Why does my boiler sound like its going to explode? (Piglet John - r u around?)

[coveredinweetabix]

We'd been away for a long weekend and when we came back last night, the house felt cold so we turned on the heating. The boiler (Vaillant ecotech) and pump are both less than 2 yrs old but I'm not sure if the control panel was replaced when they were. The control panel does both hot water & heating and you can choose between "off", "timed", "once " & "on". The hot water is always on times, last night we put the heating on "on". We forgot to turn it off when we went to bed, turned it off first thing this morning (by switching it to "off") and when I came back from drop off etc it was still boiling and some of the downstairs radiators were chucking out heat. So, I checked and yes, it was off. After a hot hour or so, it dawned on me to turn the thermostat down as low as it would go (10degrees) and to turn all of the valves on the radiators that were on down to 0. This helped with the temp. I also left a message with our boiler man but he's on holiday. I eventually turned off the hot water too in case that was somehow affecting it. I turned the hot water back on this evening (about 6pm) and for the last hour or so there have been increasingly loud clunks coming from the boiler cupboard & as well as the whoosh you get when the boiler kicks in again, there are clunks, bashes and general unhappy metal sounds. I flipped down the cover and spotted that when this happens the temperature reading on the boiler itself races up from 52degrees to 98degrees in a matter of seconds and then slowly drops back down again and then goes through the same cycle 10-15mins later. It sounded so alarming and we were so unnerved by the temperature being in the 90s that we have turned the whole thing off. Any advice? Can we use it or should we leave it off? Can I wait until my man gets back from his holiday or should I try & find someone else (have already texted some local friends asking for recommendations).
Thanks

1. you appear not to have a room thermostat

2. you have poor water circulation. This might be due to loss of water from the system; or a sediment blockage; or the pump being seized.

Unless you are fond of DIY plumbing, you need a heating engineer.

It will probably not explode, but I would be happier if you left the CH off amd tried HW on to see if the problem is the same. it probably will be. Do you have a hot water cylinder with an immersion heater, which you can use while the boiler is out of action?

Try bleeding the highest radiator in the house, and see if water squirts out vigourously.

If the radiators and pipework are a lot older than the boiler, it will probably be a sediment-related problem. Sediment can clog the pump, it can also block pipes, including the pipe that tops up the water in the system. It is much harder to clear a blockage than to clean out the sediment before it gets that bad.

Have you got a loft ladder? have you got a strong magnet?

I've changed my mind

The radiators were still hot even though CH was set to "off" and/or the room stat (yes, I see now that you have one) was turned down.

So it is most likely the 3-port valve that is allowing heat to go to the radiators even though they are supposed to be off. If you are lucky it will just be the motorised head that needs changing, this is not a water part. If the rubber ball is worn out then the system will need to be bunged or partly drained to fit it.

If you have a HW cylinder, I'm a bit surprised that the cylinder stat didn't turn the boiler off once the cylinder was hot. is the hot water from the taps very hot? Or is the boiler set very low?

Thank you Piglet. Yes, we have (yellow) HW cylinder immediately below the boiler which is also less than 2yrs old as it was replaced when the boiler was. The thermostat for that is in quite an awkward place so I will get DP to check that later (I've been awake all night worrying about this, what else can go wrong with the house & the expense; he hasn't). I'm v glad you mentioned the immersion heater as we do have that so I've just been to turn that on so at least can have a shower this morning.
As a matter of interest, over the summer, should we heat hot water with the immersion (currently not on a timer but presumably could be) or the boiler?

It will usually be more economical to heat the cylinder with the boiler, even in summer, relying on the timer (morning and evening), the cylinder thermostat, and good insulation of the pipes and cylinder. I would expect you to use between half and one cubic metre of gas per day in summer (put gas meter readings on your calendar). Electricity costs about three times as much as gas.

Modern cylinders are blue (or white casing) yellow is quite old (nearer 20 years than 2)