PulsaCoils in general are thermal stores, rather than hot water cylinders. Like conventional hot water cylinders, a thermal store is a container filled with hot water but here the similarity ends. Once filled, the water in a thermal store never changes. Instead, the heat stored in it is used to heat the tap water using a heat exchanger. This allows the hot tap water to be delivered at full mains pressure, and is one of the primary benefits of installing a thermal store instead of a conventional hot water cylinder.
The Gledhill PulsaCoil transfers heat into the tap water using a pump and an external 'plate heat exchanger'. A plate heat exchanger is a block of very thin stainless steel plates arranged so that cold mains water can flow through one set of spaces between the plates, and hot water from the thermal store core can flow through an alternate spaces. Heat transfers through the plates and heats the cold mains water on it's way to the hot tap.
How does the PulsaCoil A-Class work?
An immersion heater heats the water inside the thermal store. A thermistor (heat sensor) is attached to the domestic hot water outlet from the plate heat exchanger. When the thermistor records a fall in temperature, the circuit board runs the pump. The pump circulates stored hot water through the plate heat exchanger, heating it, and the circuit board turns it off again when the thermistor reports a temperature rise. This system is proportional. The bigger the temperature fall seen by the thermistor, the faster the circuit board runs the pump. This way the designed flow temperature (of 52 degrees Celsius, I think) can be maintained at almost any flow rate when a hot tap is turned on.
The PulsaCoil A'Class is a relatively new model so unlike the other models I've had very few calls to repair them so far. The breakdowns to which I have been called fall into one of the following categories:
1) Red light flashing once per second:
There is a red LED indicator on the front panel of a PulsaCoil A-Class. The label next to it says "Fault attention required" if flashing. Unfortunately on some early batches of A-Classes this can light flash during normal operation, even when there is no fault present, and the label incorrectly suggests a fault has occurred.
On those early versions, the red light flashing once per second simply means that the unit has cooled and may be re-heated immediately using daytime electricity if the user wishes, by pressing the black 'On Peak Boost button'. There is no particular need to do this unless more hot water is needed immediately. The unit will re-heat automatically at midnight.
2) Burned out power relay.
A short section of wiring to the Off-Peak heater on a PulsaCoil A-Class has a habit of burning out, along with the power relay. When this happens the red light flashes and the unit only produces hot water if the user presses the black button next to the flashing red light. The burned out section of wiring and relay can be easily replaced, but Gledhill now market an upgrade kit to prevent the problem recurring. It comprises a different type of relay and some replacement wires made from solid copper. I now keep this upgrade kit in the van as a stock item.
3) Depleted water in the thermal store:
The PulsaCoil A-Class is filled with water using a small header tank installed separately above the unit. This is not always permanently connected to the mains supply (usually when an overflow pipe to outside cannot be fitted), which means water lost from the thermal store through evaporation and/or leaks needs to be replaced manually. If the water level in the PulsaCoil A-Class falls too low, the pump simply does not have enough water to pump through the heat exchanger when a hot tap is turned on, and the unit will not deliver hot water. The problem starts intermittently, and the unit runs noisily. The answer is to check the water level in the header tank and top it up to the waterline moulded into the wall of the tank.
4) Immersion heater element failure.
The unit fails to heat up. Easily diagnosed by measuring the resistance of the heater element. A good element will measure 18 Ohms approximately.
5) Water scale-contaminated plate heat exchanger.
The plate heat exchanger is prone in some areas to water scaling. This presents as maximum water temperature becoming progressively lower, and in the final stages of scaling, the flow rate from the taps reducing too. The fix is to either fit a new plate heat exchanger, or to descale the existing heat exchanger using conventional descaling techniques.
If you'd rather I came and fixed your PulsaCoil A-Class, contact me by calling or texting my mobile 07866 766364. I keep a full stock of spares in the van and can fix most faults in a single visit.
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Copyright Michael Bryant 2020
Site first published 2nd January 2007
Last updated 1st February 2020
Gas Safe Register 197499, CIPHE registration number 56207