Terminology Matters – Boilers Purging, Bleeding, Venting, Draining

Boilers and Hydronic Systems Terminology

There are many terms used loosely for boilers and hydronic systems to describe activities. Here we attempt to put these terms into a context and better describe them.

1. Draining

• Definition: Draining is a general term that refers to removing ALL fluid from a system, and it's often used interchangeably with purging. Since Draining means emptying the whole system, it is usually specific to performing maintenance such as braising or soldering.
• Purpose: To empty the system for maintenance, cleaning, replacement of components, or winterizing (in some cases).
• Method: A drain valve or hose connection at the lowest point of the system is opened to allow water to exit. Gravity or a pump may assist in removing the water.
• Use Case: Performed during major maintenance, when replacing antifreeze, or before making plumbing changes.

2. Purging

• Definition: This process involves emptying the entire system or a line of fluid, often to replace it with new fluid. Purging can be used to flush out old, contaminated fluid and ensure the system is clean and operating correctly. Purging often involves opening multiple valves or connections to allow the fluid to drain completely as fast as possible.
• Purpose: To eliminate air pockets or "airlocks" that can restrict water flow, causing uneven heating or no heating in certain areas.
• Method: Fresh water is introduced at a specific point, and the air is forced out through the purge valve until water flows freely without air bubbles.
• Use Case: Commonly done during the initial system fill or after significant repairs or maintenance.

3. Bleeding

• Definition: Bleeding is the process of releasing trapped air from individual radiators or heating units in a hydronic boilers heating system. Bleeding involves opening a valve (bleeder valve) to allow the fluid to flow and push out any trapped air bubbles. Bleeding is specific in the sense that air is removed up until water pushes the air completely.
• Purpose: To restore proper heating performance and ensure radiators heat evenly.
• Method: A radiator key or flathead screwdriver is used to open the bleed valve on the radiator. Air is released until water begins to flow steadily, then the valve is closed.
• Use Case: Done regularly as part of system maintenance, especially if a radiator is not heating properly or there is gurgling noise.

4. Venting

• Definition: Venting refers to releasing trapped air or gases from specific parts of the system, such as radiators. Venting could be used specifically for removing portions of trapped air, but a bit of pocket might still exist in the vent.
• Purpose: To ensure proper pressure levels and prevent "airlocks" that hinder the flow of water.
• Use Case: Regularly done after system startup or refilling.

5. Flushing

• Definition: Flushing involves running clean water or a cleaning solution through the hot water boiler system to remove debris, sludge, or mineral build-up.
• Purpose: To improve efficiency and prevent blockages that can reduce the system's lifespan.
• Use Case: Typically performed when signs of reduced efficiency or uneven heating are observed, or as a preventative measure during annual maintenance.

6. Backflushing

• Definition: Backflushing involves reversing the flow of water through the system to dislodge debris and improve circulation.
• Purpose: To remove stubborn blockages that regular flushing may not resolve.
• Use Case: When sediment buildup is significant, particularly in older systems.

7. Filling

• Definition: Filling is the process of adding water to the heating system to maintain proper pressure levels.
• Purpose: To compensate for water lost through leaks, evaporation, or purging.
• Use Case: Regular maintenance or after draining/purging.

8. Airlocking

• Definition: Airlocking describes the presence of trapped air in the system that prevents water from circulating properly.
• Purpose: This is an issue rather than a process and typically necessitates purging or bleeding to resolve.

9. Descaling

• Definition: Descaling involves removing mineral deposits (scale) that accumulate in the boiler or heat exchanger due to hard water.
• Purpose: To restore efficiency and prevent damage to components.
• Use Case: Performed as part of maintenance, especially in areas with hard water.

10. Skimming

• Definition: Skimming is the process of removing oils or other contaminants that may be floating on the surface of water in a boiler.
• Purpose: To improve water quality and prevent foaming or surging.
• Use Case: Primarily used in steam boiler systems during initial start-up or maintenance.

11. Priming

• Definition: Priming involves preparing a pump or component to operate correctly by filling it with water.
• Purpose: To ensure proper operation and avoid damage caused by running the pump dry.
• Use Case: Before starting a circulation pump that has been drained.