Ongoing central heating problems

[Fiddler]

We moved into this house about 10 yrs ago. The central heating was working ok, and continued to do so for a few years. Then it started to play up, not getting hot enough, rads needing bleeding etc. I put some cleaner into the system and flushed it out ( the water was black as ink) and all seemed ok for a while. Eventually, the problems started to return, and after a lot of head scratching, the pump was replaced, (grundfos). All was hunky dory.........for about 14 months!..Again, much to my disbelief the pump was diagnosed as 'kaput'. Ah, no probs, I thought, it's only just over a year old, bound to be under warranty......wrong!!.....only a years warranty....!!??.....someones having a laugh surely, I thought these pumps lasted for years? Anyway, bought another, different make this time (DAB) and fitted it...off we went again....everything working again, woohoooo............That was15 months ago.....guess what?.....same probs.......!!! The downstairs rads (x2)...aren't getting hot......the feedpipes are getting hot, but the hot water isn't getting through.....it gets further when the boiler is going, but not far enough. Sometimes the rads get slightly warm, but never hot enough for the return pipes to feel warm. The pump is working, ie rotating,...I can hear it , it isnt making any unusual noises,just the occasional 'whooshing ' noise as air gets pumped through, (they have always done this). I have tried bleeding the rads, but there's hardly any air in them. What does happen tho, is that air gets into the system, and needs to be bled from the pipe that rises alonside the hotwater tank. I can only assume that it's getting drawn in from the expansion pipe. The whole system is, I think, a diy job, as the location of the pump in the airing cupboard must have been thought out by a masochist...!!...not to mention the pipework thats obscuring the pump.... I've tried turning all the rads off upstairs, and just leaving the downstairs ones on, but that still doesn't work, they get a bit hotter, but only a bit. Surely the pump hasn't gone down again, and if it has, why? Whenever I drain the system down, it's real pig to fill. I have to fill it from downstairs, to make sure there's no airlocks, but manage it, eventually.I'm convinced it isn't an airlock problem, but am at a loss as to what it is....I've tried to give all the information in one hit, but any questions will be glady answered..........thanks guys.....

[John]

SNIP tale of woe

Oh yes it can be a pump failure and the most likely cause is the sludge you are expecting it to pump. You have an ongoing oxidation problem which must be addressed or you can expect a continuing failure cycle.

[Fiddler]

It is a vented system, but, rather than pumping over, which I understand to mean the pump forcing water UP the overflow/expansion pipe, the system is sucking water Down the pipe....!...which is one way that air is getting directly into the system. As for inhibitor.......I do have some ...of a sort.....it's a soluble oil that we used in lathes and milling machines at work , for lubrication, and anti rust......not the 'right' stuff' I know, but it was free, lol........when I drain a bit of water from the system, it isn't crystal clear, but just slightly greyish.....And yes the expasion tank has got water in it. The other day, I 'backfilled' the system....ie,I forced water up the return pipe, into the tank in the loft, until it overflowed. But it's always had enough water in it even before that. Ok, so what sludge cleaner do y'all recommend, I was thinking along the lines of Superfloc.........yes, or no?.........and thanks for replying so quickly guys :-)

[Fiddler]

By 'pump failure'....do mean it's finished, or it will run ok after the sludge problem has been dealt with ?