Vokera Linea Max CH problem

[hossacd]

My parents are having poor performance from the central heating the radiators are only getting fairly warm rather than hot. They live in an old house with high ceilings so have 11 large radiators which with the recent cold weather has resulted in the heating being unable to get the temperature above 13c with the heating running 24 by 7.

If you watch the boiler in CH mode if fires up OK and heats the water to 80c where the burners then cut out and the output temperature drops down to around 30-35c then repeats the process. As it takes around 3 minutes for it to drop in temperature and less than a minute to reach 80c again I assume this is the reason the radiators are not getting hot as they are not getting a constant supply of hot water.

They have a BG contract who advised nothing is wrong with the boiler and that the system needs a power flush at £800. Rather than pay BG last week I drained the system refilled it with Sentinel X400 in the system and ran it until today, drained it again and then removed each radiator and flushed out with a hose pipe. There was a fair amount of sludge in the radiators but it didn't take too long to clear them and the water coming out of the drain outlet was clear both times I drained the system.

I have read a few posts on here that if a similar scenario happens with the DHW it may be a clogged up secondary heat exchanger (which looks tight to remove) however the hot water seems unaffected the boiler will sit at a steady 50c with the hot water tap running I did only run the tap for 3 or 4 minutes however. (They have a power shower so the hot water is never on for long periods)

I will run the hot water for longer tomorrow incase it may be a clogged heat exchanger is there anything else I can check before calling out BG again I suspect as I have flushed it out they are going to insist that they do it before looking at the boiler again.

 

[fanboy]

it sounds like poor circulation, possible causes: frozen pipe or partially frozen, lazy or stuck pump, sludge is system, more commonly frozen pipes seen a few with the same problem.

 

[nickso]

linea max's are absolute barstewards for blockages inside the boiler. if the rads had ****e in them theres a good chance theres a big choke in the boiler.

 

[hossacd]

I took the DHW heat exchanger off to clean as after 20minutes it would cycle and it was dirty but nothing extreme. I then flushed DS40 (I put X400 in my orignial post it was infact x800 I tired first) round the boiler and a couple of radiators which has improved the performance significantly however the boiler it still cycling it is just now running for 2-3 minutes before cutting out for 3 minutes allowing the return flow temperature to reach 50c. This does allow the house to now get up to 20c but the fact the boiler is cycling can not be right as it results in bursts of heat down the flow pipes rather than a constant flow.

Two things I noticed is the current pump attached is a 15-50 yet the manual says if replacing a 15-60 should be used and the boiler is rated to 30kw which seems excessive for 11 radaitors. 2 are 1600 by 600, 6 are 1400 by 600, one 1900 by 600, two 500 by 700 and a towel rail all are doubles.

Could it be that that flow from the pump and the power output of the boiler are too high (my dad does think BG replaced the pump a couple of years ago however he is not certain so the wrong pump may have been fitted) or could it still be sludge in the boiler I don't really want to put another load of DS40 through as I exhausted a full 1.5kg tub of it although it was exhausted very quickly after 2 3 hour runs the lot was green.