Drain Valves, Lockshield Valves, and Draining Down a Central Heating System: A UK Plumber's Guide

Every central heating system, no matter how modern, will need to be partially or fully drained at some point — for radiator replacement, pipework alterations, adding components, or annual servicing. The ability to isolate and drain efficiently comes down to the quality and positioning of drain valves, lockshield valves, and radiator isolation fittings installed during the original fit-out.

This guide covers the full range of drain-down components: BS1010 drain valves, hose union bibcocks, lockshield radiator valves, balancing procedures, system re-fill, and inhibitor dosing after drain-down.

Why Drain Valves Matter

A heating system that cannot be isolated and drained quickly costs time and money. Without strategically placed drain points:

• A single radiator replacement requires draining the entire system
• Pipework modifications at low points trap water even after the system appears empty
• Refilling takes longer as air-locking increases without proper vent points
• Water hammer and noise increase when air is trapped post-refill

Good practice during first fix is to install drain valves at every low point and at the base of each circuit drop — typically in the utility room or plant room, and adjacent to any zone valve or pump set that may need future servicing.

Types of Drain Valve

BS1010 Screwdown Drain Cock (Boiler Drain Valve)

The standard UK heating drain valve, manufactured to BS 1010 Part 2. Features:

• Body: DZR (dezincification-resistant) brass or gunmetal
• Connection: ½" BSP male threaded body screwed directly into a tee or elbow fitting in the pipework
• Outlet: ½" BSP male hose tail accepting a standard garden hose connector or proprietary drain hose
• Operation: flathead or cross-head screwdriver slot rotates a gate or plug internally to open/close
• Sizes: ½" (most common), ¾" (for larger pipe runs)

BS1010 drain valves are specified in virtually every domestic and light commercial heating system. Install with PTFE tape and/or boss white on the thread — never hemp, which can swell and block the valve aperture. See the APM guide to PTFE tape and thread sealants.

Hose Union Bibcock (Outdoor-Pattern Drain Tap)

Used as a drain point where a hose connection will be left permanently attached. The bibcock pattern has a long lever or cross handle that can be operated by hand without a screwdriver. Less common in central heating but useful in plant rooms with permanent drain-down connections.

In-Line Drain Valve (Service Valve with Drain Port)

A combined isolation and drain fitting — usually a ½" ball valve or service valve body with an integrated ½" drain port. Allows the section downstream to be isolated and drained without disconnecting pipework. Widely used:

• Under boilers on the heating flow and return connections
• On the inlet/outlet of circulator pumps
• On hot water cylinder connections
• On zone valve assemblies where the zone needs to be isolated for maintenance

TRV and Lockshield Combination Drain Valves

Some radiator valve ranges include a drain port in the lockshield body, allowing individual radiators to be drained without draining the full circuit. These are useful in large systems but must be compatible with the radiator valve manufacturer's thread specification.

Positioning Drain Valves — Best Practice

Lockshield Radiator Valves

The lockshield valve (LSV) is the non-user-adjustable valve fitted to one side of every radiator — typically the return connection. Its functions are:

1. Isolation — fully close to isolate the radiator for removal or replacement
2. Balancing — set a specific flow rate through each radiator during commissioning to ensure even heat distribution across the system
3. Tamper resistance — the plastic cap locks out unauthorised adjustment once balanced

LSV Types

• Standard (angled/straight/corner) — matches TRV type on the flow connection; available in 15mm compression or push-fit (JG Speedfit)
• LSV with drain point — drain port under the cap for individual radiator drain-down
• Manual radiator valve (MRV) — non-thermostatic flow control used on the flow side where a TRV is not required (e.g. bathroom towel rail, bypass radiator)
• Pre-settable LSV — calibrated setting indicator for reproducible balancing; used in S-plan and underfloor systems where accurate Kv values are required

Balancing Lockshield Valves

After any system drain-down and refill, radiator balancing should be re-checked — particularly if the order of refilling was different or if components have changed. See the APM guide to balancing a central heating system for the full procedure. In summary:

1. Open all TRVs and lockshield valves fully
2. Run the system at full demand until stabilised
3. Identify the radiator furthest from the pump (last on the circuit) — leave its LSV fully open
4. Work backwards towards the pump, progressively closing LSVs to increase resistance on nearer radiators
5. Target: 10–12°C flow-to-return temperature difference (ΔT) across each radiator under steady-state
6. Lock LSV caps once balanced

Partial System Drain-Down (Single Radiator)

To replace or move a single radiator without draining the entire system:

Method 1: Freeze the Pipework

Use a proprietary pipe-freezing kit (CO2 aerosol or freeze clamp) on both connections to the radiator, then remove the radiator between the frozen sections. Fast, no chemicals or drain-down required. See the APM guide to pipe freezing kits and emergency pipe repairs.

