Looking for an explanation

[dpm_dpmartin]

Hi all,

I moved into my current house in 2014 and for around 2 years my central heating and hot water heating worked perfectly fine. Some time in the middle of last year I had an 'incident' where I heard lots of hot water gushing out of an safety overflow pipe and out into the waste.

I called out an emergency plumber who worked on unvented systems and they pumped out the air from a smaller header cylinder. At the same time they told me that I'd been heating my water for around 2 years via 2 immersion heaters attached to the giant 300l cylinder. They suggested I turn those off and just use the gas to heat the water, as it'd be more efficient / cost effective.

Although it is certainly more noisy, as the boiler fires up more.

I had this niggle at the back of my mind that the showering experience wasn't as good. Not as hot and not as powerful. But I let it go. I thought I could get around to programming the hot water heating at some point. I never did.

Anyway, it had been impressed upon me at the time of the call-out that my system and cylinder should be serviced annually. So I heeded that advice and I had someone around in February to do just that.

As part of that, they made a recommendation that the dial on the side of the cylinder (my first picture) be turned from 50 degrees down to 40 degrees and, again, I followed that advice.

The entire household is now convinced that the showering experience is even worse. Water is not pushed out now, it more just falls out... hot water emitted from taps is still fine, but first thing this morning I took the, probably daft, decision to do 2 things. Yes, 2, at the same time!

1) I moved the dial on the side of the cylinder back from 40 to 50.
2) At the same time I also turned the top immersion heater on.

Lo and behold - the showering experience about an hour later was back on top form. I have received much praise from within the household.

Result.

But, no. Before lunchtime I heard that same water gushing sound and I could see water escaping from the system, via the see-through overflow, and going into the waste.

Damn!

So I turned the immersion heater off, I turned the dial back down to 40 and I ran off a load of hot water into a bath - about half full - then I saw the water stopped going through the pipe.

My question #1 is - what is this dial doing? Why do I think it would be a thermostat (a target temperature) for the 2 immersion heaters - if the immersion heaters are turned off, isn't it defunct?)?

My question #2 is - is there a potential fault with my immersion heater, is it continuing to heat my water even once it's reached the temperature it should get to - is this why it's 'blowing off'? Is there a separate thermostat within the immersion heater?

Like anyone, all I want at the end of the day is a good showering experience without having to wonder if my system is going to start ejecting hot water into the waste. Would anyone be able to take the time and explain to me what these things I've taken pictures of do - as I had thought that if I'd disabled the immersion heaters the hot water cylinder was just kept hot by the boiler doing its thing.

 

[Mottie]

There will be a separate thermostat for the immersions under that cap in the second picture. Remove that brass nut to remove the cover but switch it off at the spur first. The top picture is the thermostat for the boiler heated hot water only. That's all I know! I would suspect your immersion thermostat(s) are set higher than the boiler thermostat or perhaps the immersion thermostat is not shutting the immersion heater off causing it to boil the water and make too much pressure in the system, blowing the safety valve?

 

[picasso]

Put the black dial on min 55c, that will heat the water in the cylinder using the boiler, do not use the immersion heater unless your boiler is broken, what make of cylinder do you have and can you post a pic of "smaller header tank" you metioned ?

 

[Lower]

You have a problem either with the expansion vessel or with a thermostat. You need someone to look at that for you.

 

[dpm_dpmartin]

Thank you all, that's very helpful and I've now left the immersion heater off and I've moved the black dial to 60 - the boiler has fired up to handle that call (as central heating is not being called for). I will monitor and test how the shower is performing tomorrow.

I've included a bigger picture of all the stuff here. It's a Gledhill cylinder. The smaller header tank has Aquasystem written on it. When the 'incident' I refer to happens, you see scalding hot water going through that top-most black plastic connector with a hole in it. That's, so far, only happened when the immersion heater(s) is / are on - in 2016 and today. I was wondering whether it was because the thermostat in the immersion heating was duff, therefore it just continues to heat the water in the cylinder even when it's reached temperature. It's certainly not its ability to heat water that doesn't work, that's for sure.

I am wondering why the last person recommended to me that the black dial should be pulled back to 40.

It might look like the bottom immersion heater is also switched on, but it's the switch that's broken - the guy who sorted the system out on the day it first happened turned both immersions off, so said I could get myself a new switch if I wanted, or leave it. He isolated it electrically.

 

[dpm_dpmartin]

...or perhaps the immersion thermostat is not shutting the immersion heater off causing it to boil the water and make too much pressure in the system, blowing the safety valve?

This was my uneducated thinking, it's good to know it might actually be a possibility (I mean, I was potentially thinking along the right lines).

