still looking for a little help with my boiler which has been making noises or the last 18 months (since I bought house).
A bit of background...
The system is a fully pumped CH and HW system with a motorised valve, cylinder stat, 1 room stat. there is a bypass from pipe from the pump back to the inlet of the boiler with a gate valve (what is this for?????)There are 9 rads 2 of which are fitted as bypass devices (shower room and bathroom). Bolier is wall moutned on first floor in an airing cupboard right next to HW tank.
The boiler is a Thorn EMI Apollo Fanfare 30/50i fitted in 1987 when house built In the last 12 months I have replaced the pump with a suitable Grundfos Alpha repalcement, tried to descale the system and replaced the auto air vent on top of the boiler. I have also tried to make sure that the system is properly vented but I am not really sure if I have achieved this. The noise has improved but not drastically - in fact the way the boiler behaves seems slightly changeable so it is possible that all my efforts are in vain.
On switching on (firing up) from cold everything works OK for few minutes. Then I get massive bangs that wake the dead/neighbours/neighbours dogs. Then system settles down after 1-4 iterations of the start-bang-stop cycle. Having gone into the loft to investigate I can see gas (I am pretty sure steam) bubbling/exploding out of the CH header tank during the banging - there is not a safety vent pipe above the tank as I have seen in other installs. The steam appears to be coming into the tank via the outlet that feeds water from the header tank to the main system.
Recently I have come of the opinion that the pump is switching off for perhaps 20 seconds (there is a green LED which shows pump is running) - the water in the boiler really does boil, steam evolves and the expansion forces it to the header tank - then the pump kicks back in again for no real reason (green light comes on) and unless the cycle is repeated everything settles down. Immediately after the bang there is a real whooshing sound in the pipes which I presume is the sound of tiny gas (steam) bubbles being circulated this soon settles and is not present in normal operation.
So I guess my line of questioning should be around the control logic which switches the pump on/off.
I know the boiler has a lo stat, high stat and an overrun stat - you can see these on the pipes when the covers are off. The overrun seems to work as the pump will keep operating (with light on) for some time after boiler has stopped heating. With the lo stat selected the bolier never really seems to get going jut starts, does a little banging and stops... In the High position this is the only thing that seems to kickstart the boiler and get it going - as I say once the banging is over in normal operation the boiler/pump work fine.
So - how does the logic work which switches the pump on/off?
Would faulty stat(s) cause these symptoms? I could understand if a stat failed that the pump might stay on for ever or never come on but why does it come on, go off for a bit (20 seconds) and then come back on as the boiler is boling and making a racket...
Can the pump switch itself off or is it, as I believe, a fairly dumb device which lights up and spins when it has power and switches off when there is no power.
What else could be causing this pump behaviour and then boiler noise(I am sure it is not loose connection etc. fault is too consistent) can the pump itself shutoff???
I am really desperate to try and fix this as I do not think there is necessarily anything wrong with the boiler and do not want to try and find £1500 for a new one. I am also reluctant to spend lots on fernox etc until I know bolier is not about to end up in landfill.
SOrry for long post but I tried to give detail. If there are any questions/suggestions please let me know and I will try them. I have a wife and baby who are getting increasingly unhappy with me for this :(
Thanks in advance Tim