Dual immersion-long element not heating whole cylinder.

[shackec]

I have recently replaced a copper cylinder with stainless steel cylinder. I have also, at the same time, replaced a single element immersion heater with a dual element. (Sink/bath) suitable for a stainless tank. Both tanks are the same size, both vented, both insulated, and both fitted with an unused single coil. (The water has only ever been heated electrically; the coil is redundant). Inlets and outlets identical-not changed.

The original immersion heater was 27” and heated the entire tank. The new immersion heater has a short element of about 14” (2KW) and a long element of 27” (2.85KW). When the sink/bath switch is on “sink” the top half of the new tank heats up; so presumably the short immersion heater is working.
When the sink/bath is on “bath” it seems that only the top half of the tank is heated the bottom half remains cold; almost as if the bottom half of the element is not working!
I have continuity tested both elements and both seem fine. The voltage across the elements is 230V on both elements when switched on. The resistance of the short element 28 ohms and the resistance on the long element is 20 ohms.

I am at a complete loss as to understand why the original 27” element would heat the entire tank but the replacement 27” element only heats the top half. I cannot imagine a defect in the element that could cause this.
Please anyone-any ideas?

 

[ericmark]

Thermostat switching off element too early? If the thermostat is at the top and the water at the top is already warm, then it will not heat the water further down the tank.

Only the Irish seem to have got around the problem, the willis system has the immersion heater outside the tank, and heats from the top down, so the longer left on, the more of the tank is heated, but it seems rest of UK we are too tick to work out how to do it.

I have looked at it many times, but not willing to take the plunge and install it, as not sure if Welsh plumbers could get it to work.

But I would test by allowing tank to cool down first, and then switch on bottom element.

I have wondered how the Willis system would work with an iboost+ when not putting in a full 3 kW, and I note the iboost+ is designed for twin immersion heaters. I am an electrician not a plumber, so would also be interested in answers as I for first time in 40 years have started using an immersion heater as it is powered from spare solar energy.

 

[shackec]

Thanks Eric; I had thought of this so I tried cranking up the temperature setting on the thermostat. Didn't seem to make any difference. It is hard to be precise because the insulation makes it difficult to know how much of the tank is heated. I have to judge by how much hot water I can run off. I can use the external threads of the coil (which are exposed) as some indication.The bottom one never gets even warm.
The thermostat sensing element is the same size in both immersion heaters. I can't see how a different thermostat would make so much difference to the heating when, otherwise, the installations are identical.
I'm missing something ; just wish I knew what!

 

[stem]

How tall is your hot water cylinder?

If the 'bath' element is 27" long and it's mounted at a slight angle it will probably extend no more than 24" into the cylinder. Therefore any portion of the cylinder below that, whilst it might get warm, it won't get hot. This is because heat rises upwards from the element and the cold water filling the cylinder enters at the bottom. With most hot water cylinders, it's quite usual for the bottom quarter / third of the cylinder to be cool or cold.

 

[Johntheo5]

I have recently replaced a copper cylinder with stainless steel cylinder. I have also, at the same time, replaced a single element immersion heater with a dual element. (Sink/bath) suitable for a stainless tank. Both tanks are the same size, both vented, both insulated, and both fitted with an unused single coil. (The water has only ever been heated electrically; the coil is redundant). Inlets and outlets identical-not changed.

The original immersion heater was 27” and heated the entire tank. The new immersion heater has a short element of about 14” (2KW) and a long element of 27” (2.85KW). When the sink/bath switch is on “sink” the top half of the new tank heats up; so presumably the short immersion heater is working.
When the sink/bath is on “bath” it seems that only the top half of the tank is heated the bottom half remains cold; almost as if the bottom half of the element is not working!
I have continuity tested both elements and both seem fine. The voltage across the elements is 230V on both elements when switched on. The resistance of the short element 28 ohms and the resistance on the long element is 20 ohms.

I am at a complete loss as to understand why the original 27” element would heat the entire tank but the replacement 27” element only heats the top half. I cannot imagine a defect in the element that could cause this.
Please anyone-any ideas?

Just run off say 40/45L of HW, that should certainly get the stat to reset and if wired correctly, then with bath selected should heat the larger volume of water, mine certainly does, its a 150L cylinder and the top eleme t heats exactly 29L.
Can you post a photo of the immersion with cover removed, the black wire should go to the sink terminal, the brown wire to the bath terminal, both element neutrals are joined together with the stat then switching ON/OFF via the common neutral, there will also be a common earth.

