Nest Thermostat F3 Fix (Heat Link Fault)

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What This Error Means

Nest Thermostat F3 means **the Heat Link has a fault on the heating control side** (the bit that actually switches your boiler on and off). In plain English: the thermostat is calling for heat, but the Heat Link output doesn’t look safe or normal, so Nest refuses to fire the boiler.

Official Fix

Here’s the textbook route, basically what Google/Nest wants you to do.

Safety first – you’re dealing with mains voltage (230V in most regions):

  • Turn off power to the boiler and Heat Link at the isolation switch / fused spur.
  • Also kill the breaker at the consumer unit if you’re unsure which switch does what.

1. Check system power

  • Make sure the boiler’s main power switch is ON (after you’ve finished wiring checks).
  • If there’s a fused spur, pull the fuse and visually check it.
  • If the fuse looks blown, replace it with the correct rating (usually 3A for boilers, never bigger “just because”).
  • Reset any tripped breaker feeding the heating circuit.

2. Inspect the Heat Link wiring (what the manual tells you)

  • With power still OFF, pop the Heat Link cover off.
  • Check the mains side first: L, N, and Earth firmly clamped, no loose copper, no scorch marks.
  • Check the boiler control side: Common and Heating On (usually terminals 2 and 3 on on/off systems) are:
    • Actually in the right terminals.
    • Fully inserted with no copper showing.
    • Tight when you tug gently on the wire.
  • If you have T1/T2 going to the wall thermostat, make sure those are tight too (no broken strands, no corrosion).

3. Put it back together and power up

  • Refit the Heat Link cover properly (don’t leave it hanging, some devices behave badly if the cover isn’t seated).
  • Turn the spur / switch back on, then restore the breaker.
  • Check the Heat Link LED:
    • Solid green: powered and generally happy.
    • Red / off / weird flashing: that’s a Heat Link problem – official advice is to contact a Nest Pro or Google support.

4. Run the Nest thermostat test

  • On the thermostat, go to Settings > Equipment > Continue > Test (or a similar menu depending on version).
  • Run the heating test and see if F3 returns.
  • If F3 comes back immediately after wiring and power checks, the manual answer is: call a Nest Pro / electrician because the Heat Link may be faulty.

The Technician’s Trick

Here’s what a working tech actually does to separate “bad wiring” from “dead Heat Link”. Only do this if you’re confident around mains. If not, stop and call a pro.

  • 1. Prove the boiler, not the Nest
    • Power OFF first.
    • Open the Heat Link and find the boiler call terminals (typically Common and Heating On).
    • Note where each wire goes or take a photo.
    • Turn power back ON.
    • Use a short link wire to bridge Live (L) to the heating call terminal (Heating On) for a few seconds.
    • If the boiler fires, the boiler and wiring are fine. The Heat Link relay or electronics are the problem.
    • If nothing happens, the fault is in the boiler or the cable going from Heat Link to boiler – not the Nest itself.
  • 2. Tap test on the Heat Link relay
    • Set the thermostat to call for heat so it should click on.
    • Lightly tap the side of the Heat Link with the handle of a screwdriver.
    • If heating suddenly kicks in or you hear a dodgy click, the relay inside is sticking – that’s a failing Heat Link.
  • 3. Quick swap decision
    • If the boiler works when you bridge L to Heating On, but F3 keeps coming back in normal use, a pro will almost always just replace the Heat Link / Nest pack.
    • No amount of resetting will fix a cooked relay or fried circuit board.

Is It Worth Fixing? (The Financial Verdict)

  • ✅ Fix: Nest is under warranty, boiler is otherwise healthy, and a simple wiring tidy or fuse swap clears the F3 or a single Heat Link replacement sorts it.
  • ⚠️ Debatable: System is 8–12+ years old, you’ve had multiple Nest/Heat Link error codes, and a paid callout plus parts is getting close to the cost of a new smart control.
  • ❌ Replace: Heat Link is clearly dead, out of warranty, boiler control wiring is a mess, and your boiler itself is near end-of-life – put the money toward a new boiler + control package.

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