my vuw242/1e vaillant hot water supply is intermittant?

It's a complicated issue overall, but one that can be overcome.
Removal and replacement of the DHE is a simple enough process, to clean it, you will need a propriety acid cleaner, pourd into the CH side of the DHE and left for a period to dissolve any deposit build up.

The trouble is, once cleaned and returned to the system, you will soon fill it up with sludge etc from the system you have just cleaned out from it.

The proper answer is a full system clean, however a cheaper alternative is to fit a magnetic cleaner on the system CH return pipework, these act as a filter to remove much of the sludge from re-entering the boiler.

There are a few on the market that seem to work quite well.
Spirotech
Fernox
Magnatec
They are just three more popular (no doubt there are more)
I've listed the first one as a website, just so you get the idea.

http://www.magnabooster2.co.uk/

Carefree
February 2011

many thanks to you -Carefree, can you tell me the best way to go about cleaning the heat exchanger out.

cotton bud
February 2011

Blockage in domestic heat exchanger

Carefree
February 2011