Pulsacoil
Gledhill PulsaCoil water heaters are 'thermal stores', which work in a significantly different way from ordinary hot water cylinders and water heaters. Different enough to to mystify many heating engineers when it comes to fixing them, it seems. Pulsacoils are not particularly difficult to understand but they look intimidating, and without background knowledge many heating engineers understandably struggle to diagnose faults and repair them efficiently.
If you have a Pulsacoil unit that is not working there are two things to check before calling out an engineer:
1) Check the water level - there is a top-up tank at the top of the unit. The unit needs to be full of water and some, from time to time, need topping up. Very simple to do, get something safe to stand on (the unit is very tall) and remove the lid at the top. Check the water level is up to and a little bit beyond the waterline moulded into the wall of the tank.
2) The overheat protection device may have tripped. You can re-set yourself on many of the models. Some have two overheat trips. Read the user instruction booklet to find them. You may need to remove the front cover panel to access the rest button.
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 IIIs and PulsaCoil 2000s transfer 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 III work?
An immersion heater heats the water inside the thermal store. A flow switch detects when a hot tap has been turned on and tells the circuit board, which runs the pump. The pump circulates stored hot water through the plate heat exchanger, heating it, and the circuit board modulates the pump speed to maintain output temperature using two thermistor temperature sensors. The output temperature to the hot taps is regulated by a thermostatic blender valve on the outlet of the domestic heat exchanger which adds in some cold water if necessary. The blender valve output temperature can adjusted by the user.
PulsaCoil III common faults:
1) Depleted water in the thermal store.
PulsaCoil IIIs have a top-up cistern attached to the top. This may or may not have a float valve connected to the mains water supply to fill it. (When there is NO mains connection, there is usually provision for manual filling by the user by means of a tap on the wall nearby.) Water is continually lost from the thermal store through evaporation and/or leaks, and when there is no float valve to top it up, the user needs to do it manually from time to time. Low water level in the cistern at the top WILL prevent the PulsaCoil III from working because there is a float switch in the cistern that turns the unit off when the depth of water in it falls to about three inches or less. The power supply to the immersion heaters is turned off and the green "Water Level" indicator light on the front panel will have gone out, and the unit will fail to heat. The answer is to check the water level in the top-up cistern and top it up to the waterline embossed into the wall of the cistern.
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) 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) 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 we have 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.
6) Thermostatic blender valve failure.
The thermostatic blender valve is susceptible to contamination by water scale in hard water areas. There is a component inside which breaks and the blender valve delivers just luke-warm water to the taps. A new blender valve cures the problem.
7) 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.
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.
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 can be maintained at almost any flow rate when a hot tap is turned on.
Common problems:
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 we have 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 difficult.
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.
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 can be maintained at almost any flow rate when a hot tap is turned on.
Common problems:
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.
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.
