E29 = high-limit lockout. Heat exchanger temperature exceeded safety threshold. Control shuts down to protect the heat exchanger from damage. Critical to diagnose before resetting.
Why this matters
Sustained overheating warps and cracks heat exchangers — $1,500-2,500 to replace. Don\'t repeatedly reset E29 hoping it clears. Diagnose first.
Diagnostic sequence
1. Low flow in the heating system
- System pressure low: verify 12-15 PSI; bleed air pockets that block flow
- Circulator pump failure: listen for pump hum; if silent, pump failed or seized
- Closed isolation valve on the boiler loop — sometimes accidentally closed during service
- Clogged strainer — many Lochinvar combis have inline strainer that captures debris and chokes flow
- Mis-sized circulator — undersized pump can\'t maintain flow through a long radiant loop
2. Heat exchanger scale (DHW side, combi)
- Combi units exposed to hard water develop scale on the DHW plate exchanger
- Scale reduces heat transfer; flame must run harder to deliver same DHW temp
- Eventually triggers high-limit during DHW calls
- Solution: descale (annual in hard water, every 2-4 yrs in soft)
3. Stuck thermostat or sensor fault
- Supply sensor reading low while exchanger actually overheating = control over-fires
- Test sensor per E12 diagnostics
4. Mixed up sensors
- If supply and return sensors swapped during service, control reads return temp as supply — over-fires
- Common post-service issue; verify sensor location after any service work
5. Bypass loop missing
- Cold-start protection (boiler bypass / primary-secondary piping) required on Lochinvar mod-cons
- Missing bypass on radiant systems with cold return temps = condensing inside heat exchanger + thermal shock
Reset
- Wait 30 minutes for heat exchanger to cool
- Address the root cause (flow, scale, sensor)
- Hold Reset 5 seconds
- Monitor — if E29 returns within 1 cycle, more diagnosis needed
Bottom line
E29 is the most damaging fault to ignore. Almost always low flow, scale, or sensor. Get a contractor in if the cause isn\'t obvious — repeated overheating destroys the heat exchanger.