Central heating systems sometimes emit strange noises ranging from loud banging radiators to gentle gurgling or whistling sounds. There are several possible causes for this phenomenon; low water pressure, insufficient water in the system, air trapped inside radiators or pockets of steam trapped in pipes from water boiling or “kettling” inside a sludge-filled heat exchanger.
Turn your boiler off and allow it to cool down. Check the reading on the pressure gauge mounted underneath your combi boiler. If this is showing less than 1-bar, low system pressure could be causing your banging pipes.
Identify two capped pipes on your boiler's plumbing system. One may lead down from your boiler's horizontal water supply line, while the other will be connected to your unit's return water pipe, but the precise configuration of your system will depend on how and where your boiler was installed.
Twist off the caps from each pipe by hand and connect them with a filling loop. If a filling loop didn't come with your boiler, you can pick one up from a plumbing store.
Open the valves on each of your system's capped pipes and allow your boiler's pressure gauge to hit 1-bar before closing them. Once you've shut off your system's valves, place a bucket underneath your filling loop and remove it. It's illegal to leave a filling loop connected to a central heating system, according to DIY site Ultimate Handyman.
Insert a radiator key into the bleed valve situated on upper side of your radiator. Hold an absorbent rag or tissue under the bleed valve.
Open the bleed valve a quarter turn anticlockwise; you’ll hear air hissing out of the system. Close the valve firmly as soon as water comes out. Repeat this procedure on every radiator in the house.
Re-pressurise your boiler system using a filling loop after you've bled all your radiators, making sure to remove your filling loop once you've done so.
Turn off the RCD on your electrical panel controlling power to the central heating recirculating pump. Put a strip of masking tape over the switch to prevent someone turning it on while you’re working.
Locate the recirculating pump connected to the central heating plumbing; it’s about 32mm in diameter and usually coloured red. Give the pump a gentle tap with a screwdriver handle to free a possible stuck valve.
Turn the pump pressure adjuster knob on the side of the pump until its pointer lines up with the 1-bar mark on the pump housing. If the recirculating pressure is too low or too high, it may cause a banging or gurgling noise.
Restore electrical power and turn the boiler back on. Insert the tip of a flat screwdriver into the brass bleed screw on the face of the pump. Hold a tissue under the screw and open it half a turn. Tighten the screw as soon water comes out. You will not need to re-pressurise your system after this step.
Allow the central heating system to heat up. Stand next to your boiler and listen carefully. If you hear a sound similar to a kettle boiling, your boiler's heat exchanger may be clogged up with lime scale and sludge. This restricts water flow, allowing trapped water in the boiler to reach boiling point. The boiler then generates steam as well as hot water, creating pockets of trapped steam inside the pipes that can result in repetitive banging.
Employ a qualified heating engineer to descale the boiler’s heat exchanger. He will first connect a circulating pump between the boiler's inlet and outlet pipes and then introduce a descaling chemical into the heat exchanger under pressure. This process will usually clear any obstruction and solve your pipe banging problem.
Arrange for a full system powerflush if kettling and banging noise persist. Your central heating technician will drain the entire system and then then flush it out with a high-volume, low-pressure pump and flushing chemicals. These chemicals are capable of removing all traces of scale and sludge from both your boiler's heat exchanger and the inner walls of all the copper tubing in your system.
Inspect all your central heating system's accessible pipes attached to independent hangers, floor joists and rafters. Make sure they aren’t hanging loose or secured too tight to allow expansion and contraction. Both conditions will cause a clanging and banging noise.
Undo any tight clamps holding copper heating pipes too securely to hangers, joists or rafters. Wrap the pipes with flexible insulating foam and then reattach them without tightening the clamps all the way. Place spacers made of rigid insulating material between copper pipes and the sides of cutouts to restrict sideways movement if installing clamps is impractical.
Secure long stretches of loose pipes likely to bang against nearby walls, joists or rafters when stressed. Install independent pipe hangers or saddle clamps where necessary, and secure the pipes with insulating foam the same way as described above. Do not over tighten the clamps.
If you have a sealed central heating system, employ a qualified heating engineer to carry out the pressurisation procedure before and after bleeding your radiators.
Carrying out annual powerflushing is a cost-effective way of improving central heating efficiency and reducing your gas bills.