 

[JohnD]

if you follow Picasso's advice, and it cures the problem, the next thing to do would be to see if your immersion heater is faulty and is overheating. This can be costly and sometimes dangerous. If so, you should not turn the immersion heater on until it has been repaired or replaced. Modern immersion heaters have a safety overheat cutout which will operate if the thermostat fails. As you have an unvented cylinder, it should be maintained by a heating engineer with the extra qualification to work on them.

 

[dpm_dpmartin]

As you have an unvented cylinder, it should be maintained by a heating engineer with the extra qualification to work on them.

Absolutely! I have a DOMESTIC UNVENTED HOT WATER STORAGE VESSEL COMMISSIONING/INSPECTION RECORD for the service done by a qualified fellow in late Feb.. Still unsure why it was recommended to turn that black dial down to 40, from 50, though (I may well ask him)... anyway, it's up at 60 now and we'll see what happens... hopefully no further blow-off.

 

[dpm_dpmartin]

Intriguing, I'm just looking at that certificate, and it says - in Observations (if any) / Recommendations (if any) - "hot water thermostat reduced to 50° C, checked immersion settings for temp" - but that's not where it ended up - it ended up at 40° C - and not by me doing it.

 

[Mottie]

......... perhaps the immersion thermostat is not shutting the immersion heater off causing it to boil the water and make too much pressure in the system, blowing the safety valve?

That was my thinking too. I wouldn't be surprised if that had been your problem from the very first time it blew off.

 

[picasso]

I would have the white expansion vessel checked for the correct precharge, lack of pressure would cause the discharge and lack of pressure in the shower.

 

[dpm_dpmartin]

My thinking had been this...

Lack of pressure / force in the showers was due to their thermostat not being able to provide enough hot water in the mix, due to the boiler setting being at 40°. Once I turned that up to 50° and turned on the immersion (not knowing any better) the shower pressure was nice, and it was toasty.

Now the immersion is off and the boiler setting is at 60° I need to re-test the showers, probably tomorrow.

I will also probably take the cap off that top immersion heater and try to figure out if the thermostat is a) set very high or b) broken.

 

[Diver Fred]

Gosh, haven’t you been caught in the various ‘rule’ changes…

Some years ago there was a ‘ruling’ (for domestic, rented properties) given that the hot water system could not exceed 50 degrees C at point of delivery – i.e. the tap tp prevent scolding. If you visit industrial premises you see warning signs over hot water taps saying ‘caution, very hot water’ - who wants those signs in their houses…
I guess your emergency plumber was working to those rules.

Last year there was an edict to landlords that the hot water systems had to be checked for prevention of ‘Legionnaires disease’; How? By checking the hot water temperature at the hot tap – it has to be in excess of 55 degrees C…
So now the thermostats have to be adjusted to cause the water to be heated to 55/60 degrees C, (which is where we were some years ago…).

From your report the thermostat to the top immersion heater has either failed ‘closed’, not fitted or has been missed when the immersion was wired up. If it’s fitted it can be replaced without draining the system down. Switch the immersion circuit off at the fuse box, switch off at the FCU by the tank, take the cap off the immersion cover; the ‘stat has two wires and a small dial on it, disconnect the wires and pull the ‘stat out. Replace with one the same length, set 55/60 degrees C and reconnect and replace cover and switch the mains back on.

I also think you have another problem - I believe there should be a timer controller for the upper immersion to supply power to the upper element for about one hour.
The lower element should be wired to the overnight (economy 7) fuse board, the upper element to the 24-hour supply.

 

[picasso]

The expansion vessel is normally pumped up to around 3 bar (depending on incoming pressure) as the water heats up (boiler or immersion) it raises the pressure in the cylinder, the higher the temp the more hot water pressure you will have, the discharge happens when the water expands as its being heated and has no where to expand into,
that's why its important to have the expansion vessel at the correct precharge.

 

[dpm_dpmartin]

Hi - thanks for all this. It wasn't the emergency guy who changed that dial, setting it lower, it was the guy (different guy) who carried out the recent service of the boiler and cylinder - but I think I see, now, what was happening there - that's good to know - although it ended up at 40°, not 50°. If I need to reset it back to 55° or 50° then I will (after some testing of the shower).

I think the immersion heaters worked fine since the system was installed in 2011 and I bought the property in 2014 - for at least 2 years. I think something has broken around mid-2016 (whenever it was, can't recall exactly) - knowing what it is that has broken could be hard, but there's obvious options now. However, the important thing is that I now know what that black dial does and that it's not controlling the immersion heaters, which was what I originally thought. That information alone has been incredibly helpful.