Pump Valves Explained: Gate vs Ball Type, 22mm & 28mm Sizes

Pump valves are the dedicated pair of mechanical isolation valves installed directly upstream and downstream of a central heating or underfloor heating circulating pump. Each valve features an integrated captive 1½″ BSP union nut designed to mate directly with the pump housing ports, paired with either a 22mm or 28mm compression connection for direct jointing onto copper distribution pipework.

With a functioning pair of pump valves in place, replacing an aged or seized circulating pump requires less than 15 minutes and catches only a cupful of water in a tray. Without working isolation valves, swapping a circulator forces the engineer to execute a full system drain down, re-vent every radiator or manifold loop, clear severe airlocks, and dose fresh chemical inhibitor under BS 7593. Investing in a quality pair of brass pump valves at £4.16 to £6.47 each provides the most cost-effective installation insurance in the airing cupboard.

Quick answer: For modern domestic heating systems and underfloor heating manifolds, install a pair of 22mm ball type pump valves featuring 1½″ BSP captive union nuts. Fit 28mm pump valves on large heat pump or boiler primary circuits. When replacing an older 180mm pump with a modern 130mm circulator, screw a 1½″ male-to-female long pump valve adaptor into one port to bridge the 50mm gap instantly. Always replace the high-temperature EPDM union washers on every pump swap to prevent nuisance leaks.

Gate Type vs Ball Type Pump Valves: Operational Mechanics and Longevity

In domestic hydronic heating installations, pump isolation valves are manufactured in two distinct internal configurations: traditional multi-turn gate valves and modern quarter-turn ball valves. Understanding their mechanical differences is crucial for ensuring system reliability and preventing seized valves during emergency breakdowns:

Both valves are forged from heavy-duty DZR/CW617N brass, WRAS approved, and pressure-rated to PN16 (16 bar static working pressure at up to 110°C). However, for any modern installation or circulator upgrade, ball type pump valves represent the clear engineering choice: they cost less, preserve vital pump head pressure, and ensure that when a replacement pump is needed years down the line, the valves isolate fully without weeping.

Sizing Rules: 22mm vs 28mm Compression Ends with Universal 1½″ BSP Unions

A frequent point of confusion among apprentices and DIY renovators is the dual measurement system found on pump valves. Pump valves are specified with two distinct dimensions:

• The Compression End (22mm or 28mm): The male body connection that accepts standard BS EN 1057 copper tube via a compression nut and brass olive.
• The Pump Union Nut (1½″ BSP Parallel Thread): The captive female union nut that spins freely on the valve body and threads onto the male threads cast directly into the cast iron or stainless steel pump housing.

Critically, both 22mm and 28mm pump valves share the exact same 1½″ BSP union nut size. The physical dimensions of the pump volute connection do not change between domestic pipe sizes. The selection between 22mm and 28mm is determined entirely by system flow requirements and hydraulic design:

When to Specify 22mm Pump Valves

Domestic central heating circuits up to 18 kW output typically run on 22mm copper pipework. At typical domestic design temperature differentials (ΔT = 20°C on condensing boilers or ΔT = 11°C on older non-condensing systems), 22mm copper easily handles heating loads with flow velocities maintained under 1.0 m/s to prevent acoustic pipe noise. Underfloor heating blending manifolds also run almost universally on 22mm secondary pipe loops. In these setups, pair a standard 22mm pump valve on the boiler flow and another on the return.

When to Specify 28mm Pump Valves

Modern high-output systems (24 kW to 40+ kW), low-temperature air source heat pumps (ASHPs), and large unvented hot water cylinders require significantly greater volumetric flow rates. Because heat pumps operate at narrow temperature differentials (ΔT = 5°C), circulating water volume must roughly quadruple compared to a ΔT = 20°C boiler circuit to deliver equivalent heat:

Q = ṁ • C_p • ΔT   ⇒   ṁ = Q / (C_p • ΔT)

Restricting primary heating flow through a 22mm neck before a high-output circulator introduces excessive hydraulic friction, increases pump energy consumption, and causes premature cavitation. Always specify 28mm pump valves on primary boiler runs and heat pump interfaces to ensure smooth, unchoked hydronic performance.

Bridging Port-to-Port Spacing: 130mm vs 180mm Pump Adaptors

When replacing an older domestic heating circulator, the physical length of the pump body between the union sealing faces is the single most critical dimensional check. In the UK, heating circulators are manufactured in two standard port-to-port lengths:

• 180mm Face-to-Face: The traditional standard dimension found on virtually all standalone domestic heating pumps installed over the past 40 years (including older Grundfos UPS 15-50/60, Myson, and Wilo Gold models).
• 130mm Face-to-Face: The compact modern standard utilized on integrated boiler assemblies, compact heat interface units (HIUs), and underfloor heating manifold blending control packs (such as the Grundfos UPM3 Auto/Flex).

Heating engineers frequently encounter situations where a replacement pump is sourced with a 130mm body length, but the existing pipework and pump valves are locked at a rigid 180mm spacing. Re-plumbing copper pipework in a confined airing cupboard can take hours and risks disturbing existing joints.

The solution is to install a 1½″ Male x Female Long Pump Valve Adaptor (£4.41 ex VAT). This precision-machined brass extension features a 1½″ BSP female union thread on one end that connects directly to the pump valve body, and a 1½″ BSP male thread on the other that seats into the pump union nut. Adding the 50mm extension makes up the exact dimensional difference between 130mm and 180mm without cutting pipe, draining down, or using blowlamps.

