Copyright 2007 to 2015 Michael Bryant

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What IS a PulsaCoil?

PulsaCoils 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 2000 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.

Common problems:

The vast majority of PulsaCoil 2000 breakdowns to which I am called out fall into one of the following categories:


1) Depleted water in the thermal store. 

The PulsaCoil 2000 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 2000 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.


2) Thermistor failure.

The heat sensors (there are actually two) can become unreliable with age. This usually presents as unpredictable hot water performance or unstable hot water temperature. The thermal store will be hot, but the pump will not run fast enough (or at all) when the hot tap is open. The circuit board may be reporting thermistor failure via it's red LED. One flash per second indicates flow thermistor failure, seven flashes per second indicates the pumped return thermistor has failed. Two flashes per second means the circuit board thinks both thermistors are good, but this is not always true in my experience, and changing both apparently good thermistors on a unit behaving inconsistently can often cure the problem.


3) Circuit board failure.

No flashes on the circuit board LED means circuit board failure, usually. If there is 240v across the live and neutral terminals on the board yet no LED flashing, then board failure is virtually certain. 


4) Persistent tripping of the overheat protection thermostat.

 The red 'Fault' light comes on and the unit fails to heat up until the front cover is removed and the manual 'overheat reset button' is pressed. An upgrade kit is available from Gledhill to cure this. The old overheat protection thermostat is removed from the circuit and new immersion heater thermostats fitted incorporating new, separate overheat thermostats. 


5) 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.


6) Immersion heater leaking.

Older 'Skel' brand immersion heaters (fitted as original equipment) seem to suffer from leaks in the thermostat sensor pocket. On many occasions I've seen water emerging from the copper tube in which the thermostat sensor is housed. This is clearly dangerous as it introduces water into the electrical connection box on the heater element head, and it often results in thermostat failure. The only repair is to replace the whole immersion heater and thermostat.  


7) External Economy Seven time clock failure.

PulsaCoils are usually connected to an Economy Seven tariff electricity supply. When there is no separate off-peak power supply to the unit an Economy Seven timer will have been fitted. These seem to fail after a few years and no longer deliver power to the immersion heaters, even when the indictor lights on the timer say power is being delivered!. Although it's a straightforward matter to replace these timers, finding an electrical merchant who keeps them in stock can be very difficult. I keep them in stock myself as a result. 


8) 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. 


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First published 2nd January 2007
Last updated 12th January 2013

Copyright 2007-2013 Michael Bryant