Loud banging noises when boiler starts up

[wipeout]

We have an Ideal icos HE18 boiler and it’s developed a problem where every time the boiler starts up (either hot water or heating) the boiler makes loud banging noises. However, once the boiler has got going properly (which isn’t a given, see below) then it works fine without any issues.

Detailed explanation of what happens:

Once you turn the boiler on at the programmer (either on schedule or on demand) the boiler starts to fire up. The display will change from “0” to “c” and the led will start flashing as normal and then the led will turn solid. Then after about 30 seconds a banging and klunking noise can be heard from the boiler. This issue happens for both the hot water or central heating.

This banging noise (which is quite loud) will last for 15-30 seconds at which point the boiler realises something is wrong and the led starts flashing again and the banging noise stops.

Then either one of these 2 things happens after a minute or 2 has passed:

1. It will start working as normal for hot water or suppling to the radiators without an issue; or​

2. The boiler enters into a repetitive cycle of trying to start up, making a loud banging noise for 15-30 seconds, stopping for 1-2 minutes. But no HW or CH is supplied. And this just keeps on repeating itself. The only way to get out of this cycle is to turn off the HW/CH at the programmer, wait 5-10 minutes and try again from the start.​

However, once the boiler is on and past the loud banging noise with the HW/CH working then it appears to work as normal until the programmer switches it off or the water is hot.

This morning it took 3 attempts at the programmer to get the hot water working.

Any ideas as to what is causing this and how to fix?

 

[CBW]

If the definitely the boiler could be a few things, like ignition failure, or heat exchanger. If it’s on the system but sounds like the boiler then it could be a failing pump or motorised valve.

 

[Madrab]

I'd look at a lazy pump firstly, especially if it quietens down after a min or 2 or it works ok intermittently.

 

[DP]

A proper boiler service would be advisable. By that I mean the head off etc, A FULL STRIPDOWN service, not just a poke in the flue for FGA

 

[Londoner2]

My brand new boiler takes off like a jet when dhw is full on.

 

[wipeout]

Thanks for the replies.

Some more observations. When this happens, it appears that water does not start circulating in the cylinder cupboard. We have a Grundfos ALPHA2 L circulator pump and only the "power on" light comes on. None of the other pump setting lights come on.

Once the system gets working, the pump shows the III setting.

We had a British gas engineer round and they said that a power flush is necessary to fix the problem?

@Madrab The boiler definitely quietens down after a minute but if the water does start circulating the loud noises come back. To my uneducated eye, the pump doesn't seem to turn on with the boiler. Once the pump turns on and shows the III setting, then it seems to work fine.

 

[Gasplumber]

As above sounds like lack of servicing and poor ignition.
Pay someone to do a full strip down service