Overview
S.34 on a Vaillant ecoTEC Plus regular boiler is a status code indicating the boiler’s anti‑frost (frost protection) mode is active. Anti‑frost mode is not a fault in itself: when the boiler detects that the heating flow temperature has dropped very low (typically below about 5 °C) or an external frost stat signals risk of freezing, it will run the pump and periodically fire the burner to raise system water temperature to a safe level (often around 30 °C) to stop pipes and the boiler from freezing. Severity is generally low if the code appears during very cold weather and the boiler is actually keeping the system above freezing. It becomes important to act if the boiler shows S.34 but radiators remain cold, water pressure is very low, or the code stays on when outside/room temperatures are well above freezing — that can indicate a problem with sensors, controls or wiring. Basic checks and simple actions can be done by a competent homeowner, but any work involving gas, electrics, internal sensors or replacing parts should be carried out by a Gas Safe registered engineer.
Possible Cause: Anti-frost mode is active
Troubleshooting Steps
Safety precautions:
1. If you smell gas or suspect a gas leak, do not touch the boiler. Turn off the gas supply at the meter or isolator if safe to do, ventilate the building, evacuate, and call the gas emergency number immediately.
2. If you have any concerns about carbon monoxide (odd smells, headaches, soot, pilot lights behaving oddly), turn the boiler off at the mains, ventilate, leave the property and call a Gas Safe engineer or emergency services. Ensure a working CO alarm is present.
3. Do not attempt internal repairs, sensor or PCB replacements, or anything that requires removing the appliance casing unless you are a registered engineer.
Initial checks a homeowner can do:
1. Look at the display: confirm it shows S.34 (anti‑frost) rather than an F‑type fault. Note whether heating or hot water is selected and whether the boiler appears to be running occasionally (pump noise or brief burner firing).
2. Check the weather/room temperature: if it is below freezing or very cold, anti‑frost mode may be expected behaviour.
3. Confirm the boiler has power and the main switch is on. Check the fuse in your consumer unit if the boiler is unresponsive.
4. Check the gas supply: ensure the gas isolator cock is fully open at the appliance and any external stop cock is open.
5. Check the boiler pressure gauge: system pressure should normally read around 1.0–1.5 bar cold. If pressure is very low (below ~0.7–0.8 bar) the boiler may not heat the system properly.
6. Check that heating flow and return service valves (if present) are open and that any room thermostat or external programmer is not set to permanently off or to a very low temperature.
Diagnostic and simple fix steps:
1. If radiators are cold but the boiler is showing S.34, feel the flow and return pipes at the boiler (carefully — they may be hot). If the flow pipe is warm and the return is cooler, the boiler’s anti‑frost function is working. If both are cold and the boiler is showing S.34 but not operating, proceed with the next checks.
2. Check system pressure. If pressure is low, top up the system using the filling loop following your boiler manual instructions. Only do this if you know how; if not confident, call an engineer. After topping up, bleed any cold radiators at the highest points to remove air, then recheck pressure.
3. If the boiler is in Summer mode (heating off), anti‑frost protection can still trigger. Check the boiler/programmer settings: ensure heating flow temperature is not set to "Heating off" if you expect normal heating to run. If you change settings, save and monitor the display.
4. Try a single reset of the boiler using the reset button as described in the manual. Hold for the recommended time (usually a few seconds). Only reset once; repeated resets can mask a recurring fault and may cause further issues.
5. Check for a frozen condensate pipe if temperatures are below freezing (common on boilers). If you suspect a frozen condensate discharge outside, you can carefully thaw it with warm (not boiling) water or wrap with warm cloths. Do not force or use naked flame. If thawing clears the issue, monitor the boiler; persistent freezing of the condensate trap means you need an engineer to fit an insulation solution or reroute the discharge.
6. If S.34 remains displayed when temperatures are above freezing and the boiler will not return to normal heating operation, or if the boiler cycles abnormally, this may indicate a faulty flow NTC (temperature sensor), wiring fault, or PCB/controller issue.
When to call a professional:
1. Call a Gas Safe registered engineer if: the boiler will not heat the system despite checks; system pressure repeatedly falls or cannot be maintained; the S.34 code persists at moderate temperatures; you suspect a faulty sensor, wiring or PCB; or you detect any gas smell or CO concerns.
2. Do not attempt to open the boiler casing, replace sensors, or work on gas or electrical components yourself. Only a qualified engineer should carry out diagnostic tests that require continuity testers, resistance checks on NTC probes, or PCB replacement.
Final note: S.34 is usually protective and helpful in cold weather, but if the status remains on when it should not, or if the heating is not working correctly, arrange a Gas Safe engineer visit to diagnose sensors, wiring and controls. If you are ever unsure or worried about safety, stop using the appliance and call a professional.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Vaillant EcoTEC Plus Regular Gas Boiler.