Method 2: Close TRV and Lockshield

1. Turn the TRV to fully closed (0 position)
2. Count the number of turns to close the lockshield valve fully (note for re-balancing)
3. Close the lockshield valve
4. Place a suitable container under each valve connection
5. Using a radiator bleed key, open the bleed valve at the top of the radiator to allow air in, breaking the vacuum as water drains
6. Loosen the radiator valve cap nut (TRV side first) — water will drain into the container
7. Remove the radiator
8. On refitting: close bleed valve, reconnect both sides, open lockshield to counted position, open TRV, re-bleed radiator

Full System Drain-Down Procedure

Before You Start

• Turn off the boiler and allow system to cool — drain a hot system only if absolutely necessary and with extreme caution; pressurised water at 70°C+ is a serious burn risk
• Turn off the cold feed to the system (close the mains stopcock or ball valve on the cold feed if open-vented, or close the filling loop on sealed systems)
• Prepare sufficient hose to reach a drain or external gutter; note that inhibitor-dosed heating water is NOT to be discharged to soakaway or watercourse — use a soakaway gully or controlled disposal
• Have inhibitor, cleaner (if flushing), and replacement components ready

Draining a Sealed System

1. Switch off boiler and immersion heater; close filling loop
2. Connect a suitable hose to the lowest drain valve on the system
3. Open bleed valves on the highest radiators first — this breaks the vacuum and allows water to flow
4. Open the drain valve — water will flow out under residual system pressure (typically 1–1.5 bar when cool)
5. Open remaining bleed valves from top to bottom as flow slows
6. Once flow stops, open any remaining drain valves at circuit low points
7. Tip or slightly tilt pipework where possible to drain residual water from horizontal runs

Draining an Open-Vented System

1. Switch off the boiler and allow to cool
2. Tie up the ball valve in the feed and expansion (F&E) cistern to prevent refill
3. Connect hose to the lowest drain cock (usually near the boiler return)
4. Open radiator bleed valves from the highest radiator downward
5. Open the drain cock — gravity drains the system
6. For complete emptying, use a wet vacuum on each drain cock point in turn

Common Drainage Problems

Refilling After Drain-Down

Sealed System Refill

1. Close all bleed valves and drain cocks
2. Connect the filling loop (or flexible filler) between mains cold and the heating return
3. Open the filling loop slowly — cold mains pressure (typically 2–4 bar) pushes water into the system
4. As water fills, open radiator bleed valves from the lowest radiator upward, releasing trapped air; close when water flows steadily
5. Monitor the system pressure gauge — fill to 1.0–1.2 bar cold fill pressure (consult boiler manual)
6. Close filling loop once target pressure is reached
7. Run the system through a heat cycle; re-bleed any radiators that are cold or slow to heat
8. Re-check pressure after bleeding — top up if below 1 bar

Open-Vented System Refill

1. Close all drain cocks and bleed valves
2. Release the F&E cistern ball valve to allow cold feed in
3. Open bleed valves from the lowest radiator upward as system fills
4. Run the pump briefly to circulate and assist filling; check for airlocks
5. Allow F&E cistern to fill to correct level — ball valve should close when cistern is at operating level (typically 25mm below overflow)

Inhibitor Dosing After Drain-Down

Every time the system is drained and refilled, inhibitor concentration is lost. Always dose fresh inhibitor after a full drain-down:

• Dose rate: typically 1 litre of inhibitor per 100 litres of system volume (check product data sheet — varies by brand)
• Products: Fernox F1 Protector, Sentinel X100, Adey MC1+, Sentinel X200 (for hard water areas)
• Application: dose via the F&E cistern (open-vented) or via inhibitor pot/radiator bleed valve (sealed). Some engineers use a pressurised dosing vessel connected to the drain cock for sealed systems.
• Test: use test strips or a refractometer to verify inhibitor concentration after dosing

If the system shows evidence of corrosion (black sludge from radiator bleed valves, reduced heat output), consider a system cleaner (Fernox F3, Sentinel X400) before dosing inhibitor. See the APM guide to boiler system chemicals.

Magnetic Filters and Drain-Down

During system drain-down, flush and clean the magnetic filter before refilling. Most Adey MagnaClean, Fernox TF1, and Sentinel Eliminator filters have a base bowl drain valve that allows the filter chamber to be emptied and flushed without draining the full system. See the APM guide to magnetic filters.

Y-Strainer and Dirt Separator Maintenance

Y-strainers and dirt separators have a blow-down valve at the base — open briefly during refill to flush debris from the strainer basket. Do not leave the blow-down open during normal operation. See the APM guide to Y-strainers and in-line filters.

Key Products for System Drain-Down

• BS1010 drain valves — ½" and ¾" brass drain cocks for every low point
• Lockshield valves — angled, straight, and corner formats in 15mm and 22mm; pair with TRVs and manual radiator valves
• Filling loop — flexible braided stainless filling hose with double-check valve; ½" BSP fittings to most UK boilers
• Radiator bleed key — cross-slot or square bleed screw type; use with a bleed key with integrated catch tray
• Inhibitor — Fernox F1 (general purpose), Adey MC1+ (heavy-duty corrosion protection)
• PTFE tape — essential when refitting drain cocks and valve connections

Shop Related Products

Taurus Angled TRV With Lockshield 15mm

View Product

½" Hose Union Bib Tap Lockshield

View Product

15mm Filling Loop Straight

View Product

Calmag Heating Pack 22mm Filter & Inhibitor

View Product

Summary

Drain valves and lockshield valves are unglamorous but critical components for any serviceable heating system. Fitting BS1010 drain cocks at every low point, using lockshield valves with accurate balancing, and following a systematic drain-down and refill procedure protects the system long-term, prevents unnecessary full drain-downs during maintenance, and ensures inhibitor concentration is maintained.

Related APM guides: Balancing a Central Heating System | Boiler System Chemicals | Magnetic Filters | Pipe Freezing Kits | Y-Strainers and Dirt Separators | Sealed System Pressure and Filling Loops | PTFE Tape and Thread Sealants | Radiator Valves

Get It from APM Electricals

APM Electricals, 24 Western Avenue, Acton, London W3 7TZ. Call 020 8702 8080 or visit www.apmi.uk for same-day trade counter collection and next-day delivery across London and the UK.