As the colder months approach, boilers start showing their weak spots. Around Dun Laoghaire–Rathdown and greater Dublin, we at DDV Plumbing and Heating often get called out to the same recurring faults. Knowing what to look out for (or tell your engineer) can save time, stress and sometimes a hefty repair bill.
Here are the most frequent boiler problems we see, real fault codes on popular brands (e.g. Viessmann), and what you can do in the meantime.
1. Low Pressure / Loss of Pressure
What It Looks Like
- Radiators cold or only warm at the bottom
- Boiler display shows pressure below ~1 bar
- The boiler may lock out or refuse to fire
Why It Happens
- Small leaks in pipework or radiator joints
- Bleeding the radiators without repressurising
- Faulty filling loop or expansion vessel issues
In Dublin, this is one of the top issues in winter.
What You Can Try
- Repressurise using the filling loop (if your boiler allows it)
- Check for visible leaks
- Bleed radiators carefully
If it keeps losing pressure, call a plumber—there's likely something internal that needs attention.
2. Frozen or Blocked Condensate Pipes
What It Looks Like
- Boiler shows a "lockout" or fault code
- You may see no visible heating or hot water
Condensing boilers eject acidic condensate via a pipe. If it becomes frozen or clogged (especially in very cold spells), the boiler may shut down for safety.
Quick Fix
- Pour lukewarm (not boiling) water over the exposed section of the condensate pipe to thaw it
- Clear any blockages
If that doesn't work—or the pipe is buried—call a gas-engineer to sort it properly.
3. Strange Noises / Kettling
What It Sounds Like
- Gurgling, rumbling or 'kettle-like' hiss
- Occasional banging or rattling
Root Causes
- Scale or sludge build-up in the heat exchanger
- Air trapped in the system
- Faulty pump
This is often called "kettling" in boiler talk.
What to Do
- Bleed radiators (air in the system is often the culprit)
- If noise persists, a power-flush may be needed to cleanse the system
4. Fault Codes on Viessmann Boilers (and Others)
If your boiler is a Viessmann, here are some common error/fault codes you might see and what they often mean. (Always refer to your manual — codes can vary by model.)
- 5C / 5D / 5E / 5F — Communication fault between Vitotronic units. Try resetting; check wiring or contact your installer.
- 10 — Outdoor-temperature sensor shorted / reading 0 °C. Inspect sensor wiring or replace sensor.
- D4 — Emergency mode, cooling down, internal bus error. Usually requires professional diagnosis.
- F6 / F9 / 58 / F-E1 / F-E2 — Burner fault / fan not at rest. Reset, but often needs engineer intervention.
- A4 — Burner blocked / pump airlock / flow not maintained. Look at internal circulation, commissioning settings.
If your boiler isn't a Viessmann, similar error codes or symbols (e.g. "E", "F", "D") may still point to sensor, pump, flame or communication faults. Always check the manual or the manufacturer's website.
5. No Hot Water But Heating Works (or vice versa)
What Happens
- Radiators heat, but taps run cold
- Or hot water but no heat
Possible Causes
- Diverter valve failure
- Issues with zone valves or motorised valves
- Faulty thermostat or wiring
This is a fairly common issue in combi or system boilers.
6. Boiler Turning Off Repeatedly / Lockouts
What You'll See
- Boiler fires, then shuts down mid-cycle
- May display a lockout or fault code
- Radiators get cold again
What Causes It
- Low or high pressure
- Blocked flue or air supply
- Faulty thermostat or diverter valves
- Internal safety devices tripping
This is another frequent call-out fault in Dublin.
7. Leaks / Water Seepage
Where to Look
- Around boiler base
- Pipe joints, seals, pump housing
- Pressure relief valve
Why It's Serious
Even small leaks can lead to bigger corrosion or electrical damage down the line.
When You Should Call Us
If you see any of these signs, here's when to call DDV Plumbing and Heating:
- Recurring fault codes that you can't clear
- Leaks or corrosion
- Boiler refuses to fire
- You smell gas or suspect carbon monoxide
- You're unsure or unsafe to proceed
We're RGI / Gas-qualified and cover Dun Laoghaire–Rathdown and greater Dublin.
Final Tips
- Keep an eye on your boiler's pressure (aim for around 1–1.5 bar)
- Bleed radiators at least once per season
- Thaw condensate pipes before they lock out your system
- Get annual servicing before winter hits