturer's recommended design margin. Large extra safety factors on top tend to lead to oversized solutions.

- **Unit B:** the system would settle at a lower flow than designed, widening the ΔT. Look at the cheapest fixes first. For example, if the primary pipework was sized in 22 mm copper, upsizing to 28 mm may reduce the primary pipework pressure drop substantially. The new value should be recalculated from the actual pipe length, internal diameter, fittings and the flow through that section, rather than scaled from the 15 kPa allowance by a fixed ratio. If the recalculated total falls below the available head with a reasonable margin, the built-in pump may be sufficient; if the margin is still thin, a lower-resistance filter or valve, or splitting the longest UFH loop, may also be worth considering. Hydraulic separation with a secondary pump comes after these options, not before.

**What if the design ΔT were 7 K?**

Flow falls to ≈ 1.47 m³/h. Because pressure drop scales roughly with flow squared, the same system would drop to roughly half — around 22 kPa. That can solve a head problem on paper, but it is not a free fix. A wider design ΔT must stay within the heat pump manufacturer's permitted operating range and minimum-flow requirement. It must also be checked against emitter capacity: the emitters then need to deliver the same output with a lower mean water temperature, so check that the design flow temperature does not have to rise to compensate.

FAQ

**Does the built-in pump on a monobloc heat pump need to be replaced if it seems too weak?**

Usually not as a first step. Check the system resistance, pipe sizes, filter condition and pump control mode first. Replacing a built-in pump may affect the unit's warranty and controls; consult the manufacturer.

**What happens if the flow rate is too low?**

The ΔT widens, the heat pump may run at a higher flow temperature, efficiency may fall, and below the minimum flow the unit may stop on flow protection. Defrost may also become less reliable.

**Is a bigger pump always safer?**

No. Excess flow raises velocity, noise and pump energy, can disturb the heat pump's control, and in separated systems may cause unwanted mixing.

**Do I need a secondary pump with a buffer tank?**

It depends on the configuration. A 4-port buffer tank used as a hydraulic separator normally needs a pump on the heating side. A 2-port buffer in series typically does not. See the buffer tank guide linked above.

**How does antifreeze glycol affect pump sizing?**

Glycol mixtures generally need more flow for the same heat output and create more pressure drop at that flow, so the available head margin shrinks. Use corrected data rather than water values.

Continue Exploring

📖 **Hydraulic design guides**

- [Do You Need a Buffer Tank with an Air-to-Water Heat Pump?](https://nordthermglobal.com/blog/do-you-need-a-buffer-tank-with-an-air-to-water-heat-pump)

📖 **Related reading**

- [Heat Pump Weather Compensation: How to Set the Heating Curve](https://nordthermglobal.com/blog/heat-pump-weather-compensation-how-to-set-the-heating-curve)

- [Underfloor Heating Manifold Explained](https://nordthermglobal.com/blog/underfloor-heating-manifold-explained-components-function-balancing)

- [What Is the Best Flow Temperature for Underfloor Heating with a Heat Pump?](https://nordthermglobal.com/blog/what-is-the-best-flow-temperature-for-underfloor-heating-with-a-heat-pump)

🏠 **Related products**

- [R32 air-to-water heat pumps](https://nordthermglobal.com/shop?category=2)

*Check design flow, system pressure drop, available external head and operating limits before selecting a secondary pump.*

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