No hot water from Pulsa-coil A-class

Ok so I'm an idiot and stupidly let the top up tank on top of the unit become empty, i.e it was bone dry when I finally topped it up. I don't know how long it was like that but for the past weeks the heater has been noisy overnight and then also during the day. Then the hot water stopped coming, which I what prompted me to investigate and realise the tank was empty.

However having topped up there is still no hot water. We're on Economy7 but it used to be we'd get hot water all day, but now there's not even enough to do the dishes after breakfast. Our usage patterns haven't changed. It's emitting a low humming noise a bit like a central heating boiler, which I guess is a pump running, iirc it didn't used to do this when it was "ok", at least not continuously. The level in the top up tank hasn't changed since topping up. It still works on the boost. Any suggestions please?

Ah, I think this post could explain the problem:
Top tank had emptied, and I think something has blown...

What are the consequences of overheating? In the manual it mentions an "overheat stat tripped" as a fault cause. Can this be reset? Can anyone advise how I would go about doing that or is it a job for a professional?

Hi

The overheat sensor on the Pulsacoil A Class is combined with another sensor and they work on resistance values (NTC) so they don't actually trip out, they just let the control board know the exact temperature of the unit, and therefore they don't need resetting.

The most probable cause is that the unit has airlocked itself and although the tank is full, the water may not have filtered down into the unit. Have a look at the small pipe coming from the tank and see if there is a vent point (just like you would get on a central heating radiator).
If the unit itself is hot, but you still have no hot water and there is no air at the vent point, then you will need to get someone in to look at it.

Hi, many thanks for the response. I'll check for a vent when I'm at home, but I don't remember seeing one external to the unit.

Hi,

Right, so there's no sign of a vent valve. I tried to bleed it by taking the plug out of the front of the hot water pump but nothing much happened - warm water just tricked out at a rate of roughly 1 drop per second.

The digital display doesn't indicate any faults and the appropriate lights appear to come on and off in the day, night (off-peak) and when the boost is selected.

The pump seems to run continuously, or at least the light corresponding to "pump on" on the display is lit, regardless of what the taps or heater are doing. You can feel/hear the pump running but sometimes it stops with the light staying on. The pipes into and out of the pump are slightly warm but the pump body is definitely hotter than the pipes. I took out the plug while the pump was running and could see the rotor spinning and the same slight trickle of water came out.

After a night supposedly heating up on off peak electricity, there's no hot water again in the morning. The pump seems to be running whenever I check on the device and again the pipes in and out of it are relatively cool.

I'm about to call on a plumber, but can't shake the feeling the thing's still empty. Is there any other way to check water from the top up cistern's actually making it into to the heater?


-------- UPDATE -----------------
After a bit more poking around I've discovered the relay for the off-peak imm heater has burnt out. Am getting continuity across the heater itself so hopefully that's okay. Interestingly, the digital display isn't showing any faults although immersion heater failure is given as one of the 'common fault codes' in the manual so presumably the unit's capable of detecting this. Anyway, fingers crossed for what should hopefully be a cheap fix.