Pump speed and noise - overheating boiler and overflow

[jamesn6]

Having spent a lot of time worrying, asking questions, having a new cylinder and now changing the pipework and having new pumps fitted I think that I have just realised what my original problem was.

I had gravity HW and pumped CH. Every now and again when the CH clicked off there would be an almighty racket and water would pour out of the overflow.

I think this was caused by a low pump speed causing the boiler to overheat and then when the pump switched off all of this heat shot up the vent. Might I be right? The boiler had been kettling for some time.

The reason I had a low pump speed was because when set higher it woke me up - even though the pump was in the kitchen. I now have a grundfos pump plan in the airing cupboard and I cant sleep at all with the racket it is making but if I turn the CH pump down to 1 then no doubt I will be back to sqaure 1.

After the new cylinder was fitted the engineer did turn the pump speed up but I turned it down again so I didnt give it time to see if the overflow problem was likely to happen on a higher speed - and in any case I didnt know this might be connected.

Have I just spent a small fortune for nothing? And I'm still not going to get any sleep!

 

[bengasman]

One advantage of gravity dhw, probably the only one, is that your boiler should never overheat. From what you described, it sounds like that is what happened. Invariably, gravity equals open vent, equals sludge. With a system that has been corroding for that long, chances are the only way of getting it right is a powerflush with fx2. Unless you are knowledgeable enough to add various chemicals in the right way to get rid of limescale, corrosion and dirt seperately.