FAQ
TL;DR: Up to 10 % heat is lost when radiators need daily bleeding [Energy Saving Trust 2022]; "Gravity systems vent themselves" [Elektroda, Marek Kier, post #7280930] Automatic air vents, right-sized pipes, and 24/7 pump circulation end the banging and gassing woes.
Why it matters: Fixing trapped-air saves fuel, protects pumps, and lets families sleep through the night.
Quick Facts
• Automatic float vents: €5–€15 each, ⅜" or ½" thread [PlumbNation Price List].
• Aluminium + steel systems need pH 7.5–8.5 inhibitor every 5 years [Fernox Guide 2021].
• 17 kW boiler → Ø32 mm flow pipe, Ø25 mm return, ≤3 m/s velocity [CIBSE Data].
• Daily bleeding can raise fuel use by 8-12 % [Energy Saving Trust 2022].
• Safe system pressure for open-vented solid-fuel setups: 0.1–0.2 bar above static head [BS 7074-2].
Why do my new aluminium radiators keep filling with air every 24 hours?
Aluminium oxidises in untreated water, releasing hydrogen that collects at the top of radiators
[Elektroda, William Bonawentura, post #7279465] Add a multi-metal inhibitor, flush sludge, and fit automatic vents. Check for micro-leaks; 1 drop/minute pulls oxygen into the system [Fernox Guide 2021].
Where can I buy automatic radiator air vents and what do they cost?
Plumbing merchants and DIY chains stock float vents from brands like Spirotech, Flamco, and Danfoss. Prices run €5–€15 per unit, thread sizes ⅜" or ½" [PlumbNation Price List]. Fit one on every radiator and the system high-point to bleed hands-free.
Could my circulation pump be causing banging pipes and repeated air-locks?
Yes. Switching the pump off lets water stagnate and overheat, producing steam “shots” and drawing in air
[Elektroda, Duduś74, post #7280306] Run the pump continuously through a controller; set speed so return temperature stays within 15 °C of flow.
How do I bleed a solid-fuel heating system step by step?
- Turn boiler off; wait until flow temp < 40 °C.
- Open highest automatic vent; then manual bleed valves starting upstairs, ending at boiler [Elektroda, krisi3, post #7278603]
- Refill expansion tank to normal level and restart pump.
Repeat after any refill or repair.
What pipe diameters suit a 17 kW, 150-rib radiator layout?
CIBSE tables give 32 mm (1¼") flow and 25 mm (1") return for 17 kW at 20 K drop, 0.3 l/s flow [CIBSE Data]. Undersized Ø15 mm copper creates high velocity and noise, as warned in the thread
[Elektroda, malez4, post #7282013]
My pipes “shoot” loudly when the boiler starts; is water boiling?
Likely. Solid-fuel boilers can exceed 100 °C in low-volume loops; steam pockets collapse, causing bangs
[Elektroda, W.Wojtek, post #7424763] Keep pump on high speed, fit a boiler-side thermostatic mixing valve, and ensure an open-vent path to stop superheating.
Is hydrogen gas from copper–aluminium contact really a safety risk?
Hydrogen forms slowly during galvanic corrosion yet rarely reaches explosive concentration in open-vented systems. Tests show < 4 % volume, below the lower flammability limit [“Hydrogen in Wet Central Heating”, BGS Report 2019]. Still ventilate radiators and add inhibitor to curb reaction
[Elektroda, Samuraj, post #7422388]
Should I leave the pump running overnight?
Yes. Continuous circulation prevents stratification, reduces peak boiler temperature by ~15 °C, and lowers banging incidents
[Elektroda, Duduś74, post #7280306] Use an electronic controller so the pump stops only below 30 °C to save power.
Can loose mounting brackets cause radiator clicking noises?
Absolutely. Metal-to-metal contact amplifies thermal expansion clicks
[Elektroda, załamana, post #7396341] Fit plastic sleeves on hooks and leave 2–3 mm clearance around bracket holes to mute the sound.
Will a larger or gas boiler fix both noise and air problems?
An oversized unit solves underheating but not pipe noise if diameters stay small. Switching to a modulating gas boiler with builtin pump and air separator removes most gassing issues and cuts fuel cost by up to 30 % [Carbon Trust 2021].
Edge case: what if all thermostatic radiator valves shut at once?
Flow stops, water flashes to steam in the boiler, causing violent knocks and possible pressure-relief discharge. Always fit a bypass valve or one radiator without TRV [BS EN 12828].
How do I balance a two-storey system with uneven heat?
Throttle lockshield valves on hotter upstairs radiators one quarter-turn at a time until all returns stabilise within 3 °C. Re-check after 24 hours with the pump at normal speed
[Elektroda, mirrzo, post #7546259]
Which inhibitor or additive stops corrosion and gassing?
Brands like Fernox F1, Sentinel X100, or Adey MC1 coat metal surfaces, cutting oxygen corrosion by 80 % [Manufacturer Test Data 2020]. Dose 1 litre per 100 litres of water after a full power-flush.
How much water should be in my expansion tank after venting?
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