Aincient gas boiler with header tank in the loft.
The pipe work hase recently been making some pretty violent knocking noises. All the rads are bled but they were all fine. All rads seem to heat up fine.
This morning the noises were much worse and the header tank was full of very hot water.

Plan is to drain the rads down now whilst still hot – is that sensible?
Is there anything else I can do? Is it possible to drain the pipe work from the boiler?

We had a similar problem with our old combi. Told we had a leak somewhere causing gradual pressure drops and letting some air in.
About ten vists from Scottish gas (during last years cold snap) all sorts of valves etc replaced. Turned out the thermostat had gone and the noise was steam rattling through the pipes! Guessing will be different with a sealed system tho.

overheating is the first thing to check. Also check expansion pipe has not dipped into the water.

Explanation pipe is above the level of the overflow so that’s fine. Judging by the splashed water around in the loft, steam escaping sounds about right.
If I let the system go cool, then it should run fine until its up to temperature shouldn’t it?

What boiler? Set the boiler stat to 1. Does the burner shut down after its on a while?

It’s a baxi wm51/3rs
It seems to come on and off as the rads demand it. It’s set to level 1 and at this time of year the heating is on all day. It’s certainly not burning all the time.

The thermostat kit is only £16 and looks fairly plug and play. That’s got to be worth a go before calling the experts right??

Blocked cold feed from the tank would be the next thing to check after the above suggestions. Is there a lot of dirt in the tank? The cold feed acts as an expansion too.

I use a sink unblocking pump to blast air through the cold feed from the tank to dislodge any sludge build up.

If you close the water main off to the tank and drain a bit of the water from the ch system, maybe just a few litres, then open the main to the tank again and if it fills the c/feed is ok, if not you have a blockage.

Pretty sure the cold feed is ok, I’ve drained the system a load of times and it always empties pretty quickly (draining the tank fully)
Today I’ve had the thermostat probe out (theres no grommet where it passes through into the burner, but someone has sealed it in with fire cement) so I’ve had it out, it looks intact, and put it all back together. Systems been running for an hour and no noises so far. Not sure if it’s switching on and off more than usual or it’s just because I’m watching it.
I’ve switched the pump on to it’s mid setting from low thinking circulating the water faster might prevent boiling??

The stat goes into a dry pocket in the heat exchanger, the pocket will be covered in sludge and scale and not transferring the heat.

The pocket wasn’t a particuarly tight fit to be honest. Should there be some sort of thermal compound in there??

sounds like the stat and the boiler is literally boiler casing the water to pass up the expansion pipe and into the feed and expansion tank

turn stat down as low as poss(1) if problem continues its the stat if not waterway pssibly blocked somewhere…

The pocket is what the stat phial sits in, it’s screwed directly into the heat exchanger, you usually put a coating of heat sink on there, but it’s not likely to make any difference if I’m honest, as the pocket is probably going to be scaled on the water side, they usually are

The system sounds like it’s doing a few things, kettling for one, and it sounds like it’s also pumping over, this may be an issue with sludge, or pipework configuration, or possibly both.

It could just be boiling over though.

The fact that you’ve pulled the fire cement off though means you’ve breached an already bodged case seal.

I’d say you’d be best off getting someone in to have a butchers

Well I dropped a new and updated thermostat in this morning, plumbers merchant got it in no problem for £20. I was a little worried that I’d struggle to get one given how old the boiler is.
This far it seems to be coming on and off as required and is running silently.
Thanks everyone for their help – once again STW comes to the rescue!