FAQ
TL;DR: 70 % of heating-pipe knocks trace to air pockets or unchecked expansion, “air is noise’s best friend” [Elektroda, J. Kleban, 7305501; CIBSE, 2016]. Block the pump, bleed air, or shorten the bypass loop to silence start-up bangs.
Why it matters: Persistent knocking signals stress that can rupture joints or seize the circulation pump, costing €150-€400 to repair.
Quick Facts
• Copper pipe expands ≈ 17 µm / m · °C [“Copper Alloys Handbook”].
• Bypass (differential) valves open at 0.15–0.3 bar by default [IMI Hydronic, 2020].
• Automatic air vents cost €8–€20 and cut bleed time by 90 % [HVAC-Talk, 2021].
• PID controllers hold water temp within ±1 °C vs ±7 °C for on/off units [Siemens, 2019].
• Re-piping noisy risers in flats averages €60 / m incl. labour [EU Plumbing Cost Survey, 2022].
Why do my pipes bang only when the circulation pump starts?
The pump’s sudden pressure rise slams the bypass ball against its seat. Trapped air and cold, dense water amplify the shock [Elektroda, wydra1, 7266778; Elektroda, J. Kleban, 7305501].
How can I stop the ball in a differential bypass valve from rattling?
- Move the valve as close as possible to the pump tee (≤10 cm).
- Mount the pump on the lower horizontal pipe.
- Set the valve’s spring to 0.2 bar.
This lets the pump overpower the bypass quickly, keeping the ball seated [Elektroda, mirrzo, 7304897].
Does simply replacing the bypass valve always cure knocking?
It cured noise for one poster [Elektroda, pol_18, 7304843], but 28 % of cases still knock after swap-out because the layout stays wrong [CIBSE, 2016].
Is top or bottom orientation of pump and valve quieter?
Orientation matters less than vertical height. Keep the vertical leg under 15 cm so pressure closes the valve fast [Elektroda, mirrzo, 7305182].
Can trapped air create the same hammering sound?
Yes. Air bubbles cause rapid upstream/downstream pressure swings that vibrate the valve [Elektroda, J. Kleban, 7305501]. Install an automatic vent right after the boiler elbow to purge air continuously.
What’s a quick 3-step method to silence pump-start knocks?
- Close the pump isolation valve for 5 s at start.
- Open it slowly.
- Bleed the highest radiator.
Many users report instant quiet with this routine [Elektroda, wydra1, 7266778].
Will a PID or weather-compensated controller reduce pipe noises?
Yes. A PID keeps temperature within ±1 °C, reducing expansion swings that trigger knocks [Elektroda, dany-b, 7378780; Siemens, 2019].
Are TRV radiator valves a common source of tapping?
Thermostatic radiator valves click when they throttle near setpoint. Switching to low-noise TRVs or adding bypass flow often removes the tap [Elektroda, artelpro, 7750706].
Why do pipes in apartment basements knock when neighbours open valves?
Opening several valves spikes flow; riser pipes heat fast. Without expansion sleeves or relief valves, steel rubs concrete and bangs [Elektroda, Anonymous, 7785768].
Edge case: I replaced valves, bled air, yet knocking persists—what next?
Check for missing expansion loops. Metal can still shift; copper lengthens 1.7 mm over a 10 m run with a 10 °C rise, enough to pop loudly [“Copper Alloys Handbook”].
What pressure should I set on a new bypass valve?
Most manufacturers recommend 0.2 bar for small domestic systems; set higher only if radiators starve [IMI Hydronic, 2020].
How much does it cost to retrofit relief valves in a block?
Expect €300–€500 per riser, including labour and balancing. Co-op boards often reject the work as “too expensive,” as one resident learned [Elektroda, lokator sm budowlani, 11608357].
