Immersion Heater, thermal cut out (not thermostat)

[tugby]

I have a twin coil calorifier.

Coil 1 on engine coolant circuit,

Coil 2 on Eberspacher heating circuit.

Horizontal calorifier under double bed in aft cabin.

 

Additionally, there is a 2kw electric immersion heater. This heater has a controlling thermostat set about 60C and a safety thermal cut out to prevent tank boiling in the event of the controlling thermostat failing on.

 

I understand this is now a legal requirement in domestic/housing situation. I think the cutouts were brought in when a tank boiled, went up into the plastic loft tank which then burst, with the result of scalding water pouring through the ceiling below into a bedroom and onto a sleeping person.

 

Anyhow, the thermal cut out on here has popped out a couple of times recently needing the little black button pressing back in to reset it in order to get the immersion to go on. We have only just started using the immersion seriously now the eberspacher is back on part time duties.

 

I THINK, the cut out is tripping because the calorifier tank gets above the immersion safety trip temperature when the engine or the eberspacher has been running and presumably taking the tank temperature up to 85C or so?

 

Does this sound plausible? If so, can I safely link out / bypass the safety thermal cut out on the immersion heater given that its purpose is not relevant in the location the calorifier is fitted?

 

What do you think?

 

Many thanks.

[nicknorman]

I think bypassing safety devices is a very bad idea. Consider what would happen if the thermostat stuck on. Firstly the water would boil and spray out of the PRV - depending on where the PRV exits to that might or might not be a problem. After a while the heater element is no longer immersed in water, now 2kw of heat has to go somewhere! If you are lucky, it will overheat the heating wiring and stop working. Unlucky and it will get red hot, heat up the calorified to well above 100 deg and maybe set fire to something.

 

Perhaps you are right about the engine being too hot though I doubt it because the cutouts are set fairly high and other boats don't have this problem. More likely its limescale build up on the heater element which is causing it to get too hot due to reduced heat flow away from it into the water. Why not remove the element and descale it? - although whilst you are doing that might be better to just fit a new element.

[Phoenix_V]

I've had this problem from new, cut out fine until you start drawing water off then it goes.

I assume air is getting into the top of the tank some how causing a build up of heat but I have vented it many times through the prv with no improvement.

The rest of the tank is certainly not getting too hot.

 

Any ideas welcome!

[nicknorman]

I've had this problem from new, cut out fine until you start drawing water off then it goes.

I assume air is getting into the top of the tank some how causing a build up of heat but I have vented it many times through the prv with no improvement.

The rest of the tank is certainly not getting too hot.

 

Any ideas welcome!

Perhaps the immersion element (incorporating the cut-out) is simply defective?

 

Do I understand you correctly that with the immersion on, the problem only arises when you draw off hot water? Is the element vertical or horizontal (ie boss at the top or the side?)

 

For info, our engine runs fairly cool at around 70 deg C so hot water from that is not that hot. Our Mikuni runs at nearer 85. But the thing that gets the water hottest is the immersion. In fact I turned the thermostat down a bit because it seemed close to boiling. However, the cut-out has never operated, so its not a "usual problem". I believe the cut-outs operate at about 95 deg so if your water is getting that hot, something is wrong.

 

Only other thought is do you have insulation right over the immersion cap? I suppose its possible that this could cause overheating right at the top where the cutout is.

[Phoenix_V]

Perhaps the immersion element (incorporating the cut-out) is simply defective?

Always possible but it has been like this from new, are they easy to change?

Do I understand you correctly that with the immersion on, the problem only arises when you draw off hot water? Is the element vertical or horizontal (ie boss at the top or the side?)

yes thats right it cuts out shortly after, it's a vertical one

For info, our engine runs fairly cool at around 70 deg C so hot water from that is not that hot. Our Mikuni runs at nearer 85. But the thing that gets the water hottest is the immersion. In fact I turned the thermostat down a bit because it seemed close to boiling. However, the cut-out has never operated, so its not a "usual problem". I believe the cut-outs operate at about 95 deg so if your water is getting that hot, something is wrong.

Engine, Alde and immersion are all set fairly low, neither the water nor top of the cylinder have ever felt over hot

Only other thought is do you have insulation right over the immersion cap? I suppose its possible that this could cause overheating right at the top where the cutout is.

No, not covered at all

All I could think is air gets in to the very top of the tank where the cut out is as water is drawn off and causes a local hot spot, but I cant see how as said I have bled it lots of times with the prv

Edited April 30, 2013 by Phoenix_V

[nicknorman]

Always possible but it has been like this from new, are they easy to change?

 

Easy in principle in that they just screw in. You would need the appropriate tool - a wide box spanner. You have to bear in mind that the copper tank is fairly thin and so apply an even force to both sides of the box spanner. Maybe some heat would be required (blowtorch) but hopefully not if its fairly new. Of course you have to be careful to isolate the mains power and remove and refit the wiring correctly and safely!

 

yes thats right it cuts out shortly after, it's a vertical one

 

I suppose that when you draw off hot water, cold replaces it and reduces the temperature to the point the heater cuts in.

