Professional guide: bleeding water pumps in heating and pressure-booster systems

As a pump manufacturer's design team, we help users and installers every day with issues such as bleeding a central-heating pump, how to bleed a central-heating pump, and how to carry out the pump-bleeding process in pressure-booster systems or with heat pumps. Below, you will find an expert explanation of when the pump should be bled, what the symptoms of an airlocked pump and symptoms of an airlocked system are, and what a step-by-step guide to bleeding a circulation pump looks like. Remember: safe pump bleeding and proper diagnosis are essential for ensuring proper pump operation, efficiency, and a long pump service life.

Why does the system become airlocked, and what does this mean for its operation?

Air enters the system for several reasons: filling it “too quickly,” leaks on the suction side, a drop in static pressure, dissolved gases released by changes in temperature and pressure, and during servicing before the start of the heating season. As a result, an airlocked system loses flow efficiency, the pump operates loudly, air bubbles circulate through the system, and radiators and underfloor-heating loops heat unevenly. In extreme cases, an airlocked pump and the entire heating system experience cavitation, which can accelerate damage to the pump and fittings.

Modern air-venting systems (automatic air vents, microbubble separators) support the process, but in practice pumps also require manual venting—especially after servicing, upgrades, or a longer shutdown before the heating season.

Quick diagnosis: when does it require bleeding and what should you check first?

• Noise / humming / knocking – characteristic “gurgling,” metallic knocks, or cyclic scraping indicate bubbles in the impeller. These are classic symptoms of air in the pump.
• Uneven heating – some radiators are cold in their upper sections; the heating system’s operation is disrupted.
• Reduced performance – the circulation pump’s performance decreases, heating time increases, and heating system performance drops.
• Pressure gauge and flow meters – fluctuations or lack of stabilization after starting the circulation pump.

If you notice the symptoms above, the pump should be vented. A properly vented pump still requires a brief check of the parameters and tightness to confirm that the system continues venting automatically.

Safety and preparation

Every safe pump venting procedure requires basic precautions: disconnecting the power supply if the procedure requires it (some circulation pumps are vented while running—check the manual), protecting your eyes and hands, and securing the work area against hot water. Before you begin venting the pump, make sure that the static pressure and water level in the expansion vessel comply with the design of the heating system. For pressure booster systems, check the check valve and the tightness of the suction line—without this, filling the pump with water and restarting it will be ineffective.

Central heating circulation pump – manual procedure using the vent screw

In many devices, the central heating circulation pump has a capped vent screw on the front. The following pump venting steps are standard for many central heating pumps and constitute a practical circulation pump venting guide:

1. Locate the vent screw on the front of the housing (usually a slot for a flat-head screwdriver).
2. Depending on the model, the pump should operate at low speed or be disconnected. If the manufacturer allows venting “with the impeller running,” this enables quick pump venting.
3. Gently loosen the screw – you will hear the escaping air hiss. When a steady stream of water without bubbles appears, tighten the screw.
4. Check for leaks, top up the water in the system (if it is an open system/one with an expansion vessel), and run it at maximum speed for a few minutes – the system will remove the remaining microbubbles.

This type of system bleeding is quick and effective, especially after short shutdowns or servicing. Remember to bleed the system at radiators/floor-heating loops as well (if they have manual air vents), because the circulation pump alone will not remove air pockets in remote branches.

Automation and the “self-venting” function

Many modern central heating circulation devices support automatic air removal. After initial start-up, set a high speed for about 30 minutes, and the air will escape through the air vents in the boiler and manifolds. This is a fully valid method of bleeding the heating system and ensures that the properly bled pump reaches its nominal operating curve.

Table – pump bleeding procedures and post-procedure checks

Bleeding a circulation pump – when manually and when “systemically”?

If the system has microbubble separators and automatic air vents, the system usually bleeds itself while circulating. However, after major work (e.g., boiler replacement), the need to bleed the pump manually often arises. In that case, you should bleed the pumps and simultaneously bleed the system at the manifolds and radiators. This approach results in a properly bled pump and even temperatures throughout the building.

Bleeding the heat pump and ground-source circuits

In heat pumps (air-to-water, ground-source), venting the heat pump concerns both the heating circuit and, in ground-source systems, the brine circuit. Using a flushing station (a high-flow service pump) is recommended to vent the pump and the main lines in continuous circulation mode with recirculation. After initial flushing and venting, briefly running all circulation pumps at maximum speed helps remove microbubbles to the separator. Best practice includes high-point air vents and a microbubble separator installed where gas solubility is lowest (high temperature, low pressure), which accelerates venting the heating system.

