Descaling a pulsacoil 2000

Hi Mike and all,
I have a pulsacoil2000 which has been giving me hassle eversince I moved in nearly 2 years ago.
I have replaced both immersion heaters as these were corroded and leaking in to the caps.
I'm still getting unstable water temperatures which I now believe is to do with the thermistors.
On opening the hot tap, there is no hot water at all, sometimes for up to 5 or 6 minutes. Then close the hot tap, Wait a second or two and open it again and the water will be luke warm to hot, then go cold. This loop can continue up to 4 or 5 times. Then all of sudden - hot water. This sounds to me like one of the thermistors is not immediately sensing that the heat exchanger has dropped below temperature and so the pump is not running. Can any one confirm my theory and let me know which thermistor(s) need replacing if this is the case?

The immersion heaters are both VERY noisy when they turn on so my next wish is to descale the unit to see if this will make the unit more reliable.

Which leads to my question:

Can anyone help with descaling tips for these units?
Which descaler / inhibitor is the best one to use?
Do I need to drain the core and start over completely (Nightmare) or is there a descaler I can add to the header tank?

Any advice here would be much appreciated

Many thanks and all the best
Tim W.

you cannot descaled a p/c 2000.
i suggest that you completely empty the unit via the drain point half fill agai and flush, replace both heater elements and stats, when you replaced them before are they genuine gledhill/shel units ? refill store and add some inhibitor [ fernox f1 / sentinel x100 ] when the system is running on the control board ] top right hand corner is a red led .
this should in normal operation be pulsing like a heartbeat, or is it erratic and a low buzzing humming noise present, if so i would say defective sensor and or wiring to the pcb. most p/c 2000 have a wiring diagram on the back of the front panel.
hope this helps
mark.
www.gledhill-repair.co.uk

Hi Mark,
Apologies for the long delay in responding.
Both immersion heaters were replaced with genuine shel units.
I don't think they need replacing as they are only just over a year old.

The buzzing sound from the PCB you mention only happens when the pump is running.

Could I sentinal X200 boiler descaler? I realise that there is no way around draining the core again.

The products described as boiler descalers or inhibitors all seem to be described working in the same way, as if they are one and the same -

Sentinal X200 boiler descaler is described as:
"A unique, life-long system treatment for the elimination of limescale and boiler noise.
Safe to use in all types of indirect central heating systems and will quickly pay for itself in reduced fuel usage.
Restores boiler efficiency.
Eliminates "kettling".
Safe and non-corrosive.
A much faster, simpler method of de-scaling.
No need to flush - stays in system."

Sentinal X100 inhibitor is described as:
"Non-acidic and non-caustic - easy to handle.
Ensures optimum boiler efficiency.
Prevents boiler noise.
Prevents pin-holing.
Non-staining, colourless formulation.
Once added, Sentinel X100 never needs to be replaced unless the system is drained down."

Your further thoughts would be apreciated.
Many thanks
Tim