Types of Central Heating Circulator

Wet Rotor Circulators

The vast majority of domestic central heating pumps are wet rotor circulators. In this design, the motor's rotor and bearings are immersed in the system water — the water itself acts as coolant and lubricant. This eliminates the need for shaft seals, making wet rotor pumps virtually maintenance-free and very quiet. They are the standard choice for domestic radiator circuits, underfloor heating circuits, and hot water cylinder primaries.

Fixed-Speed vs Variable-Speed (ECM) Pumps

Fixed-speed pumps (3-speed)

Traditional circulators offer three selectable speed settings, each delivering a fixed head and flow rate. The installer sets the speed at commissioning and leaves it. Speed 1 is typically used for small systems or mild weather; speed 2 or 3 for full heating demand. These pumps run at full energy use regardless of system demand.

Variable-speed (ECM) pumps

Modern electronically commutated motor (ECM) pumps modulate speed in response to system pressure demand. They use up to 80% less electrical energy than fixed-speed equivalents. The EU ErP Directive 2009/125/EC prohibited sale of non-variable-speed circulator pumps (≤2,500W input) for residential heating after January 2013. Any replacement pump purchased since 2013 is legally required to be an energy-class A rated ECM pump.

Inline vs Flanged Circulators

Domestic circulators are typically inline — installed directly in the pipework with the pump body centred in the pipe run. Connection is via BSP-threaded unions (1¼" BSP female unions for the standard 1" bore pump head) or compression fittings. Flanged circulators are used in commercial and high-flow applications where PN16 flange connections are used. Domestic replacement almost always uses union connections.

Boiler Integral Pumps

Most modern combination and system boilers incorporate the pump within the boiler casing. These boiler-integral pumps are typically Grundfos UPM or Wilo Para units customised for the boiler manufacturer. Replacement requires removing the boiler casing and accessing the pump within. Procedure varies by boiler brand — always consult the boiler manual and consider whether pump replacement or full boiler service is more cost-effective.

Major Brands — Grundfos, Wilo, Salmson, DAB

Grundfos

Grundfos (Denmark) is the world's largest pump manufacturer and the historic default for UK central heating. The UPS range (fixed 3-speed) was the dominant domestic pump for decades. The modern replacement is the ALPHA range:

• Grundfos ALPHA2: variable-speed A-rated pump, 180mm pump/union spacing. Available in 15-50/60, 25-40/60 configurations. Autoadapt mode adjusts speed to system demand automatically.
• Grundfos ALPHA3: latest generation with Bluetooth connectivity, flow measurement, and diagnostic LED. Replaces ALPHA2 in current production.
• Grundfos MAGNA: commercial circulator, higher flow and head ratings, flanged options.

Wilo

Wilo (Germany) is a direct Grundfos competitor with a strong UK market presence:

• Wilo Para: common OEM pump inside Worcester Bosch, Vaillant, and Ideal boilers. Available as replacement part.
• Wilo Yonos PICO: variable-speed domestic circulator, very compact body, 130mm centre distance option (shorter than standard 180mm Grundfos). Popular for tight boiler/airing cupboard spaces.
• Wilo Stratos PICO: premium variable-speed pump with digital pressure setpoint display. Suitable for underfloor heating manifold pumps.

Salmson (Nuaire/Xylem)

Salmson pumps (part of the Xylem group) are popular in the social housing and commercial sector. The 2SAH and SAL ranges are direct replacements for older fixed-speed pumps. The Salmson NSS2 ECONOM is an ECM A-rated replacement with three control modes.

DAB

DAB Pumps (Italy) produce the Evoplus Small range — compact ECM circulators with electronic setpoint control. Growing market share in UK plumbing trade due to competitive pricing.

Like-for-Like vs Upgrade

When replacing a failed pump, always assess whether a like-for-like replacement is appropriate or whether an upgrade to a higher-efficiency ECM pump is warranted. An ECM pump replacement on a domestic system typically pays back in energy savings within 2–3 years and may be required under the ErP rules regardless. If the existing pump is a pre-2013 fixed-speed unit, a variable-speed replacement is the only legal option.

Pump Sizing — Head and Flow Rate

Key Parameters

A circulator pump is sized on two parameters:

• Flow rate (Q): the volume of water moved per hour (l/h or m³/h). Determined by the heat output required and the temperature differential across the system.
• Head (H): the pressure the pump must overcome to circulate water through the circuit. Expressed in metres of water column (mWC) or kPa. Determined by the pipe index circuit resistance.

Flow Rate Calculation

The required flow rate is derived from the heating load:

Q (l/h) = P (kW) × 860 / ΔT (°C)

Where P = total heat output of the system in kW, ΔT = design temperature differential (typically 10–20°C for radiator circuits, 5–10°C for UFH circuits).

