In Uniclima Solutions we carry out over 1,200 boiler maintenance services each season in Madrid and Corredor del Henares. The reality is uncomfortable: 8 out of 10 winter faults that we attend to during emergency hours could have been avoided with a check-up done in September or October. This article summarizes, updated for 2026, the 5 key points that make the difference.
These are not generic listings taken from a manual: they are the actual faults we encounter in the field, with ranges, frequencies, and how to detect them before they leave you without domestic hot water on a Sunday.
Why does professional maintenance make a difference?
LegislationRITE IT3) requires a technical inspection every 2 years for boilers >70 kW and an annual recommended inspection for domestic ones. Beyond the obligation:
- SavingsA boiler with a scaled heat exchanger loses between 8% and 15% efficiency.
- LifespanA well-maintained boiler lasts 18-22 years; without maintenance, it rarely exceeds 10.
- SafetyCO in flue gases, gas leaks, uncalibrated safety sensors — all are silent faults that an inspection detects.
- WarrantyExtended warranties (Vaillant, Viessmann, Bosch) require documented official maintenance.
1. Check the circuit pressure
The pressure of the primary circuit should be between 1.2 and 1.8 bar cold (the boiler turned off and lukewarm). You can see it on the front pressure gauge or, in modern models, in the app.
How to safely refill a modern boiler:
- Boiler off and cold.
- Open the filling valve under the boiler very slowly.
- Observe the pressure gauge: stop at 1.5 bar.
- Close the valve completely (this is the point where we see the most faults: a half-open valve saturates the expansion vessel).
2. Bleed the radiators (and the boiler)
If a radiator stays cold at the top and hot at the bottom, there is trapped air. This air prevents the hot water from circulating and also oxidizes the inside of the radiator.
Correct order of steps:
- Turn off the boiler and wait 30 minutes.
- Start with the radiator furthest from the boiler (usually the furthest in the house).
- Open the bleed valve with the key or a flat screwdriver until stop the whistling and continuous water flows out.
- Close and move to the next one, going up by floors.
- Check the pressure at the end: it has probably dropped. Refill to 1.5 bar.
Modern boilers usually have their own automatic air vent at the top — check that it is not blocked by limescale (if so, it needs to be replaced during the inspection).
3. Cleaning of the heat exchanger and filters
The most important point, and the one that only a professional can do. Over the years it accumulates:
- Limescale / lime in the DHW heat exchanger (reduces the hot water flow).
- Sludge in the primary circuit (the typical black sludge from rust — reduces efficiency and clogs the 3-way valve).
- Soot in the burner (boilers with combustion problems).
Cleaning is done with specific productsSentinel X400 + X100 / Fernox F3 + F1) and it is advisable to complete it with the installation of a magnetic filter (Fernox TF1, MagnaClean) on the return: traps ferrous sludge and protects the pump and heat exchanger for life.
4. Burner inspection and flue gas analysis
The combustion analysis With an approved analyser, it is the only objective way to know if your boiler is burning well. We measure:
- CO₂ (8-10% in condensation)
- CO (target < 50 ppm — high CO is a symptom of a dirty burner or obstructed air intake)
- Flue gas temperature (50-65 °C in condensation; if it exceeds 80 °C, it is not condensing)
- Actual performance about PCB
5. Check of sensors, 3-way valve, and antifreeze
The components that cause the most faults out of warranty:
- NTC flow/return sensor: over time they become uncalibrated. Result: the boiler cycles poorly and consumes 10-12% more.
- 3-way valveThe motorized valve gets stuck; the boiler provides DHW or heating, but not both.
- Expansion vessel: loses precharge over the years; be careful if you refill pressure often.
- Antifreeze (only boilers with external circuit or hybrid aerothermal): glycol expires — it is advisable to check pH and concentration every 3 years.
Recommended annual schedule
| Month | Action | Who |
|---|---|---|
| September | Complete annual review | Official technician |
| October | Bleeding radiators | User |
| Nov – Feb | Check pressure every 2-3 weeks | User |
| March | Post-season review | Technician (optional) |
| June | Analyse DHW flue gases | Technician (biannual) |
Frequently Asked Questions
Hire our annual maintenance plan: preventive maintenance, flue gas analysis, cleaning, bleeding, and fault coverage. 15 years maintaining boilers in Madrid, own technicians and no subcontracting.
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