 

[Johntheo5]

I have recently replaced a copper cylinder with stainless steel cylinder. I have also, at the same time, replaced a single element immersion heater with a dual element. (Sink/bath) suitable for a stainless tank. Both tanks are the same size, both vented, both insulated, and both fitted with an unused single coil. (The water has only ever been heated electrically; the coil is redundant). Inlets and outlets identical-not changed.

The original immersion heater was 27” and heated the entire tank. The new immersion heater has a short element of about 14” (2KW) and a long element of 27” (2.85KW). When the sink/bath switch is on “sink” the top half of the new tank heats up; so presumably the short immersion heater is working.
When the sink/bath is on “bath” it seems that only the top half of the tank is heated the bottom half remains cold; almost as if the bottom half of the element is not working!
I have continuity tested both elements and both seem fine. The voltage across the elements is 230V on both elements when switched on. The resistance of the short element 28 ohms and the resistance on the long element is 20 ohms.

I am at a complete loss as to understand why the original 27” element would heat the entire tank but the replacement 27” element only heats the top half. I cannot imagine a defect in the element that could cause this.
Please anyone-any ideas?

Just run off say 40/45L of HW, that should certainly get the stat to reset and if wired correctly, then with bath selected should heat the larger volume of water, mine certainly does, its a 150L cylinder and the top eleme t heats exactly 29L.

Thanks Eric; I had thought of this so I tried cranking up the temperature setting on the thermostat. Didn't seem to make any difference. It is hard to be precise because the insulation makes it difficult to know how much of the tank is heated. I have to judge by how much hot water I can run off. I can use the external threads of the coil (which are exposed) as some indication.The bottom one never gets even warm.
The thermostat sensing element is the same size in both immersion heaters. I can't see how a different thermostat would make so much difference to the heating when, otherwise, the installations are identical.
I'm missing something ; just wish I knew what!

Thermostat switching off element too early? If the thermostat is at the top and the water at the top is already warm, then it will not heat the water further down the tank.

Only the Irish seem to have got around the problem, the willis system has the immersion heater outside the tank, and heats from the top down, so the longer left on, the more of the tank is heated, but it seems rest of UK we are too tick to work out how to do it. View attachment 332125 I have looked at it many times, but not willing to take the plunge and install it, as not sure if Welsh plumbers could get it to work.

But I would test by allowing tank to cool down first, and then switch on bottom element.

I have wondered how the Willis system would work with an iboost+ when not putting in a full 3 kW, and I note the iboost+ is designed for twin immersion heaters. I am an electrician not a plumber, so would also be interested in answers as I for first time in 40 years have started using an immersion heater as it is powered from spare solar

Can you post a photo of the immersion with cover removed, the black wire should go to the sink terminal, the brown wire to the bath terminal, both element neutrals are joined together with the stat then switching ON/OFF via the common neutral, there will also be a common earth.

 

[Johntheo5]

Thermostat switching off element too early? If the thermostat is at the top and the water at the top is already warm, then it will not heat the water further down the tank.

Only the Irish seem to have got around the problem, the willis system has the immersion heater outside the tank, and heats from the top down, so the longer left on, the more of the tank is heated, but it seems rest of UK we are too tick to work out how to do it. View attachment 332125 I have looked at it many times, but not willing to take the plunge and install it, as not sure if Welsh plumbers could get it to work.

But I would test by allowing tank to cool down first, and then switch on bottom element.

I have wondered how the Willis system would work with an iboost+ when not putting in a full 3 kW, and I note the iboost+ is designed for twin immersion heaters. I am an electrician not a plumber, so would also be interested in answers as I for first time in 40 years have started using an immersion heater as it is powered from spare solar energy.

I assisted someone who installed the Willis with Iboost but a pipestat had to be installed on the willis outlet to do the switching on/off as the willis stat just couldn,t handle it as the temperature outlet was extre.ely high, so used the pipestat as the control stat and the willis stat as the hi limit stat, the Iboost works well I thi k and only diverts any export to the willis, the element is a 3kw but if i wS installing one I would try and get o e with a 2/2.5kw element as the e cessive temerature rise only seems to occur if eleme t power is > 1.25/1.5kw.

 

[ericmark]

As @Johntheo5 has pointed out Willis install is not straight forward, I thank him for his reply, and since I want to sink the excess power when it is available think for me a longer element is the answer, mine goes in the top.

