Trouble diagnosing boiler fault

Hi there,

I'm having trouble working out what's going wrong with my Pulsacoil 2000. The boiler is connected to a Grasslin Ecosave timer which flicks the boiler between on-peak and off-peak modes and has an optional boost.

Basically, I don't seem to be getting any hot water. When I run the taps, it sometimes feels like the water starts to heat up but then it gets colder again. I've tried setting the timer so that it is off peak the whole time and I've tried setting it so that it is on peak the whole time but neither seems to change the temperature (I wouldn't expect on peak to heat the tank but thought I'd give it a go). The boost button however, does work and we've been able to have hot showers and wash up etc using this.

I opened up the boiler and the LED on the circuit board was flashing once per second but having left the boiler to look up the LED meanings I returned to find that it was flashing twice per second. It spends most of the time flashing like this but occasionally goes back to flashing once per second and I did catch it, just once, flashing rapidly (could be 5, could be 7).

I could just jump in and start replacing sensors but I thought I'd post a question first just to see if there were any other likely causes for the problem (and because it seems to be an unclear intermittent fault). Any thoughts/suggestions would be gratefully received!

Thanks

Hi Mark

The problem you are describing indicates sensor faults.
Flashing once per/sec indicates a faulty PHE sensor (the grey one that's below the heat exchanger), whilst flashing sevent times/sec indicates a faulty hot water sensor (the white one in the mains water above the heat exchanger).

Both sensors can give intermittent and erratic fault patterns that can be tricky to trace. Replacing these sensors should cure your trouble.

Hi David,

I have replaced the PHE temperature sensor (which I presume is no more complex than sticking the sensor into the dry hole in the pipe and sealing the top with the rubber seal - because that's what I've done!) but this does not appear to have solved the issue.

Given that the boost button gives us hot water reliably, presumably this means that the DHW sensor is working okay and there is no need to replace this (as the system seems to be able to regulate hot water temperatures once we have hit boost)? Similarly, presumably this also means that the pump is okay?

I was wondering therefore if it was possible that the immersion coil for the off peak supply was broken? Is there an easy way to check this? Alternatively, if there are any other likely or even possible causes for the fault I'd be happy to hear suggestions!

Thanks!

Hi MArk

I seem to have missunderstood your initial fault. Reading your reply seems to say that you can get perfectly good hot water with the on peak element (boost) buty not with the off peak (overnight) element.

Testing the element resistance should only be done by someone who is competent with electrticity and ALWAYS with the power isolated. The resistance value is approx 18 Ohms, and this will give a current draw of around 12.7A.

It is also probalbe that the bottom element overheat thermostat has tripped out. This can be reset, but if it continually trips then the unit will need checking by an engineer.

Hi David,

Okay, so I have done a few checks around the immersion coils. Having removed the cover over the immersion wiring, I checked the impedance between the neutral/earth point and both sides of the bottom immersion. I get 18ohms between the negative terminal (of the plastic device in-line with the live) and the neutral but an open circuit between the positive terminal and the netural (i.e. there is an open circuit across the plastic device and 18 ohms through the coil). Carrying out the same set of measurements on the top immersion coil I get 18 ohms between both terminals and neutral (i.e. a short circuit across the plastic device and 18 ohms through the coil).

Would I be right in assuming that the plastic device is the element overheat thermostat that you were referring to? Given that I am able to measure 18 ohms through the coil I would also assume that this means that the coil is okay but the thermostat (or fuse or what ever that plastic device is) is tripped/broken. It's not entirely clear to me how I might reset this however, there is a circular dial on the back of the device with an arrow (currently pointing from - to +) but turning it so that it points the other way makes no difference.

Is this device something that I can replace without having to replace the whole coil, alternatively am I missing a trick with resetting it?

Many thanks, feel I'm close to getting there now!!

Just an update, ignore my vague ramblings, I've gone off and done a bit more research and am now happy that the problem is the bottom immersion heater thermostat (and that the dial that I was twiddling was the temperature control!). I've ordered a replacement part and hopefully this will take care of the problem.

Many thanks.