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.
The vast majority of Pulsacoil 2000 breakdowns to which I am called out fall into one of the following categories:
1) Tripping of the overheat protection thermostat.
On the early unmodified Pulsacoil 2000 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. A upgrade kit is available from Gledhill to cure this and most Pulsacoils will have had this kit fitted, which then sometimes leads to a different problem where the individual overheat thermostat in the lower heater trips. When this happens, the unit stops heating up overnight but the daytime boost heater still works. Re-setting the tripped thermostat is straightforward and described in my video here:
2) 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
3) Water dripping onto the floor.
As the Pulsacoil 2000 model grows older, a problem is emerging where the copper tank inside leaks, and water oozes through the outer steel case near the bottom and drips slowly onto the floor. Water also emerges from where the pipe feeding the pump passes through the outer steel case at the bottom. This is never an emergency as the leak is always very slow, and people call me thinking a joint needs tightening or a seal replacing but sadly this isn't the case. The leak is a pin-hole in the copper tank inside could be anywhere on the tank. The leaking water percolates down to the base through the rigid foam insulation filling the space between the inner copper tank and the outer steel case, saturating it. Finding this leak is impossible as there is no access to the inner copper tank without stripping off the outer steel case and all the foam insulation. Technically feasible (just), but always cheaper to buy and fit a new unit. If your Pulsacoil is showing these symptoms but the leak is manageable, i.e. not making the floor wet enough to damage anything, there is perfectly feasible to just live with it. The water leaking out will mean the header tank on top needs topping up every few weeks as in 1) above, but there are no other consequences. If you have this problem, see Replacing a Pulsacoil.
4) 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.
5) 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.
6) 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.
7) 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.
8) 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.
9) 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 2000, call or text me on my mobile, 07866 766364
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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