Replace Plate Heat Exchanger?

My PulsaCoil III is 9 years old.
LIFF limefighter fitted at installation (Water hardness 250 - 325).
Top-up cistern filled to embossed line.

30th June
Not enough hot water for shower.
Afterwards Plate Heat Exchanger (PHE) is cold - other parts feel hot/warm.
PHE takes few hours to return to warm. Thereafter, again, a little fairly hot/warm water only.
Next day same - very quick shower only!
Today the flow from bathroom basin hot tap is OK though initially slow to respond, while kitchen hot tap flow OK.
=> Do you agree this is a scaled PHE that needs replacing (not mixing valve)?

13th July
Loud knocking noise during early hours as PulsaCoil operates off-peak.
=> Possible corrosion deposits on bottom immersion heater?
Next day adding 2 pints of water to top-up cistern reduced the noises.
After 4/5 days the knocking noises are not noticeable.


I am tempted to replace the PHE (or mixing valve) myself as is external component.
In simple terms the PHE process could be as follows:

1. PulsaCoil mains power off
2. Mains water supply off (stop cock)
3. Close the three valves
4. Remove four nuts on PHE with adjustable spanner - catch residual water
5. Remove old and locate new PHE (PHE not attached to unit?)
6. Replace the four nuts for the water flows to/from PHE
7. Open valves
8. Mains water back on
9. PulsaCoil mains power back on


So, do you agree with the scaled PHE diagnosis?

Any prognosis on the short-lived knocking noises?

If replacing the PHE, any advice to avoid pitfalls - particularly WRT steps 4 and 5?

Thanks.

Hi

I would advise that you DO NOT replace the PHE at this point in time as the symptoms you are describing indicate a faulty temperature probe.
Although a scaled heat exchanger would result in poor hot water transfer, it woulod not prevent the PHE from becoming hot. This would only occur if a] the store itself was cold, or more likely b] the pump is not running.
The pump is controlled by the temperature probes and the pump speed board and is the most probable cause of your trouble.

If you do decide to replace the PHE, your method is correct, however, be aware that the pump valves do not always shut off properly due to lack of use, and they almost always leak through the valve gland when turned either off or on (this is because the O ring seals are not moved for many years and become inflexible and are unable to re-adjust to the new surfaces that they are presented with inside the valve).

Try it by all means if you have to replace the PHE (although we normally descaled them using a descaling pump), but I would always drain the unit initially as it removes the need to do the work twice and the cost of charging customers for new pump valves. As for the noisy elements, this is due to sediment around them preventing the heat transfer to circulate correctly.

Regards
Dave Hickey
07973 624179
service@midlandboilercare.co.uk