FAQ
TL;DR: On this thread, 1 confirmed fix came from cleaning a scaled 90° elbow; "there was silence after cleaning the elbow." Humming during heating often indicates pump load or flow restriction. Start by feeling the pump and inspecting the intake path. [Elektroda, ptak01, post #17583846]
Why it matters: For owners and technicians chasing a humming ecoTEC 186/3-5 Plus during CH, this FAQ pinpoints causes and safe first checks.
Quick Facts
- Hum typically appears during active central heating, not DHW production, per user report. [Elektroda, ptak01, post #17056506]
- The circulation pump is the main mechanical component that can produce a constant hum. [Elektroda, Darek A., #17055598]
- Quick check: remove front cover, touch the pump; feel for abnormal vibration. [Elektroda, ls_77, post #17054864]
- Confirmed fix in thread: descaling/cleaning a clogged 90° elbow below the boiler restored quiet. [Elektroda, ptak01, post #17583846]
- If noise disappears with casing off, test combustion air path and gas tightness during operation. [Elektroda, kz61, post #17058407]
Why does my Vaillant ecoTEC 186/3-5 Plus hum during heating?
Humming during CH often points to hydraulic stress or a pump issue. A confirmed case here traced the noise to a scaled 90° elbow. The pump struggled to push water through the restriction. Replacing the pump did not cure it until the elbow was cleaned. After cleaning, the boiler ran quietly. [Elektroda, ptak01, post #17583846]
How do I check if the circulation pump is causing the hum?
Remove the casing and place your hand on the pump. Expect slight vibration in normal operation. Excess buzz or resonance suggests bearing wear or cavitation. Compare sound with and without heating demand. If vibration matches the hum, investigate the pump and flow path. [Elektroda, ls_77, post #17054864]
Why does it hum in CH but not when running hot water (DHW)?
During DHW heating, the boiler circulates its internal water in a short loop. The pump faces low resistance there. Central heating sends flow through the full radiator circuit. Higher resistance can load the pump and amplify noise. That difference explains a hum during heating but not DHW. [Elektroda, ls_77, post #17057041]
Could the fan or air-intake muffler be the source?
Yes. Intake components can hum if the muffler or fan resonates. Listen near the air intake while the boiler runs. Remove the front cover and compare sound levels. If the hum localizes to the intake, service the muffler and fan. Check for blockage or damage. [Elektroda, ls_77, post #17055629]
The humming stops after I reboot the boiler—what does that mean?
Power-cycling temporarily stops the hum for some users. That suggests a state-dependent vibration or debris shift. It may reset pump speed or break resonance. If the hum returns under heating load, inspect the pump and flow path. Document when and how it reappears. [Elektroda, ptak01, post #17054137]
What fix actually solved it for the original poster?
In the confirmed case, cleaning a scaled 90° elbow solved the humming. The pump had already been replaced. The elbow was below the boiler and restricted flow. "There was silence after cleaning the elbow." In this thread, 1 of 1 confirmed fixes involved descaling that elbow. [Elektroda, ptak01, post #17583846]
How do I check and clean a clogged 90° elbow below the boiler?
Focus on the 90° elbow below the boiler and nearby fittings.
- Isolate and drain the section below the boiler; remove the 90° elbow.
- Inspect and descale the elbow; flush debris.
- Refit, refill, bleed, and test under heating load.
If the hum disappears, the restriction was the cause. [Elektroda, ptak01, post #17583846]
The hum goes away with the cover off—what should I test?
Run a combustion analyzer check and inspect what the boiler "sucks" for air. Then check gas tightness with the unit operating. Cover-off silence points to intake resonance or leakage. Address air path restrictions or sealing issues. Schedule service if you lack test gear. [Elektroda, kz61, post #17058407]
Is a constant hum normal on this boiler?
In a gas boiler, only the pump can produce a constant mechanical hum. Continuous hum means the pump or its hydraulic load needs attention. Inspect for restrictions and verify pump condition. Then test under CH demand to reproduce the noise. "Apart from the pump, there are no mechanics that would emit a constant hum." [Elektroda, Darek A., #17055598]
Does the pump run during hot-water production?
Yes. The pump circulates boiler water through the main exchanger and the DHW exchanger. The incoming cold water then gains heat across the plate or coil. That loop uses short piping and less resistance. Noise there is less likely unless components fail. Hum under CH load points back to the heating circuit. [Elektroda, ls_77, post #17057041]
My filter is clean—can a restriction still cause humming?
Yes. The filter can be clean while an elbow or valve is scaled. In this case, a 90° elbow below the boiler was clogged. The pump struggled until the elbow was descaled. Cleaning that elbow restored quiet operation. Inspect elbows and local fittings even if the filter looks fine. [Elektroda, ptak01, post #17583846]
How much pump vibration is normal?
A small amount of vibration is normal when touching the pump. Excessive buzzing or rattling suggests wear or cavitation. Use a mechanic’s stethoscope or screwdriver as a probe. Compare vibration with and without heating demand. Replace or service the pump if vibration increases under load. [Elektroda, ls_77, post #17054864]
What should I check before calling Vaillant service?
Open the front cover and localize the sound. Place a hand on the pump to detect abnormal vibration. Listen at the intake for resonance. Note whether the hum happens only in CH mode. Share these findings with the technician to speed diagnosis. [Elektroda, ls_77, post #17054864]