Pressure-boosting pump – “venting” by filling (priming)

A water pressure-boosting pump will not operate properly if there is air in the suction line. Therefore, the pressure-boosting pump is vented by completely filling the pump with water and filling the suction line through the dedicated priming plug. In practice, these are the key steps in the venting process for pressure-boosting systems:

1. Close the discharge valves and fill the casing until air bubbles stop coming out.
2. Make sure the check valve at the end of the suction line is working properly; a leaking valve is a common reason for having to vent the pump “over and over.”
3. Start the pump – if the suction line does not “catch” the water column, add more water and try again. Sometimes the pump operates for several cycles before achieving a stable water column.

Although colloquially called “venting,” this process is actually priming—restoring a continuous column of liquid so that the pressure-boosting pump can operate normally again. For added safety, use modern pressure-boosting pumps with dry-run protection and a flow sensor—you will reduce the risk of pump damage.

Installation tips

• Keep the suction line as short and leak-free as possible; minimize bends – this supports proper pump operation.
• A manual air vent at the highest point of the discharge line makes it easier to vent the system after servicing.
• Regular pump maintenance and inspection of the check valve will extend the pump’s service life.

Central heating – on-site practice

During the heating season and at the start of the heating season, we receive the most questions about how to vent a circulation pump and how to vent a central heating pump in single-family homes. In brief: we vent central heating pumps locally first (using the plug), then along the system (manifold/radiators), and finally activate the automatic system air-venting systems to complete the process. Properly selected circulation pump parameters (head versus resistance) guarantee that the circulation pump operation will “carry” the remaining microbubbles through the separators.

Frequently asked questions and answers

How do I know that it requires venting?

Noise, reduced comfort, and disrupted circuit regulation are typical symptoms of air in the system. If humming can still be heard after a brief run at maximum speed and bleeding the radiators, you should vent the pump and sections. After the procedure is complete, a properly vented pump operates smoothly and quietly.

Is venting the fuel system the same as venting a central heating system?

No – this is a completely different procedure concerning the fuel supply of oil-fired boilers and engines. In central heating hydraulics, we are dealing with water/glycol solution. This guide covers only venting water systems and how to vent the circulation pump and the booster pump.

Is regular pump venting necessary?

If the system is leak-free and has functioning separators and automatic air vents, regular pump venting comes down to an inspection after servicing and filling. However, after any major intervention in the heating system, you should always vent the system.

What if the central heating circulation pump has a self-venting mode?

Use it – it is quick and safe. After startup, set the maximum speed and let it run for several to several dozen minutes. This is a fully acceptable way to vent the heating system. If in doubt, additionally vent the circulation pump using the plug.

Checklist – before you start and after completion

• Before: verify the heating system filling pressure, the condition of the air vents, access to the plug, and occupational safety protection.
• During: carry out safe pump venting and monitor the temperature and leaks.
• Afterwards: perform a pump operation check (silence, flow, no vibrations), assess the circulation pump performance, and adjust the settings.

Good design and service practices

Use microbubble separators at points where gas solubility is low, install automatic air vents at the highest points, and for pressure-boosting pumps, ensure the check valve, pipe diameters, and suction-line tightness are correct. This way, the properly vented pump will maintain its curve, the pump operates more quietly, and the pump’s service life increases. If cavitation is suspected, review the circulation pump parameters and system resistance.

Summary and quick “cheat sheet”

If you notice signs of air in the pump, air removal should be carried out from the pumps through the radiators to the manifolds. Central heating pumps and every central heating circulation pump should provide access to the plug; some models allow air removal while operating. In pressure-boosting systems, we vent the pressure-boosting pump by completely filling the pump with water. Afterward, perform a pump operation check. Only an effectively vented pump and properly selected circulation pumps to use will ensure comfort in central heating and reliable pump operation for years.

Our service teams emphasize that the circulation pump in central heating systems is the heart of the circulation: proper air removal, stable filling, and suitable automation are essential for the circulation pump and other central heating pumps to maintain their efficiency. Once the circulation pump has been properly diagnosed and vented, the circulation pump operates quietly, and the heating system reaches its target parameters more quickly.

Appendix: glossary and key terms

• Bleed screw – a plug on the front of the pump for releasing air.
• Microbubble separator – an installation device that “captures” microbubbles and supports system air removal.
• Priming – completely filling the pump housing and suction line in a pressure-boosting system; this is how we effectively vent a pressure-boosting pump.
• Self-venting – an automatic function of many pumps; it speeds up the pump air-venting process.