Help in replacing the heat exchanger block

To help out my stepson.... We've figured out that the heat exchanger plate must be firred up after 5 years.... Full bore and the hot runs for a bit and then goes cold. Run the hot at a trickle and it comes out hot and stays hot. C/H rads is fine and switching to electric backup is the same so definitely not a boiler issue.
So although I'm competent at DIY plumbing, this system is a bit more complex so I could do with some guidance to save flooding everywhere.
Cold feed in isolation is fine as is switching everything off..
Do that and run off all the hot taps to drain one side of the heat exchanger.
I'm guessing that the hot water pump circulates the reservoir water from the top, through the heat exchanger and back into the bottom of the reservoir. Is that right ?
There seems to be an inline service valve above the HW pump, but not below the outlet of the heat exchanger.( poor design?)
But what about draining down the main heat reservoir so the other side of the heat exchanger can be disconnected ? Are the boiler and rads both indirect ? If so, is it just a case of draining out the reservoir from the bottom drain valve.?
Or is it more complex and requires draining down both the CH rads and the boiler ?
Thanks for any help on this...it will be much appreciated

Well looking in the manual in the parts list it states Key no's 3 and 4
PHE pump isolating valve - inlet
PHE pump isolating valve - outlet
So that makes the design pretty good!
Perhaps bigger books and or print!
Don't forget the new PHE washers!
Mike
MGC/H2O
Norfolk

Thx, Mike. RTFM I guess, but what about draining down ?
Also, can the pump speed be checked with a digital meter set to AC RMS ?

Well if you have the 2 pump valves.
Or if not turn the header tank supply off and drain the unit using the doc.
Check pump has supply using volt meter AC.
In use the DHW pump will speed up and down.
See my post "warning on use of 15/50 pumps.
Also you need inhibitor at the rate of 1 litre to every 100 litres.
But I would fill the unit first to check for leak, then if ok.
Drop some more water out and add the inhibitor.
Mike
MGC/H2O
Norfolk

Sorted. Checked out everything after seeing if the header tank had any water in it..... Yep, it was empty.
Filled that up and as a matter of course checked all the thermistors and the DHW pump modulation voltage and the fill rate of hot versus cold. 5 seconds difference to fill a bucket full flow.
Guess that proves the limescale level is OK. The system will now deliver a nice shower full time and a bath with the tap on 75%.
Thanks for the help and hope the solution helps someone else.
Why they didn't fit a ball valve I have no idea.

Fit a ball valve.
Then fit an overflow!
That is the reason.
Should not need filling that often
Mike
MGC/H2O
Norfolk

I would, but there's absolutely no exit route for the overflow. The unit is right in the middle of the house..Pobably explains why it was done that way
First time the tank has been filled/checked since new -6 years.