Step-by-Step Trade Guide: How to Change a Circulating Pump Without Draining Down

Follow this trade-verified procedure to exchange a circulating pump swiftly while keeping the primary heating system fully charged:

Stage 1: Electrical Isolation and System Safety

Before touching hydronic components, isolate the heating system at the fused spur. Turn off both the boiler and programmer controls. Remove the 3A fuse or lock off the miniature circuit breaker (MCB). Use an approved two-pole voltage indicator to prove dead at the pump terminal box. Disconnect the electrical wiring (Live, Neutral, Earth, and any PWM speed control cables) noting cable polarities.

Stage 2: Isolate the Flow and Return Pump Valves

Close both pump isolation valves:

• For Ball Type Valves: Use a wide flathead screwdriver or adjustable spanner to rotate the control slot exactly 90 degrees until the slot lies perpendicular to the pipe centerline.
• For Gate Type Valves: Rotate the handwheel clockwise until firmly seated. Do not apply excessive cheater-bar torque to a stiff handwheel, as old brass gate spindles can shear inside the bonnet.

Stage 3: Controlled Depressurization and Union Disconnection

Position a shallow plastic catch tray and absorbent plumber's towels directly beneath the lower pump union. Using an adjustable spanner or water pump pliers (minimum 52mm jaw opening):

• Apply a holding spanner to the octagonal flats of the pump valve body.
• With a second spanner, carefully loosen the top 1½″ captive union nut by turning anti-clockwise. Allow trapped internal pressure and a small volume of water (typically 100 to 200 ml) to weep out.
• If water continues to flow under mains or head pressure without diminishing, one of the isolation valves is passing. Re-tighten the nut immediately. You must execute a partial drain down before continuing.
• If flow ceases, proceed to back off the lower union nut completely, then release the top union nut.

Stage 4: Pump Removal and Gasket Surface Prep

Carefully pull the old pump clear. Scrape away remnants of the degraded rubber or fiber union washers from both valve mating faces using a soft brass wire brush or utility blade. Never leave pitted, hardened gasket material on the sealing face, as surface imperfections guarantee persistent weeping under 2 bar operating pressure.

Stage 5: Installing the New Circulator and Fresh Gaskets

Insert fresh 1½″ pump valve washer seals (£0.18 ex VAT) into each union nut. Place the new pump into position, observing three vital physical orientation rules:

1. Flow Direction: Check the cast directional arrow on the pump volute body to ensure it matches the system flow direction (towards radiators/manifold).
2. Motor Shaft Orientation: The motor shaft must remain horizontal (parallel to the floor). Operating a wet-rotor pump with a vertical motor shaft traps air in the rotor can, resulting in dry bearing friction and rapid motor burnout.
3. Terminal Box Position: Position the electrical terminal enclosure on the upper side or lateral flank of the pump—never facing downwards where condensation or union leaks could enter the electronics.

Tighten the 1½″ union nuts hand-tight. Using two spanners, tighten each nut a further ¼ to ½ turn until the EPDM gasket firmly compresses. Avoid extreme overtightening, which distorts the rubber washer into the water path.

Stage 6: Valve Opening, Hydronic Venting & Commissioning

Slowly reopen the pump valves:

• Open the return valve first to allow system water to push air upwards into the pump head.
• Open the flow valve second to equalize hydraulic pressure across both sides.
• On traditional pumps, loosen the large center chrome bleed screw on the motor face until water emerges, then retighten. On modern automated pumps (e.g. Grundfos UPM3 or Wilo Yonos PARA), activate the automatic de-aeration routine via the control button.
• Inspect both union faces and adjacent compression nuts with a dry tissue. Reconnect electrics, set the required differential pressure or proportional curve, and recheck joints after 15 minutes of thermal cycling.

Recommended Trade Hardware for Pump Swaps & Retrofits

Fitting Pump Valves for the First Time and System Filtration Protection

If an older heating system was installed without pump isolation valves, or if seized gate valves refuse to budge, you must execute a drain down to below the pump level. Cut out the rigid pipe section with a rotary pipe cutter, deburr the copper cleanly, and install fresh 22mm or 28mm ball type valves. Follow strict compression best practices: hand-tighten the compression nuts plus ½ turn with a spanner while bracing the valve body with a second wrench.

Hydronic System Protection (BS 7593 Compliance): When installing or replacing a circulating pump, always inspect upstream filtration. Circulating pumps rely on system water for wet-rotor bearing lubrication. Circulating black iron oxide (magnetite) sludge rapidly accumulates on the powerful permanent magnetic rotors found in modern ECM pumps, destroying bearings and jamming impellers. To safeguard the pump warranty, fit an inline magnetic filter or high-capacity brass strainer on the heating return immediately before the pump. Consult our comprehensive Y-Strainer and Heating System Filter Guide for sizing and micron mesh standards.

Pump Isolation on Underfloor Heating Manifolds

Underfloor heating installations utilize dedicated mixing shunts where high-temperature boiler or heat pump flow is blended with cooler return water via a thermostatic mixing valve. In pre-assembled manifold pump packs, the pump unions screw directly into the blending body and manifold header.

Where an auxiliary pump is installed remotely to circulate primary water from the plantroom to a distant UFH manifold, it must always be flanked by 22mm ball type pump valves. MEP Stock stocks genuine Grundfos UPM3 130mm Auto & Flex pumps along with fully assembled commercial manifolds in our underfloor heating product range.

Frequently Asked Questions About Pump Valves and Circulators