Drain Valves, Manual Air Vents, and Automatic Air Eliminators: A Plumber's Guide to Heating System Accessories

Every sealed heating system needs a way to fill, bleed air, and drain when service or repair is required. Drain valves, air vents, and automatic air eliminators are among the least glamorous products in a plumber's van, but specifying or fitting the wrong one causes problems long after the job is signed off — a corroded drain cock seizing during a future flush, an automatic air vent weeping onto a boiler casing, or persistent air locks in a system that should vent itself. This guide covers the main product types, practical selection checks, and the situations where each may apply. Final selection and installation must follow the system design and the current instructions for the exact product and appliance.

Need the right valve or vent for the job? Use the Quick Shop below, browse APM's automatic air valves, or call 020 8702 8080 to check suitability and counter availability before travelling to 24 Western Avenue, Acton, London W3 7TZ.

Quick Shop: Drain Valves, Manual Air Vents and Automatic Air Vents

These links go directly to the exact APM products checked for this article. Confirm the connection size, orientation and system suitability from the live product information and manufacturer instructions before fitting.

Drain Valves and Drain Cocks

Drain-Tap Standards and Product Labels

BSI lists BS 2879:1980 as the specification for draining taps of the screw-down pattern. Do not infer the operating pattern, outlet arrangement or permitted service from a Type A, B or C label alone: check the current manufacturer data for the exact valve. The two APM products in the Quick Shop retain their manufacturer-supplied Type A descriptions, but this article does not redefine the standard's type classifications.

For a service drain point, verify the inlet connection and size, operating method, hose or discharge arrangement, pressure and temperature suitability, material, orientation and access. The selected valve must suit the designed drain route and allow controlled discharge without relying on an improvised connection.

Positioning Drain Points

Every section of pipework that could trap water and prevent complete draining must have a drain point at its lowest accessible position. Key locations include:

• The lowest point of the boiler primary circuit (typically near the boiler return manifold or flow/return connections at floor level)
• The lowest point of each zone in a multi-zone system
• The base of each radiator circuit in gravity-fed systems (though modern sealed systems often use TRV lockshields instead)
• Beneath any section of pipework that drops below the primary circuit level and cannot drain by gravity to the main drain point

In practice, most domestic boiler installations provide a single drain cock at the lowest point of the primary circuit — typically a 15mm or 22mm hose union fitted in the flow or return immediately at the boiler. Ensure the drain cock is accessible without removing the boiler casing and that a drain hose can reach a suitable drain point or bucket position.

Material and Size Selection

Drain cocks for heating systems are most commonly specified in:

• DZR (dezincification-resistant) brass: Specify it where the water conditions, system design or applicable product standard require it. Verify the material designation, markings and current declaration for the exact fitting rather than assuming that every product from a named brand carries the same approvals.
• Chrome-plated brass: Decorative finish for exposed installations. Same function, cosmetic difference.
• Gunmetal (red brass): Legacy material, rarely specified on new work but encountered on older systems.

Standard drain cock sizes:

• 15mm male iron (3/4" BSP) fitting with 1/2" BSP hose union outlet — most common for domestic heating
• 22mm male iron (1" BSP) for larger commercial circuits
• 15mm compression inlet versions are available where soldering is impractical

Connecting to Radiators for Draining

Closing the radiator valves isolates the radiator from the circuit, but it does not empty the radiator. Switch the heating and circulation off, allow the system to cool, note the lockshield setting, protect the floor, close both valves, release pressure carefully at the bleed point, and drain the radiator in a controlled way through a suitable connection into trays or a hose before loosening it for removal. If either valve passes or the radiator cannot be drained safely in isolation, drain the relevant system section instead. Follow the radiator, valve and boiler manufacturer's instructions throughout.

Manual Air Vents

Radiator Bleed Valves

The most common manual air vent in domestic heating is the radiator bleed valve — a simple 1/8" BSP plug with a slotted or square key-operated bleed screw. When air is trapped in the top of a radiator body, the bleed valve is opened with a radiator key (available in brass or plastic) until water runs from the bleed point, indicating the air has been displaced.

If a bleed valve is damaged or continues to leak, identify the radiator and exact valve before replacing it. Do not assume that thread, seal, key pattern or replacement assembly is interchangeable between brands; designer and panel radiators can use different plugs, vents and operating tools.

