Lukewarm water after overheating. Curious case.

A week or so ago I heard the PulsaCoil 2000 making quite a racket - I went to check it and found the cupboard it is in to be very steamy! The header tank was nearly full and the pipe going into it was splurging boiling hot water (and air) into it. I turned it off at the wall and left it overnight. The next day I turned it on; however, the water only ever came out lukewarm. This did not change for a few days and it didn't make its normal "roaring" sound when I turn it on and it's heating up.

I got a boiler repair man out; sadly, he didn't seem to know the PulsaCoil 2000 too well. He said the thermostat was broken in the lower immersion heater - I could see it was as it had in fact nearly melted! He changed the thermostat and assured me it'd work. He checked the electricity through the thermostat, the pump (and maybe the plate exchange - can't remember if he did this one) and it was all ok - hence why I assume he thought he'd fixed it - because he had not. I pointed out it was not making the roaring noise it usually does but he assured me it would do shortly.

My PulsaCoil only uses the one immersion and I have to manually put it on about 30 minutes or so before I wish to use hot water. For a while I had left it on 24/7 - the repair man tells me this is what broke the thermostat as it would've caused it to overheat. When he opened her up I noticed the manual override switch needed resetting - he told me it's working correctly but the temperature of the manual override is just higher than the other thermostats and it allowed it to get a little too hot before it popped the reset button (hence the steam in the cupboard, etc).

Anyway, I got the guy back out and he suggested there could be an airlock. This made sense to me as the system kept making sounds as if water was rising and popping inside , something like "glub glub glub" - the boiler had never made these sounds before. We bled the pump but to no avail. However, curiously, the pump makes a much louder sound when the bleed screw (sorry, unsure of its name) is 1/4 turn or so unscrewed - it sounds more like it usually does when it is running. I can confirm the pump is spinning (prodded it with a screwdriver). Strangely, when I undo the pumps screw somewhat and then start the boiler it sounds like the heat exchange is filling up with water and air is rising in the immersion heaters (sounds like air is going up and to the box on top!)

I have completely drained and replenished the water in the system and as I was filling it up it made the same sounds (water bubble rising and popping: "glub glub glub") as when it runs with the pump bleed unscrewed. I am at a loss as to what the issue is and what it is causing the sounds. I thought completely draining and refilling the system would rid it of any air locks but it doesn't seem to have.

For what it's worth, the red light is flashing quite rapidly - I think it's telling me the DWH Sensor is knackered. But am unsure how that being broken would cause these symptoms?

Any help would be greatly appreciated - even if just so I can talk on a better level to the repair man coming out tomorrow.

Thanks in advance.

As an update to this: I moved the DHW sensor wire and the fault code has now gone. I also checked the new thermostat the plumber fitted and saw that it’s max temperature was 60 set and it was set one notch back (so, as there are 5 notches I am guessing at 48 deg). The plumber did tell me he’d put it on slightly less than maximum; however, 48 deg is too low IMO - I was under the impression that the standard Gledhill therms max our at 73/76 deg so unsure why he fitted a therm with a max temp of 60! Anyway, the issues still persist: lukewarm (albeit slightly warmer than before) water and lots of “bubbling” of air. I must say though, that since I drained and refilled the system the lukewarm water lasts longer - I can get a full shower (3-5 mins or so) without getting absolutely frozen (maybe I’ve just become accustomed!)

Any help would be greatly appreciated.

Thanks

A sad and complicated tale. Unfortunately, choosing who to get around to fix your Gledhill is the most critical part of actually getting it fixed.

But starting from here, the thermostat in your heater really should be set to MAX. 75c roughly as you say, but 60c will do at a push.

I'm puzzled about the 'bubbling' of air you mention. Is this air coming from the hot taps? Bubbling out of the open vent? Or something else? Whatever, it isn't right.

How hot do the two pipes out of the top at the back, that go to the plastic header tank? The fat one should be piping hot to the touch.

As a shot in the dark I'm inclined to suspect the fault is water scale in the heat exchanger. Before the overheat, the tank was prolly heating up to near 100c which is why it 'roared'. This disguised the lack of heat transfer inside the hex when you have a tap running. Now the store is only at 48c, the tap water isn't being heated enough due to the scale. As an experiment turn a hot tap ON and feel the temperature of the water in the pipe through the pump. It should be no more than luke warm. I bet yours is really hot. If really hot, a new heat exchanger is the fix.

Hope that helps...