The internal heat exchanger of the Pulsacoil Original was eventually abandoned in favour of an external ‘plate’ heat exchanger (probably due to scaling problems with the internal one), and the Pulsacoil III was the result. There was no Pulsacoil II, I don’t know why.
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 further 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 faults I've encountered:
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 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.
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.
If you'd like me to visit and carry out a repair for you, contact me by calling of texting my mobile 07866 766364
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The top-up cistern (header tank) on a Pulsacoil III, showing the embossed water level marker line just above the middle, for topping up to the correct level.
Copyright Michael Bryant 2020
Site first published 2nd January 2007
Last updated 21st March 2020
Gas Safe Register 197499, CIPHE registration number 56207