Boiling banging When thermostat high

I have a glowworm 18hxi condensing boiler, over the last 10 days or so if i turn the boiler thermostat above 50 deg then the boiler starts to rumble and bang and then cuts out. There are no error codes. It happily runs below 50. a bit of background....

a couple of weeks ago i noticed that the downstairs rads where not heating up, so did some balancing on the upstairs ones as they were pretty much fully open. The boiler stat was set to about 55. Now heat was going to downstairs rads but still not hot as upstairs. bleeding the rads and pump didnt make much difference, though if i turned off all the rads upstairs then the downstairs get hot. There was a lot of gushing and whooshing around the pump area whenever the boiler came on, which i dont recall previously

Turned up the boiler stat to about 65 deg and let it run, after about 45 mins the pipes around the pump starting creaking and then the boiler started the bang and switched off. This was the first time it happened. after turning down the boiler stat and letting the system cool everything worked again, but now turning the boiler above 50 causes the same problems.

Its a vented system, with 3 way motorised value, boiler is downstairs and pump and HW cylinder in airing cupboard upstairs. Grundfos pump currently running on speed 3, used to be on speed 2.

Leaving the boiler at 50 is not producing enough heat to warm the house up and when the CH and HW are both on then everything takes forever.

Any suggestions would be greatly appreciated.

Hi, I have exactly the same boiler and open vented system as you, had it for about 10yrs now... so have a bit of experience with it. I'm not a professional plumber or fitter... so I expect my response to get shot down by the experts..... but hey/ho

The pressure drop on these boilers is quite high, meaning that to get a good flow through the boiler the pump has to have a good head.. I have a 6M head grundfoss set on speed 3 (18 rads connected in 10mm piping from the flow/return)

If the flow through the boiler is too low, either because the pump is not working correctly, the system isn't balanced properly or the heat exchanger is scaled or partially blocked with crud, the boiler will "kettle" or bang because actually these boilers have a low volume of water, so overheat quite easily.

The maximum temperature difference (at full power) across the flow & return should be 20 degrees C

If you had the system working correctly in the past, and then suddenly the downstairs radiators stopped getting hot, it's unlikely that it's a balancing problem... the system doesn't just unbalance itself ..

Sounds like a pump problem. Is it the rads furthest from the pump that stopped getting hot ?

if you switch the pump from speeds 1-3 whilst it's running can you hear the speed change ?


Gushing & whoosing noises usually indicate air in the system ?

Its all the downstairs ones, they get are less hot than the ones upstairs now but atleast come on.

Ive only moved in about 3 months ago so dont really know the history, but i do remember at first when switching the pump to speed 3 it would be quite loud and could be heard in the adjacent room, but now the noise on speed 3 isnt so loud and more of a humming. I havent heard it recently but i do recall that when the pump use to stop running i would hear an abrupt grinding in the pipework in one of the bedrooms.

There doesn't seem to be much of a difference when flicking between speed 2 and 3.

I have bled that rads, pump and boiler several times now and dont get so much of the gushing sounds, my initial thoughts were too that there was a lack of water flowing from the boiler into the pump hence the whoosing.

I only have 9 rads and 4 of those are small.

can the boiler thermostat be faulty?

Sorted. Pump propeller was all but clogged with magnetite.

Thanks for all the responses.

I would be looking to get a system cleaner in your system. The magnetite is because your radiators are rotting.

chimpboy said:

I would be looking to get a system cleaner in your system. The magnetite is because your radiators are rotting.

Click to expand...

thanks, that is next on the list. Whats the best one to use given that the pipeline under the floors could be ancient?

The best thing to do would be to remove all the radiators and flush them through with a hose pipe. And treat the water with a good quality inhibitor. Regards Robiow

If you are not confident taking the rads off then run a bottle(two if larger system) of sentinel x400 in your system for three weeks. Your system will have to be flushed with mains water then treating with x100. If your system is heavily sludged then I would be calling someone in to sort it out.