The correct procedure for bleeding a sealed system radiator:

1. Use any initial warm-up only to identify a cold-at-the-top radiator, then switch the heating and circulating pump off and allow the radiator and water to cool before opening a vent.
2. Protect the wall and floor, hold a cloth or container below the bleed point, and use the correct key or tool.
3. Open the valve slowly until trapped air escapes. Keep hands clear of the outlet and close the valve when water begins to flow steadily; do not overtighten it.
4. When the system is cool, check the pressure against the boiler manufacturer's instructions. Repressurise only by the approved method and to the value specified for that appliance and installation.
5. Run the system, check for leaks and investigate repeated air entry or pressure loss rather than repeatedly topping up without finding the cause.

Manual Air Vents for Pipework

Air can also be trapped in pipework high points — particularly in microbore systems, horizontal runs with inadequate flow velocity to carry air through, and in systems where the pump has been oversized or sited incorrectly. Manual air vents for pipework (as opposed to radiators) are screwed into 1/8" or 1/4" BSP bosses welded or tapped into the pipe at the high point.

Lever-key air vents (using a standard radiator key or flat-blade screwdriver) are the common specification. They are available in:

• 15mm tee fittings with integral 1/8" BSP bleed point — fitted inline at a high point
• Brass angled vent bodies for installation in pipework bends or at appliance connections
• Combined fill/bleed points (Oventrop, Altecnic) which incorporate a Schrader valve, drain point, and manual bleed in a single fitting

Automatic Air Vents (AAVs) and Air Eliminators

How Automatic Air Vents Work

Automatic air vents (AAVs) remove air from heating systems without manual intervention. A float mechanism inside the body rises and falls with water level: when air collects inside the chamber, the float falls, opening a needle valve that releases air to atmosphere. When water fills the chamber, the float rises and closes the valve.

Standard AAVs are available in two configurations:

• Upright AAV: Installed vertically with the float chamber above the pipework connection. Most common. Sizes: 1/8" BSP or 1/4" BSP male thread, or inline 15mm/22mm. Altecnic, Honeywell, IMI Heimeier, Roca, and most boiler manufacturers supply these.
• Alternative-orientation AAV: Use only where the exact product is designed and approved for that orientation. Do not turn a vertical float vent on its side unless the current manufacturer instructions expressly permit it.

Cap operation is product-specific. On the Flamco Flexvent featured above, the cap is captive and must remain fitted: use its open/close position exactly as the current Flamco instructions describe. For any other AAV, confirm whether its cap should be loosened, opened or closed during filling, commissioning and normal service from that product's instructions.

Micro-bubble Air Eliminators

Standard float-type AAVs remove free air — large air bubbles that collect in the chamber. They are less effective at removing dissolved air (micro-bubbles that are carried throughout the system by the circulating water). Dissolved air causes noise (kettling, gurgling), accelerates corrosion, and contributes to magnetite formation.

Micro-bubble air eliminators (also called deaerators or combined air and dirt separators) address this by:

1. Creating a low-velocity zone inside the body where micro-bubbles can coalesce and rise to the float chamber
2. Using a stainless steel mesh or similar media to promote micro-bubble agglomeration
3. Incorporating a dirt separator that allows magnetite and suspended solids to settle and be flushed out via a dirt drain at the base

Combined air-and-dirt separators are a separate product class from a basic automatic air vent and from a magnetic-only system filter. Where one is specified, select and position the exact model from the system design and its manufacturer instructions. Do not treat a Fernox TF1 Omega as an air separator: Fernox classifies it as a hydronic and magnetic filter for system debris, while its air-separation products are listed separately.

