Heating Pump Faults: Air Lock, Wrong Speed Setting, Cavitation, Flow Rate and Replacement Decision Guide

Quick Answer: Most central heating pump failures are caused by air locks, incorrect speed settings, or sludge-induced cavitation — not a failed pump motor. Diagnose by checking system pressure (minimum 1 bar), bleeding air via the bleed screw, and verifying speed selection against the system's design flow rate before condemning the pump. Heating pumps in the UK are governed by ErP Directive 2009/125/EC (implemented from 2013), which mandates A-rated circulator efficiency for replacements.

Summary

A failing heating pump announces itself with cold radiators, noisy operation, or a boiler that short-cycles — symptoms that overlap with a dozen other faults. Misdiagnosis is expensive: pumps are frequently replaced when the real cause is an air lock, a sludge-blocked strainer, or a speed setting left on maximum after a powerflush. Understanding the failure modes saves a return visit and protects your reputation.

Pump faults divide into three categories: hydraulic (air lock, cavitation, restricted flow), mechanical (seized impeller, bearing failure), and control (wrong speed setting, wiring fault). Each has a distinct signature and a distinct fix. The ASCII decision tree below is the fastest path through diagnosis on site.

The ErP Directive has made A-rated variable-speed pumps (Grundfos Alpha, Wilo Stratos, Salus equivalents) the standard replacement for fixed-speed units. These pumps adjust differential pressure automatically, eliminating the trial-and-error of manual speed selection. Understanding how to commission them — setting the correct constant pressure or proportional pressure mode — is now a core competency for heating engineers.

Key Facts

Quick Reference Table

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Symptom Most Likely Cause First Check Fix
No heat, pump motor warm, pump outlet cold Air lock Bleed screw on pump face Bleed air, check system pressure
Grinding/rattling noise Cavitation System pressure gauge Repressurise to 1.0–1.5 bar
All radiators cold, pump cold Pump not running Fused spur, wiring Check 230 V supply to pump
One zone cold, others warm Motorised valve, not pump Zone valve actuator Check valve actuator 230 V
Pump runs, radiators warm but noisy Speed too high Speed selector Reduce to Speed 1 or PP mode
Intermittent heating Seized impeller, starts with heat Manual restart needed Descale or replace pump
Pump leaks at union Corroded olive/seal Visual inspection Drain, replace olive or pump
Boiler short-cycles Insufficient flow rate, overpumping Delta-T across boiler Reduce speed or fit bypass

Detailed Guidance

ASCII Fault Diagnosis Decision Tree

START: Heating system not working correctly
│
├─ Is the pump running? (feel for vibration / listen for hum)
│   │
│   ├─ NO: Pump not running
│   │   ├─ Check 230 V at fused spur → No power: check programmer/wiring
│   │   ├─ Power OK → Check pump terminals L/N/E
│   │   ├─ Voltage OK at pump → Pump motor failed → REPLACE PUMP
│   │   └─ No voltage at pump → Check motorised valve end-switch, room stat
│   │
│   └─ YES: Pump running
│       │
│       ├─ Is pump body HOT on INLET side but COLD on OUTLET?
│       │   └─ YES → AIR LOCK
│       │       ├─ Bleed pump via bleed screw (¼ turn, cloth ready)
│       │       ├─ If air keeps returning → low pressure (check gauge)
│       │       ├─ Repressurise to 1.0–1.5 bar via filling loop
│       │       └─ If pressure OK and air persists → leak in system, find and fix
│       │
│       ├─ Is there a GRINDING or RATTLING noise?
│       │   └─ YES → CAVITATION or SEIZED IMPELLER
│       │       ├─ Check system pressure: <1.0 bar → repressurise
│       │       ├─ Pressure OK → check strainer for blockage (drain and inspect)
│       │       ├─ Strainer clear → speed too high for system → reduce speed
│       │       └─ Noise persists, pressure OK, speed correct → impeller worn → REPLACE
│       │
│       ├─ Is pump running but FLOW RATE LOW? (radiators warm but not hot)
│       │   ├─ Check speed setting: on Speed 1 when should be Speed 2/3?
│       │   ├─ Check magnetic filter / strainer: blocked with magnetite?
│       │   ├─ Check for partially closed isolating valves
│       │   ├─ Check lockshield valves haven't been over-closed
│       │   └─ If all clear → system undersized pump head → reassess pump spec
│       │
│       └─ Is pump NOISY (high-pitched whine, not grinding)?
│           ├─ Variable-speed pump in wrong mode → switch to PP mode
│           ├─ Fixed-speed pump on Speed 3 with TRVs closing → reduce to Speed 1
│           └─ Pump bypassing: fit or open automatic bypass valve

Air Lock: Causes and Clearing

Air enters the system at commissioning, after draining, or through micro-leaks at fittings. It accumulates at high points and at the pump volute. Symptoms: pump warm on the casing (motor turning), no or low flow through radiators, system pressure correct.

