Use model-specific instructions to protect components, renew the right seals and complete the checks after servicing.
A service can create faults if a seal is reused, a vulnerable part is wetted or a pipe carries a load it was never designed to support. Plan the work from the exact appliance instructions, including replacement parts and the checks needed before returning it to use.
The examples below apply to the Vaillant ecoTEC plus VU/VUW ..6/5-5 in the 2021 instructions. They are examples to revise from, not the full servicing procedure. Other models may use different materials, renewal rules, cleaning methods and torque values.
Isolate, cool and support
Follow the specified gas and electrical isolation procedure and allow hot components to cool. Electrical isolation includes securing against reconnection, waiting at least three minutes for stored charge to discharge and proving that no voltage is present. Work within the competence needed for the task.
When removing the compact thermal module, support its weight independently. Leaving it suspended from its flexible corrugated gas pipe can damage that pipe. Flexibility allows movement; it does not make the connection a lifting or supporting point.
Keep leak-detection spray and fluid off the covering cap on the Venturi filter. Liquid there can block the filter and destroy the mass-flow sensor. This warning is about one vulnerable part: still leak-check the gas joints by the set method, keeping the fluid off it.
Decide what must be renewed
Every opening of the burner flange requires a new flange seal and new self-locking nuts, even if the removed parts look sound. The insulating mat follows a different rule: inspect it and replace it if damaged. Inspect the burner too; visible damage calls for replacement.
When refitting the module, tighten the four new flange nuts diagonally so the flange meets its mating face evenly. If a torque spanner is available, the stated setting is 6 Nm. These are the burner-flange nuts; do not confuse them with the four stud-bolt nuts on the heat exchanger, which the cleaning instructions say not to loosen or tighten.
Clean without damaging nearby parts
Protect the open electronics box against spray before cleaning the heat exchanger. The specified cleaning liquid is water, or vinegar with no more than 5% acid; allow the vinegar 20 minutes to act. That does not allow a stronger acid cleaner, or the same treatment on another boiler.
Rinse loosened dirt with a water jet or use a plastic brush as instructed. Keep the direct jet away from the rear insulating mat. Do not substitute soaking or wire-brushing the insulation. Rinse water leaves through the condensate path, whose condition and reassembly also need attention.
Restore the water paths
Remove and clean the cold-water inlet strainer by sending the rinse jet opposite to its normal flow direction. This pushes caught dirt back out. Replace a damaged strainer or one that cannot be cleaned sufficiently, and use the new seals required for refitting.
After cleaning the condensate trap, fill its lower section with water to about 10 mm below its top edge, then refit it. This establishes the intended water content before operation; it is not a measurement of an external siphon's required seal depth.
Complete the checks
Refitting parts is only one stage. Restore connections and seals in the specified sequence, prevent twisting the gas connection, and complete the required gas and water leak checks, operating-pressure checks, combustion checks and functional tests. Leave the casing and sample points correctly closed. A clean exchanger or a correctly tightened nut does not make the whole appliance ready for use.