Positioning Automatic Air Vents

AAV positioning is a system-design decision; not every high point automatically requires the same vent. Check the exact product's permitted orientation and operating limits, the appliance instructions, pump and expansion-vessel arrangement, access, and the consequences of any discharge or leak. Practical checks include:

• Use only the connection point and orientation allowed by the designed system and the exact vent instructions
• Do not mount in inaccessible locations — the cap or operating control must remain usable in the way the exact manufacturer instructions require, and the unit may need inspection or replacement
• Ensure the manufacturer-required clearance above and around the AAV for cap operation, inspection and safe service access
• Assess the risk to the boiler, electrics and finishes if the vent discharges or leaks, and provide protection only where it is safe and compatible with the appliance instructions
• Confirm the vent position against the pump and expansion-vessel arrangement rather than applying a universal flow/return rule

Common AAV Failures

The most frequent AAV failure is a weeping needle valve — the valve does not fully seat due to debris on the seat face. Symptoms include a constant drip from the cap area or salt deposits on the body indicating long-term slow leakage. Corrective action:

1. Close the lockshield cap — if dripping stops, the valve needle is contaminated
2. Follow the system-cleaning and product instructions if contamination is suspected; do not assume continued leakage will resolve by itself
3. Isolate and replace or service the AAV only by the method allowed for that exact model if the fault remains

A second failure mode is a seized float — the float corrodes or deposits cause it to stick in the open or closed position. A stuck-open float results in water loss; a stuck-closed float means air is not vented. Test by shaking the body (off-system) — the float should move freely.

Filling and Pressure Test Points

Combined filling/draining units (Altecnic Combiflex, JG Speedfit Fill Point) integrate a fill valve, system pressure gauge, drain cock, and sometimes a dirt trap in a single assembly. They are commonly fitted on the return pipework near the boiler, providing a single service point for:

• Filling and pressurising the system from a temporary hose connection
• Reading static and operating pressure
• Draining for service or inhibitor dosing

Pressure test points (also called Schrader test nipples) are small push-fit blanked valves inserted at circuit monitoring positions. A push-on pressure gauge (Magnehelic, Comark, or system gauge) can be connected and disconnected without draining. These are useful for setting differential pressure across zone circuits or for commissioning pump head calculations.

System Inhibitor Dosing Points

Use only a treatment chemical and dosing method approved for the system and appliance. Depending on the product and installation, dosing may use a dedicated point or a manufacturer-approved adapter after controlled partial draining. Record the product and dose used, then test and maintain the system water at the intervals required by the current treatment-product, boiler and applicable water-treatment guidance.

Commissioning Records and Manufacturer Instructions

Commission the installation against the current requirements that apply to the work and the exact boiler, heat pump, controls and water-treatment products fitted. Use the manufacturer's commissioning procedure and the recognised record required for that installation. Relevant checks commonly include:

• Flushed clean (or powerflushed if retrofitting to an existing system with sludge)
• Filled with inhibited water
• Vented and set to the cold-fill pressure specified by the appliance and system designer
• Checked for air locks and bled as required

Keep the completed commissioning record with the installation documents. Installer registration, installation notification, commissioning records and warranty conditions are separate requirements; verify each against current official guidance and the exact appliance terms instead of assuming one record satisfies them all. Drain points and vents should remain accessible where the system design and manufacturer instructions require inspection, operation or replacement.

Key Products and Brands

• Drain cocks: Altecnic (Type C hose union, DZR brass, 15mm MI), JG Speedfit push-fit drain valve, Reliance Valves, Pegler Yorkshire
• Radiator bleed valves: Myson, Pegler, Oventrop — standard 1/8" BSP, replacement packs of 10
• AAVs (float-type): Altecnic AAV 1/8" BSP, Honeywell T103A, IMI Heimeier, Roca
• Combined air and dirt separators: Select only a manufacturer-designated combined separator sized and installed for the designed circuit. A Fernox TF1 Omega is a system filter, not an air-separator example.
• Fill/drain assemblies: Altecnic Combiflex, JG Speedfit fill point, Heatrae Sadia (unvented cylinders)

Summary

Drain valves, air vents, and air eliminators are not afterthoughts — they are service components in a properly designed and commissioned heating system. Provide accessible drain points and suitable manual or automatic venting where the system design and current manufacturer instructions require them. A combined air-and-dirt separator may be appropriate on some circuits, but it is not interchangeable with a basic AAV or a magnetic-only filter. Correct selection, controlled draining and clear commissioning records make later inspection and maintenance more straightforward; they do not by themselves guarantee compliance with BS 7593, Part L or any appliance warranty.

Related guides: expansion vessels, sealed system pressure and filling loops, heating inhibitor and system flushing, central heating circulator pumps, powerflushing, radiator balancing, thermostatic radiator valves (TRVs), and magnetic filters.