Engine, Alde and immersion are all set fairly low, neither the water nor top of the cylinder have ever felt over hot

No, not covered at all

All I could think is air gets in to the very top of the tank where the cut out is as water is drawn off and causes a local hot spot, but I cant see how as said I have bled it lots of times with the prv

 

Sounds to me more like the heating element is defective. Either the thermostat not cutting out, or the cut-out operating far too soon. When I drain the calorifier for winterisation (we tend to go down to the boat a fair bit in winter but have to winterise in the mean time) on refilling there is loads of air at the top - even after most of the air has been pushed out when cold, because the outlet is not right at the top, once the water has heated up using the immersion (and heated up / expanded the air) loads more air comes out. But we have never had the cut-out operate.

 

<Disclaimer> However, bear in mind I am not a professional plumber nor heating engineer and you might be better waiting for some other views before taking action!

[ditchcrawler]

 

Always possible but it has been like this from new, are they easy to change?

 

Easy in principle in that they just screw in. You would need the appropriate tool - a wide box spanner. You have to bear in mind that the copper tank is fairly thin and so apply an even force to both sides of the box spanner. Maybe some heat would be required (blowtorch) but hopefully not if its fairly new. Of course you have to be careful to isolate the mains power and remove and refit the wiring correctly and safely!

 

yes thats right it cuts out shortly after, it's a vertical one

 

I suppose that when you draw off hot water, cold replaces it and reduces the temperature to the point the heater cuts in.

Engine, Alde and immersion are all set fairly low, neither the water nor top of the cylinder have ever felt over hot

No, not covered at all

All I could think is air gets in to the very top of the tank where the cut out is as water is drawn off and causes a local hot spot, but I cant see how as said I have bled it lots of times with the prv

 

Sounds to me more like the heating element is defective. Either the thermostat not cutting out, or the cut-out operating far too soon. When I drain the calorifier for winterisation (we tend to go down to the boat a fair bit in winter but have to winterise in the mean time) on refilling there is loads of air at the top - even after most of the air has been pushed out when cold, because the outlet is not right at the top, once the water has heated up using the immersion (and heated up / expanded the air) loads more air comes out. But we have never had the cut-out operate.

 

<Disclaimer> However, bear in mind I am not a professional plumber nor heating engineer and you might be better waiting for some other views before taking action!

If you are changing the element then crack the thread with the cylinder still full of water, just a quarter turn or less, it wont leak but the water supports the cylinder.

[smileypete]

Usually the stat just sits in a 'pocket' so it's easy to pull out and replace, check the new stat has a high temp on the secondary cut out, eg see these halfway down:

 

http://www.bes.co.uk/products/107a.asp

 

While you're at it check the cable to the immersion is the proper heat resisting rubber not PVC, and the connections are secure and haven't overheated.

 

ETA: Usually the stat is set to 60-65°C in hard water areas to help reduce the buildup of limescale.

 

cheers, Pete.

~smpt~

Edited April 30, 2013 by smileypete

[Phoenix_V]

This is the unit, http://www.surecal.co.uk/documents/Immersion%20heater.pdf the safety stat is completely separate to the main stat and has to be reset manually when it trips which is a nuisance and one day I will forget to switch the mains off before going there!!

 

I presume it doesnt go deep into the cylinder like the main stat so is more likely to be affected by air pockets?

 

I did not know you could get a stat with the safety stat built in I think that might be worth a try (bypassing the old one). But I will have a word with Surcal first and see what they have to say.

 

Good point about heat resisting cable, I will change that, but at the moment it is certainly not getting too hot!

[nicknorman]

I'm confused! Are Phoenix-V and tugby the same person?

[ditchcrawler]

I'm confused! Are Phoenix-V and tugby the same person?

I don't thinks so, just got the same problem is the way I read it but I could well be wrong. Different boat names as well

[Phoenix_V]

I don't thinks so, just got the same problem is the way I read it but I could well be wrong. Different boat names as well

Sorry I hijacked the thread when I saw that he had the same (almost) issue.

Mine is vertical and always had a problem, his is horizontal and problem appears to be new so may not be related after all.

Thanks everyone for their help (though any more suggestions welcome). I will take the steps above and report back.

[tugby]

Nope, Pheonix V and Tugby are different (my boat is now Andromeda though, no longer Tugby).

 

I have looked at the stat and cut out settings on my immersion heater today. The stat is set to 60C and the cut out was set to 85c which may be too close to the Eberspacher output temperature causing the immersion safety cut out to operate regularly. This means I have to take all the bed stuff off to get to the tank to press the little reset button on the cut out, and like the poster above, one day I will forget to turn the power off first as it becomes a routine and boring activity. I see that a link above shows heater elements that have a the cut out set at 98C and coincidentally I have wound mine up to 95C today so will see what happens when the Eber runs for a couple of hours first thing tomorrow morning.

 

Thanks for all the input. More people must experience this unless they have immersion elements without the safety cut out as they are still widely available and permitted in certain arrangements.

 

Bob.

[AndrewIC]

I had a similar problem, after a long day's running the engine would get the calorifier hotter than the immersion cutout. I found that the cutout on my immersion was adjustable (hidden under a sticker), and upping it a few degrees was enough to stop it tripping.

 

The key thing, as someone else has noted, is to stop the water boiling in the event that the regular thermostat contacts fail to open, especially on a pressurised system like boats have.