Pump is sorted.How do i clean dhw heat exchanger Vaillant part no: 065053?

Pump is sorted by now. I have back hot water and central heating. But some problems with hot water persists.
When turn the hot-tap on I can feel the hot water coming out, but soon it becomes only warm,when suppose to be hot (because only hot water tap is open and opened to the end). It does that,when the flow is good. When i run tap down low, then I get “proper“hot water. And another thing: I waited untill radiators cool down, then put on my boiler only hot water mode and turned on hot water tap for about 10 minutes – radiators were hot (as you set on max heating). At the begining I presumed, that diverter not doing it‘s job. So i was observing diverter, when you turn the hot water on you can see the spindle pushing up to the midle and when there is demand for heating you can see the spindle pulling down and stays there. So now, I assume that DHW heat exchanger is blocked, as carefree mentioned such posibility. QUESTION is:
• how do i clean it?
• do i need take apart this dhw heat exchanger? Take filter or something out? Or just soak whole exchanger in something?
• With what I can give good and quick cleaning? Is citric acid ok? Or maybe hydrochloric acid, phosphoric acid or sulphamic acid? Do i need to dilute acid or should use undiluted? If dilute - how much and how long to soak or clean before rinsing? If undiluted - how long to soak or clean before rinsing?
• And with what to clean black bits, if i find any there?
My boiler is vaillant ecomax824e and DHW heat exchanger Vaillant part no: 065053.
THANK YOU

Jane
September 2010