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 product types, standards, installation requirements, and the situations where each applies.

Drain Valves and Drain Cocks

BS 2879 and Product Standards

Drain cocks (also called drain valves or draining cocks) in heating systems are specified under BS 2879:1980 — the British Standard for draining taps. The standard defines three types:

• Type A: Simple plug-type drain cock, opened by turning a slotted head with a screwdriver. Used on lower-pressure circuits and cold water storage.
• Type B: Key-operated drain cock. A square-head pattern requiring a drain cock key (BS 2879 key). Common on central heating systems. The key-operated design prevents accidental opening.
• Type C: Hose union drain cock. Similar to Type B but with a threaded hose union outlet for attaching a drain hose. Standard specification for heating system drain points where the circuit must be fully drained into a container or drain.

For central heating systems, Type C hose union drain cocks are the correct specification for service drain points. Type B key-operated drain cocks are appropriate for auxiliary drain points on radiator tails or low points in pipework runs.

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: Required where water has a dezincification tendency (typically in areas with pH above 7.5 and chloride above 30mg/l). DZR brass is marked with a triangle on the fitting body. BS 6920 and BS EN 13828 compliance is standard for reputable brands (Altecnic, JG Speedfit, Reliance).
• 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

Where individual radiators need to be isolated for removal without draining the system, a combination of isolating valve (or TRV with lockshield) and radiator bleed valve is sufficient. For radiators in exposed or frost-risk positions, a drain cock can be fitted to the lower tapping in lieu of a blanking plug. This allows the radiator to be drained without affecting the remainder of the circuit.

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.

Bleed valves are non-repairable consumables — when the internal packing fails, replace the entire valve assembly. Most radiators use a standard 1/8" BSP thread; replacement valves are interchangeable between brands. Some modern designer radiators use a hex key socket in lieu of a square key pattern.

The correct procedure for bleeding a sealed system radiator:

1. Turn the circulating pump off (or run it briefly to push air to radiator tops)
2. Open the radiator bleed valve slowly (1/4 turn) — listen for hissing air
3. Wait until water dribbles from the bleed point — close valve
4. Check system pressure — pressure will drop when air is released. Repressurise to 1.0–1.5 bar cold via filling loop if needed
5. Run system and check bleed valves again if air persists

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.
• Horizontal AAV: Less common, used where vertical installation is not possible. Float mechanism is horizontal. Performance is less reliable than upright pattern.

A lockshield cap is supplied with most AAVs. The cap closes the valve outlet and is used during system filling to prevent the float chamber from becoming contaminated with flux, solder debris, or inhibitor before the circuit is clean. Once the system is filled and circulating, remove the cap to allow normal venting operation.

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 (Flamcovent, Spirovent, Caleffi 546/547 series, Fernox Omega) are now considered best practice on primary circuits, particularly where magnetic filters are not separately installed. They mount inline on the flow pipe, close to the boiler or heat pump. Sizes: 22mm–54mm compression or end-feed connections, or flanged versions for commercial plant.

Positioning Automatic Air Vents

AAVs should be positioned at circuit high points — typically the top of a vertical flow from the boiler, or at the highest point in each zone circuit. Key positioning rules:

• Mount on the flow pipe (hot side) where possible — water temperature promotes micro-bubble release
• Do not mount in inaccessible locations — the lockshield cap must be removable, and the unit may need replacement over its service life
• Ensure adequate clearance above the AAV body for the cap removal and for any drip from the valve outlet
• Where the AAV is mounted inside a boiler cupboard or airing cupboard, provide a small drip tray — float valves do occasionally weep, particularly in the first months of operation while the system water is being degassed
• On sealed systems, the AAV should be on the correct side of the pump — typically downstream on the flow to prevent the pump from drawing air back through a defective needle valve. Pump positioning relative to the expansion vessel (pressure side vs. suction side) affects this — consult system design guidance

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. Flush the system if the needle contamination is from flux/debris — this is typically a new-system problem that resolves within weeks of operation
3. Replace the AAV if flushing does not resolve the issue — repair is not practical on most designs

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

Heating system inhibitors (Fernox F1, Sentinel X100, Adey MC1+) should be introduced into the sealed system via a dedicated dosing point — not via the filling loop, which introduces diluted inhibitor mixed with mains water. Most fill/drain assemblies include an inline port for a dosing bottle. Alternatively, a radiator can be partially drained and the inhibitor bottle connected to the bleed valve using a proprietary adapter. Following BS 7593:2019 (code of practice for water treatment of heating systems), inhibitor should be dosed at first fill and checked annually during boiler service.

Part L and Commissioning Requirements

Under the Domestic Building Services Compliance Guide (which supports Part L of the Building Regulations), a new boiler or central heating system must be commissioned using a recognised methodology — typically the HHIC / HPA checklist or the boiler manufacturer's own commissioning procedure. The commissioning record must include confirmation that the system has been:

• Flushed clean (or powerflushed if retrofitting to an existing system with sludge)
• Filled with inhibited water
• Vented and set to the correct cold-fill pressure (1.0–1.5 bar for most domestic systems)
• Checked for air locks and bled as required

This commissioning data is required for CORGI-Gas (now Gas Safe) registration, HIES warranty compliance, and manufacturer warranty purposes. An air vent or drain cock that is inaccessible — buried in a floor screed, hidden behind fixed cabinetry, or inside a sealed unit without an access panel — will prevent this commissioning from being properly completed and may void the boiler warranty.

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: Caleffi 546 series (15–28mm), Flamcovent Clean Smart (15–22mm), Fernox Omega Filter
• 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 part of a properly designed and commissioned heating system. Fitting a hose union drain cock at every serviceable low point, an automatic air vent at every permanent high point, and a combined air/dirt separator on the primary flow circuit gives the next engineer attending the system the access and information needed to service it correctly. Getting these basics right reduces callbacks, facilitates future powerflushes and repairs, and satisfies BS 7593 and Part L commissioning requirements without additional effort.

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