Example: A system with 12 kW total heat output and 20°C differential:
Q = 12 × 860 / 20 = 516 l/h ≈ 0.52 m³/h

Head Loss Calculation

Head loss calculation requires full pipe and fitting resistance data for the index circuit (the circuit with the greatest resistance). For a typical domestic system using 22mm and 15mm copper pipe:

• Approximate specific resistance: 100–250 Pa/m (depending on velocity and pipe diameter)
• Total index circuit length: sum of all pipe run to the furthest radiator and back
• Fittings allowance: typically add 30–50% to pipe run resistance for elbows, tees, valves

For most domestic properties (3–4 bedroom, standard radiator circuit), a pump rated at 3–4m head at 600–900 l/h flow is typically adequate. Variable-speed pumps reduce sizing criticality by modulating to match actual system conditions.

Standard Pump Dimensions

Always measure the existing pump's centre-to-centre union distance before ordering a replacement. 180mm is by far the most common.

Diagnosing Pump Failure

Symptoms of Pump Problems

• Cold radiators throughout: circulation has stopped. Check pump is running — listen for hum or feel for vibration.
• Hot boiler, cold radiators: pump running but not circulating. Possible seized impeller, air lock, or closed isolating valve.
• Noisy pump (rumbling, grinding): bearing wear, cavitation, or entrained air in system. Bleeding the pump may help; persistent noise indicates bearing replacement or full pump swap.
• Pump running but overheating: check pump for limescale buildup on impeller, low system pressure (check pressure gauge, repressurise to 1–1.5 bar cold), or system airlocking.
• No electrical supply to pump: check fuse in pump wiring centre, check wiring centre relay/motorised valve position, check boiler controls — pump may not energise if zone valves are not open.

Testing the Pump

1. With system at normal pressure, set boiler to call for heat.
2. Listen at the pump body — a functional pump produces a low hum or gentle rush of water.
3. Use a contactless temperature probe along the flow and return pipework — a functioning pump creates a measurable temperature differential.
4. If no sound: check voltage at pump terminals (should be 230V AC when system calling for heat).
5. If voltage present but pump silent: pump motor has failed. Replace pump.
6. If no voltage: trace back through wiring centre, motorised valves, and boiler controls.

Seized Impeller — Manual Restart

Pumps that have been inactive (e.g., summer shutdown) sometimes have impellers seized by lime-scale deposits. Many Grundfos and Wilo pumps have an access screw on the front face that allows manual impeller rotation with a flat-blade screwdriver. With pump isolated from the electrical supply, insert the screwdriver into the slot and rotate the impeller 1–2 turns to break the deposit. Re-energise — the pump may restart. This is a temporary fix; if the pump seizes repeatedly, replace it.

Step-by-Step Pump Replacement

Materials and Tools Required

• Replacement circulator pump (correct centre distance, head/flow rating)
• PTFE tape or thread sealant (Fernox LS-X or similar)
• Pump isolation valves (if not already fitted)
• 22mm compression or push-fit fittings for making good
• Towels and small bucket for water spillage
• 22mm and 15mm pipe cutters
• Adjustable spanner and slip-joint pliers
• Multimeter for electrical verification

Isolation Valves

Modern pumps should have pump isolation valves — a pair of 22mm or 28mm isolating valves immediately upstream and downstream of the pump body — allowing pump removal without draining the full system. If the existing pump does not have isolation valves, install a pair (Taurus or equivalent) as part of the pump replacement. This makes future servicing far simpler.

Procedure

1. Isolate the electrical supply to the central heating system at the programmer/wiring centre and at the consumer unit (LOTO). Verify isolation with a multimeter at the pump terminals.
2. Allow the system to cool. Attempting to work on a hot system risks scalding from hot water and steam. Allow at least 30 minutes after last heat demand.
3. Close the pump isolation valves using a flat-blade screwdriver in the valve slot. If no isolation valves are fitted, drain the system to below pump level (attach a hose to the drain cock at the lowest radiator or system low point).
4. Place a towel and bucket beneath the pump to catch residual water.
5. Disconnect the electrical connections at the pump terminal box. Note the wiring arrangement (Live, Neutral, Earth) or photograph before disconnecting.
6. Unscrew the pump union nuts (typically 1¼" BSP) using a large spanner. Expect some water to drain — this is normal residual water from between the isolation valves.
7. Remove the old pump. Note the pump orientation (flow arrow on the pump body must align with system flow direction).
8. Install the new pump: apply PTFE tape (4–5 wraps) to the union tails or use thread sealant. Hand-tighten then torque the union nuts firmly but do not overtighten — most union connections are brass-to-brass and do not require excessive force.
9. Orient the motor head: most pumps allow the motor head to rotate in 90° increments. Position it for convenient access (terminal box upward is conventional).
10. Reconnect the electrical supply. Most pumps have a live, neutral, and earth; some ECM pumps have additional signal wires (for OpenTherm or external speed control — check the pump manual).
11. Open the isolation valves and repressurise the system if required (sealed systems: 1.0–1.5 bar cold). Bleed the pump using the bleed screw on the front of the motor head if fitted.
12. Restore electrical supply and call for heat. Verify pump operation, check for water leaks at unions, and verify circulation by monitoring radiator temperatures.

