Removing secondary heat exchanger from combi

Hi,
I planning on descaling the DHW heat exchanger on my combi. It's an Ariston Microgenus 27 and the rate of flow has gradually declined since it's been installed (over three years ago). The descaling looks like an easy enough job, and removing the heat exchanger itself is dead easy, but before I make a complete fool of myself I've never really understood exactly how the combi does the DHW. Do I need to drain the boiler completely (and all the rads) or is just turning off the cold water feed to the combi enough?
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On Wed, 19 Apr 2006 11:49:33 -0700, Martin Pentreath wrote:

Have you got the service manual? It will almost certainly involve draining the water out of the boiler but not the rads.
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The service manual does indeed say "the boiler must be drained of all water", but that's all it says, it doesn't say how to do that. I wasn't sure whether this means draining the rad circuit as well, but from what you're saying it doesn't. There's no obvious draining point underneath the boiler. Any advice on the usual procedure for draining just the boiler would be very welcome.
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On Wed, 19 Apr 2006 11:49:33 -0700, Martin Pentreath wrote:

You'll need at least to isolate the primary side as well, but most boilers have valves on the flow & return so you shouldn't have to drain the rads.
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