Central Heating Pumps — Grundfos UPS, Wilo Paco, and Pump Replacement for UK Plumbers

The circulation pump is the heart of every wet central heating system. When it fails, radiators stay cold, hot water cylinders lose heat-up performance, and combi boilers error-out. Replacing a central heating pump is one of the most common plumber call-outs — and with the right stock, it takes under an hour.

This guide covers the two dominant brands in UK replacement pump supply — Grundfos UPS and Wilo Paco/Yonos — plus pump sizing, energy efficiency ratings, and step-by-step replacement for 15mm, 22mm, and 28mm systems.

How a Central Heating Pump Works

The pump circulates heated water from the boiler through the primary circuit — passing through radiators, zone valves, and (in S-plan systems) the hot water coil of the cylinder before returning to the boiler. Without circulation, heat cannot transfer and the boiler will rapidly exceed temperature limits and lock out.

Modern pumps are wet-rotor centrifugal types: the rotor spins in the water itself, requiring no external seals or lubrication. The impeller creates a pressure differential that drives flow around the circuit.

Key pump parameters

• Head (metres) — the pressure the pump can develop, measured in metres of water column. A typical domestic system needs 2–4m head.
• Flow rate (l/min or m³/h) — volume pumped per unit time. A 15kW domestic boiler circuit needs roughly 20–25 l/min.
• Speed settings — traditional pumps offer 3 fixed speeds; modern ECM (electronically commutated motor) pumps modulate continuously.
• Connection centres — the distance between inlet and outlet flanges. UK standard is 130mm for 15mm/22mm domestic pumps.

Grundfos UPS Series — the UK Standard

Grundfos UPS pumps have been the default replacement choice in UK domestic heating for decades. The UPS2 15-50/60 range covers the vast majority of domestic boiler systems.

UPS2 15-50 130

• Connection: 15mm/22mm with adaptors (130mm centres)
• Max head: 5.0m, max flow: 45 l/min
• 3-speed: typically set to speed 2 for most domestic systems
• Power: 45–75W
• EEI: 0.25 (pre-ErP — still widely stocked for direct replacement)
• Suitable for: combi boiler circuits, S-plan, Y-plan

UPS2 15-60 130

• Higher head (6.0m) — use where circuit has high resistance (underfloor heating manifolds, long pipe runs)
• Otherwise identical form factor to 15-50 for plug-in replacement

Grundfos MAGNA3 — ErP Compliant

The MAGNA3 series replaced the UPS3 as the ErP-compliant ECM pump range. Auto-adapt mode automatically adjusts speed to match system demand. EEI ≤ 0.20. More expensive upfront but significantly lower running costs — relevant where energy-conscious customers ask for upgrades.

Wilo Paco / Yonos Series

Wilo is the other major brand found in UK properties, often fitted as OEM in Vaillant, Worcester Bosch, and Ideal boilers. Wilo pumps have the same 130mm centre-to-centre standard.

Wilo Paco 25/1-5

• Direct equivalent to Grundfos UPS2 15-50
• 3-speed, 130mm centres, 15–22mm connections
• Often cheaper than equivalent Grundfos — competitive choice for like-for-like replacement

Wilo Yonos PICO

• ECM variable-speed pump — ErP compliant (EEI ≤ 0.20)
• Dynamic pressure control automatically adapts to system demand
• Available in 25/0.5–6 sizes to cover domestic through light commercial

Pump Sizing — Getting it Right

Over-sizing is the most common mistake. An oversized pump creates excess velocity noise, increases pressure differentials across TRVs, and wastes energy. Under-sizing leaves radiators cold on the highest demand day.

Rule of thumb for domestic systems

• Up to 15kW boiler: UPS2 15-50 on speed 2 — covers most 3-bed properties
• 15–25kW boiler: UPS2 15-60 or equivalent, speed 2–3
• 25kW+ or underfloor heating circuits: check manufacturer's pump curve against system resistance curve; MAGNA3 25-100 or equivalent

Estimating system resistance

Index circuit resistance = pipe friction loss × equivalent length. For a typical 15mm microbore domestic system with 10 radiators, total equivalent pipe length is roughly 60–80m. At 0.5m head per 10m run, that's 3–4m total head — well within UPS2 15-50 capability.

ErP Regulation (since 2015)

Since 1 August 2015, fixed-speed glandless circulators with rated hydraulic power ≤ 2.5W are no longer permitted to be placed on the market. All new domestic pumps must meet EEI ≤ 0.23 (or ≤ 0.20 from 2020). UPS2 series pre-ErP pumps are valid for replacement of like-for-like; new installations require ErP-compliant ECM pumps.