To clear: isolate pump, loosen bleed screw ¼ turn with a cloth under it, allow air to escape until water runs clear, re-tighten. Do not remove the screw under pressure — the thread pitch is coarse and the screw will eject. If air reappears within hours, the system is either under-pressured (below 1.0 bar cold fill) or has a suction leak somewhere on the pump inlet side. A persistent air lock that returns after bleeding is often a sign of a pinhole in a compression olive or a failed auto air vent stuck shut.

Cavitation: Recognition and Prevention

Cavitation occurs when local pressure at the pump impeller drops below the vapour pressure of water (3.2 kPa at 25°C, 19.9 kPa at 60°C — note: boiling point drops significantly at low pressure). In practice this means:

The noise is distinctive: a high-pitched grinding or crackling, like gravel in the pump. Left unchecked, cavitation erodes the impeller within months. Fix: repressurise system, remove flow restrictions, reduce pump speed.

Speed Selection for Fixed-Speed Pumps

Fixed-speed pumps (Grundfos UPS, older Myson, Salmson) have a 3-position selector. Most domestic systems run correctly on Speed 2. Speed 3 is rarely needed and causes noise with modern TRV-fitted systems.

Rule of thumb for speed selection: if the boiler delta-T (flow minus return temperature) is above 25°C, the flow rate is too low — increase speed. If delta-T is below 10°C and the pump is noisy, reduce speed or the system is oversized. Target delta-T for a conventional gas boiler system is 10–20°C; for heat pump-compatible low-temperature systems, 5–10°C.

A-Rated Variable-Speed Pump Commissioning

Grundfos Alpha2, Alpha3, Wilo Stratos Pico, and equivalent pumps ship with a default mode (typically proportional pressure, curve 2 of 3). For most UK domestic systems with TRVs:

  1. Set Proportional Pressure (PP) mode, Curve 2
  2. Start system, all TRVs open, run for 20 minutes
  3. Check all radiators reach temperature
  4. Reduce to Curve 1 if noise develops when TRVs close in warmer weather

For underfloor heating or systems with no TRVs: use Constant Pressure (CP) mode. The pump maintains a fixed differential pressure regardless of flow — prevents noise in low-flow systems.

The pump's display will show flow rate (l/min), power consumption (W), and active curve. Log these on handover documentation.

Pump Replacement Decision Guide

Replace the pump rather than attempting further diagnosis when:

  1. Motor failed — no rotation even after manual start attempt with a screwdriver in the bleed screw slot
  2. Shaft seal leaking — water tracking from around the pump body (not unions). Seal kits exist but cost versus new pump rarely justifies repair
  3. Impeller erosion confirmed — black metalite debris visible in drained system water, noise persists after all other checks
  4. Pump >10 years old with fixed-speed EEI >0.23 — ErP-compliant replacement will reduce pump energy consumption from ~80 W to ~5–15 W (saving £20–£50/year at 25p/kWh)

Do not replace based on noise alone without completing the diagnostic tree above. Noise is almost always a settings or pressure issue, not a failed pump.

Frequently Asked Questions

How do I know if my pump is the right size?

Calculate: flow rate (l/hr) = boiler output (kW) × 860 ÷ delta-T (°C). For a 24 kW boiler with 20°C delta-T: 24 × 860 ÷ 20 = 1,032 l/hr. Then calculate system resistance (head loss) through the longest circuit — roughly 100–150 Pa/m for 22 mm copper, more for 15 mm. A Grundfos 25-60 or Wilo Stratos 25/1-4 covers most domestic systems up to 30 kW. If in doubt, a variable-speed pump with auto mode will self-regulate.

The pump runs but radiators are stone cold — is it the pump or the motorised valve?

Test the motorised valve first: it's more likely. Feel the motorised valve body while calling for heat — if it stays cold, the actuator isn't opening. Remove the head from the valve body and check for 230 V at the actuator terminals. If voltage is present and the actuator doesn't move, replace the actuator (£15–30), not the pump. Pump faults cause insufficient heat, not total absence of heat (unless the pump fails to start entirely).

Can I use inhibitor with a new pump, and what type?

Yes — BS 7593:2019 requires inhibitor to be added to all systems at commissioning and after refilling. Use a glycol-free inhibitor (Fernox F1, Sentinel X100) at the correct concentration per the manufacturer's data sheet (typically 1% for standard systems). Check inhibitor concentration annually with a refractometer or test strips. Never mix brands — different chemistries can react and precipitate, blocking the pump strainer.

How long should a heating pump last?

A quality circulator (Grundfos, Wilo, DAB) in a clean system with inhibitor correctly dosed should last 15–20 years. In a sludged system without a magnetic filter, 3–5 years is realistic. Fitting a magnetic filter (Adey MagnaClean, Fernox TF1) upstream of the pump is the single biggest factor in pump longevity. The filter must be cleaned annually.

Is pump replacement notifiable under Building Regulations?

Replacing a like-for-like heating pump is not notifiable under Part P or the Building Regulations in England. However, if you are installing a new heating system or making a material change (e.g., adding a zone, changing from vented to unvented), the full installation becomes notifiable. Always check with your local building control if scope is unclear.

Regulations & Standards