Horizontal vs Vertical Installation

Most wet rotor circulators can be installed horizontally (pump shaft horizontal) or vertically (pump shaft vertical, flow upward). Check the pump manual — some models have restrictions. Grundfos ALPHA and Wilo Yonos can be installed in any orientation. Installing a pump with the shaft vertical and motor pointing downward should generally be avoided as it can trap air in the motor cavity.

Commissioning and System Balance

Setting the Variable-Speed Pump

ECM variable-speed pumps offer several operating modes:

• Autoadapt (Grundfos): the pump automatically learns the system over several heating cycles and adjusts head setpoint. Recommended for most domestic systems.
• Proportional pressure: head varies proportionally with flow — increases as demand increases. Good for radiator systems with TRVs.
• Constant pressure: maintains a fixed pressure setpoint regardless of flow. Better for systems without TRVs or for underfloor heating distribution pumps.
• Constant speed (manual): sets the pump to a fixed speed equivalent to traditional speed 1/2/3. Not recommended for ECM pumps as it bypasses energy savings.

System Inhibitor Top-Up

After pump replacement, top up the system inhibitor concentration. Pump replacement involves some water loss and dilution. Use a Fernox F1 or Sentinel X100 inhibitor at the manufacturer's recommended dose (typically 1 litre per 100 litres system volume). Test concentration with an inhibitor test strip — correct concentration protects the new pump impeller and heat exchanger from corrosion.

Balancing Radiators

After replacing the pump, particularly if upgrading from fixed-speed to variable-speed, re-balance the radiator circuit. Open all lockshield valves to full open. Run the system at full heat demand. Identify radiators that are significantly hotter or colder than others. Partially close lockshield valves on radiators closest to the pump (high flow) to redirect flow to radiators further along the circuit. See Lockshield Valves and System Balancing for full technique.

Checking System Pressure

Sealed systems should be pressurised to 1.0–1.5 bar cold (system at ambient temperature). Check the pressure gauge after filling — the system may require topping up via the filling loop if pressure dropped during pump removal. Verify the expansion vessel pre-charge pressure matches the cold fill pressure (see Expansion Vessels and System Pressure). If the expansion vessel has lost pre-charge, re-pressurising it at this stage is good practice.

FAQs

How do I know what size pump to buy?

For a like-for-like replacement, use the model number on the existing pump's data label. The model number encodes the pump parameters — for example, UPS 15-60 means 15mm nominal port size, 6m maximum head. A 180mm centre-to-centre distance is the standard domestic size. For a new installation, calculate flow rate from system heat output and temperature differential, and head from index circuit resistance — or use a pump selection tool from Grundfos, Wilo, or DAB websites.

My pump hums but nothing circulates — what's wrong?

A humming pump that isn't circulating usually indicates a seized impeller. The motor is running but the impeller isn't turning. First, try manually rotating the impeller via the access screw on the pump face (with power isolated). If this frees it, run the system with corrosion inhibitor — scale may re-seize it. If the impeller is badly scaled or repeatedly seizes, replace the pump and power-flush or chemically clean the system to remove accumulated sludge and scale.

Can I replace a 3-speed fixed pump with an ECM variable-speed pump?

Yes — this is the standard replacement path and is required by law (ErP Directive) in most cases. ECM pumps are a direct drop-in replacement: same electrical connections (L, N, E), same union connections (with adapter if needed), same pipe size. Set the ECM pump to autoadapt or proportional pressure mode. Energy savings are immediate — typically 30–80W continuous saving compared to a fixed-speed pump on speed 2.

How often should a central heating pump be replaced?

Wet rotor circulators typically last 10–20 years in a clean, properly inhibited system. Annual inhibitor checks and periodic power-flushing extend pump life. A pump that is becoming noisy, drawing high current, or showing intermittent operation should be replaced proactively rather than waiting for complete failure — a failed pump in winter can cause a boiler lockout and leave a property without heating.

Does pump orientation matter?

Most modern wet rotor pumps (Grundfos ALPHA, Wilo Yonos) can be installed in any orientation — horizontal shaft, vertical shaft up, or vertical shaft at an angle. Avoid mounting the pump with the motor directly below the pump head (motor pointing down) as this can trap air in the motor cavity and cause noise. Always check the installation manual for the specific model.

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