Why you need a circulation pump
The pump moves heating water from the boiler to the radiators and underfloor heating. Without it the water moves slowly, distant rooms stay cold and the boiler overheats.
Wall-hung gas boilers already have a built-in pump. A separate pump is needed if:
- the boiler is floor-standing or solid fuel without its own pump;
- the house has several circuits: radiators, underfloor heating, water heater;
- the built-in pump is not enough for a large area or long pipe runs.
How to read the model code
Take a pump marked 25/60 180:
| Number | What it means |
|---|---|
| 25 | Connection size, DN 25 (1½» thread) |
| 60 | Maximum head — 6 m of water column |
| 180 | Installation length of the pump body, mm |
Head shows how much resistance from the pipes and radiators the pump can overcome. The pump reaches its maximum head at zero flow, so you need to look at the duty point on the performance curve.
How to calculate the flow rate
The flow rate is calculated from the system output:
Flow rate (m³/h) = 0.86 × output (kW) / difference between flow and return (°C).
For a house with a 20 kW boiler and radiators with a 20 °C difference: 0.86 × 20 / 20 = 0.86 m³/h. For underfloor heating the difference is smaller, usually 5-10 °C, so the flow rate in that circuit is higher.
How to estimate the head
For a private house of up to 200-250 m², a pump with a 4-6 m head is usually enough. A more accurate head is calculated for the longest branch: pipe length, number of fittings, resistance of the radiators, valves and underfloor heating manifold.
An oversized pump is also a mistake. It causes noise in the pipes and valves and increases electricity consumption.
Standard or energy-efficient pump
A classic pump (for example, the UPS series) has 2-3 fixed speeds. The speed is set manually during commissioning.
An energy-efficient pump (for example, ALPHA2) adjusts its speed to the system's demand automatically. When thermostatic heads close some of the radiators, the pump reduces its output. Power consumption is several times lower and it runs almost silently.
Installation rules
- The motor shaft must be horizontal. The arrow on the body shows the direction of flow.
- A filter is fitted before the pump and isolation valves on both sides, so the pump can be removed without draining the system.
- The pump is usually installed on the return before the boiler, but many modern models also work on the flow. Check the manual.
- After start-up, bleed the air from the system. Running dry quickly destroys the pump.
Important: flow rate and head are best calculated together with the hydraulics of the whole system. Several circuits sometimes need a hydraulic separator or a manifold with separate pumps.
Tell ThermoHouse your boiler output, house area and number of circuits. We will select a GRUNDFOS pump using its performance curve, not by guesswork. Contact us →