Diagnosing a Failed Pump

Common failure modes

Quick diagnostic steps

1. Check pump indicator light/LED — if on, pump has power
2. Touch pump body — should be warm to touch if circulating
3. Listen for hum — hum + no flow = seized impeller
4. Use flat-blade screwdriver in the bleed/spindle slot on pump head — manually rotate to free a seized impeller (if accessible)
5. Check isolation valves on each side are fully open
6. Bleed air via pump bleed screw (small brass screw on pump head — open half a turn, close when water runs)

Step-by-Step Pump Replacement

Tools and materials

• Replacement pump (UPS2 15-50 or equivalent)
• Pump adaptors (15mm or 22mm with fibre/rubber washers)
• Adjustable spanner × 2
• PTFE tape
• Small container or towels (residual water)
• Magnetic system filter check (see Magnetic System Filters — MagnaClean, Fernox TF1)

Procedure

1. Isolate electrical supply — switch off at junction box or FCU. Confirm dead with tester.
2. Close isolation valves on both pump unions. If there are no isolation valves (older systems), drain the primary circuit via the lowest drain cock.
3. Note pump orientation — photograph the existing pump showing arrow direction, speed setting, and wiring colours before removal.
4. Undo pump unions — hold the valve body with one spanner, turn the union nut with the second. Work slowly; residual water will drain when unions release.
5. Remove old pump — check pump flanges and union seats for corrosion or debris.
6. Fit new pump — install new fibre washers in unions (supplied with pump or purchased separately). Do not reuse old washers.
7. Set pump orientation — pump motor head can usually rotate 90° in 4 positions. Horizontal pipework: motor horizontal. Vertical pipework: motor can face any direction except downward (lubricates bearings).
8. Set speed — start on speed 2 for most domestic systems. Adjust after commissioning if noise or cold radiators persist.
9. Open isolation valves — slowly, checking for leaks at unions.
10. Bleed pump — open bleed screw until water appears, then close.
11. Reconnect electrical supply — L to pump live terminal, N to neutral, E to earth. Check pump runs and system circulates (radiators warm from flow).
12. System check — verify boiler pressure holds (see Boiler Pressure Problems), all radiators heat evenly, no noise from circuit.

Wiring — single-phase 230V pump

Central heating pumps are single-phase 230V AC, wired via a 3A fused spur or switched junction box, usually controlled by the programmer/timer via the boiler relay or motorised valve end-switch. Ensure the pump is not live before disconnecting — use a non-contact tester on all terminals.

Pump Position in the Circuit

Pump position relative to the feed-and-expansion cistern (in open-vented systems) or the expansion vessel (sealed systems) affects system balance and noise.

• Pump-over-boiler (open-vented): pump should push water away from the vent pipe connection to avoid pumping over — position pump on the flow (boiler outlet) side of the vent tee
• Sealed system (combi or unvented): pump position is fixed within the boiler — external pumps on S-plan systems should be on the boiler flow side
• S-plan systems: pump receives a live call from the motorised zone valves' end-switch when either zone opens (see Motorised Zone Valves)

Inhibitor and Filter — Protect the New Pump

A new pump fitted into dirty system water will fail within 2–3 years. Magnetite (black sludge) and limescale are the two main killers.

• Check the system filter before fitting the new pump — clean or replace the magnetic filter cartridge (see Magnetic System Filters)
• Dose with Fernox F1 inhibitor after pump replacement — 500ml covers up to 100 litres system volume (typical 10-rad domestic system)
• In hard water areas (above 200ppm TDS), dose with a scale inhibitor such as Fernox Protector F1 or add a polyphosphate dosing pot on the cold fill
• Log the inhibitor dose and date on the system logbook — required evidence for warranty claims on new pumps

Central Heating Pump — Related Controls and Pipework

The pump works within a system that includes controls, valves, and pipework — all of which affect its performance:

• Central Heating Controls — Programmers, Thermostats, and Smart Controls
• Motorised Zone Valves — 2-Port and 3-Port Valves
• Magnetic System Filters — MagnaClean, Fernox TF1
• Boiler Pressure Problems — Low Pressure, Leaks, and PRV Repair
• Copper Pipe and Compression Fittings — End Feed, Solder Ring, and Pushfit
• Hot Water Cylinders — Unvented, Vented, and Immersion Heaters

Product Cards

APM Electricals stocks central heating inhibitor, heating system accessories, and filtration products for UK trade plumbers. Order online for next-day delivery or collect in Birmingham.