hot water pipe makes banging noises for about 5 minutes when hot water or central heating is switched on?

It certainly does sound like trapped air, can be a pig to bleed out.

But it is definitely worth trying to trace where the banging is coming from. I have lost count of how many times I have been called out to a customer because of knocking and creaking noises, 9 times out of 10 its where a piece of copper is tight against a floorboard or joist, as it heats up and expands it creaks. Usually a quick fix is to pull the pipe away from the floorboard etc and wrap a piece of cloth around it.

Good luck!

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What's the make and model of the boiler?

If it's an old one with a cast iron heat exchanger then the noise you are hearing could be kettling.This would only be cured by renewing the heat exchanger.

If the radiators and water are heating then i doubt it will be an air lock in the pipework.

Adding cleansing chemicals to an old system can remove the carbon coating from the inside of the heat exchanger.This creates cavities where small pockets of air can become trapped.When you run the boiler the air pockets start to pop and bang.This is the kettling noise which can vibrate along the pipes,giving you a false sense of where the noise is coming from.

If it went quiet after you bled the air from the system, it may just be air again. When you have air trapped in a heating system, it can take ages for it all to work its way to the top where it can be bled out of the radiator.

Or it might just be the sound of the water heating up? When water starts to heat, it expands and moves faster. When it's been especially cold you can often hear the heating making noises when it goes on, and the water starts to move around the system.

Or it might just be the same problem that I once had. I found a loose pipe that only moved slightly when the heating went on and the water started to heat. Once it was all preety much of the same temperature around the system it stopped. It sounded all the louder because it was right by the water tank and vibrated round the entire system.

Hot Water Pipes Knocking

Enough people have said they believe it is trapped air, and I agree - somewhat. HOWEVER, some of what you are saying sounds to me like plumbing expansion. A 3/4 inch copper pipe run of 50 feet can expand as much in length as one inch. The pipes moving cause that popping or banging noise you describe. The solution is to make sure that at all points where the plumbing is supported it has freedom of movement. This will eliminate any noise that is coming from pipe expansion due to heating.

As for the appearance of the problem being fixed and returning, it may just be coincidence that on those particular days, the temperature difference was not that great to cause the banging noise. Thus, you might erroneously conclude that the problem was fixed but has returned.

Investigate ThisOldHouse.com and research your situation. You will find plenty of information on trapped air, hammer AND pipe expansion.

Good luck.

:)

Water Hammer for sure. The steam condenses and the water moves with the steam and when it hit a bend in the pipe it makes the noise you are hearing. Eventually it will dry out and go away. This is very common in old systems and it is very unlikely to be air as air is less dense than the steam in the heating pipes.

the sound you hearing is called water hammer due to remaining water in the heating system turns cold when you open hot water the cold water react with cracking noise suggest next time when you need to start the heating system just crack open the hot water for at least ten minutes depend on the volume of heating coils, this will flush out all the water from the system if you wish you can also feel the heating coils with finger when you feel warm then open the the hot water valve full, this will also save you heating coils from cracking or fracture which will start leaking, hope this will assist good luck

Almost certainly..........air in the system..... You need to get something like a garden hose, connected to mains pressure, and flush system completely. If your can run it in forward & reverse circuits (flow & return circuits).......once water starts running clear, you have cleaned the system fully.....Vent rads after the system has been running for an hour or so.......Job done...

Wishing you all the Best....!!!

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