I am no plumber, I am an electrician, and working out thermals within a hot water cylinder is a bit beyond my ken. We have had timers like this for years the idea is to heat all the tank over night, at cheap rate, and just the top during the day, we did in the 50's have selection for bath or sink, but since then we have improved the insulation on the tank, I know my tank will stay warm for 2 to 3 days, so the saving by only heating top is minimal unless the whole tank is rarely heated. I note the instructions However what it does not show is how to set the thermostats, in the systems I have used the bottom thermostat is always set hotter than the top one. This allows one to have the top auto switch on during the day when the water is running out, and for water to wash hands to be cooler than water for a bath, as in the main there is not enough hot water to fill a bath, it relies on one adding some cold so when set to bath normally set hotter.

I have not seen a duel immersion heater for years, this picture from the best practice guide warns against immersion heaters without a safety cut out, to fit a safety cut out takes up room so most are built into the main thermostat, even then looking at this although two pockets for the thermostats that would be rather crowded, I would not like to try and get them all in. I have hunted the internet for a picture of an immersion heater with two thermostats, can find two elements, but not two thermostats.

Can you post a photo of the immersion with cover removed

I would also be interested to see what it looks like. If only one thermostat then it will only heat whole tank if starting from cold.

I am intrigued as to how one gets two elements and two thermostats into the space available?

 

[Johntheo5]

I have continuity tested both elements and both seem fine. The voltage across the elements is 230V on both elements when switched on. The resistance of the short element 28 ohms and the resistance on the long element is 20 ohms.

I am at a complete loss as to understand why the original 27” element would heat the entire tank but the replacement 27” element only heats the top half. I cannot imagine a defect in the element that could cause this.
Please anyone-any ideas?

There's a good photo of what yours might look like, also a schematic. As stated above, the hi limit stat might be combined with the control stat.

 

[shackec]

Thank you for replies:
"Stem" the cylinder is 900mm high; and the heating element is at a slight angle. The issue, is that the previous cylinder had the same length element; and every other parameter was identical. The first one heated virtually the whole tank; the new one doesn't; hence the mystery?
Thank you "Johntheo5". The last photograph you sent is identical to the immersion heater I have fitted. I have checked the wiring very carefully. All of the electrical checks suggest that everything is working as it should.
Thank you "ericmark". I haven't taken the immersion heater apart but , from the photograph above, you can see the two live inputs (L1 and L2) which are sink (black) and brown (bath) coming in from the left and connecting to the two heating elements. The neutral connects to the hi-limit stat and from that to the control stat; and from that to the exiting nodes of the two heating elements; which are linked to form a common N. The two pink nodes are the bath element and the two white nodes are the sink element.
I cannot be sure but I believe the High limit stat is a separate unit from the control stat and it all seems to fit in quite snugly. The unit came prewired and I simply wired it up to the sink/bath switch through an appropriate junction box.

I had a thought concerning the heating elements. It is not uncommon for them to short out and fail but I have never experienced a failure with a new immersion heater. I assume the internal wiring is a continual coil running from one end of the element to the other. Could there be a possibility of a manufacturing defect such that part of the wiring in the bath element is not consistent through the entire length: The resistance measures at the value it should and there is continuity but this would not reveal such a defect. Has anyone ever experienced this?

 

[stem]

"Stem" the cylinder is 900mm high; and the heating element is at a slight angle. The issue, is that the previous cylinder had the same length element; and every other parameter was identical. The first one heated virtually the whole tank; the new one doesn't; hence the mystery?

Then I think the mystery is why the old cylinder did it, and not why the new one doesn't

 

[Johntheo5]

Thank you for replies:
"Stem" the cylinder is 900mm high; and the heating element is at a slight angle. The issue, is that the previous cylinder had the same length element; and every other parameter was identical. The first one heated virtually the whole tank; the new one doesn't; hence the mystery?
Thank you "Johntheo5". The last photograph you sent is identical to the immersion heater I have fitted. I have checked the wiring very carefully. All of the electrical checks suggest that everything is working as it should.
Thank you "ericmark". I haven't taken the immersion heater apart but , from the photograph above, you can see the two live inputs (L1 and L2) which are sink (black) and brown (bath) coming in from the left and connecting to the two heating elements. The neutral connects to the hi-limit stat and from that to the control stat; and from that to the exiting nodes of the two heating elements; which are linked to form a common N. The two pink nodes are the bath element and the two white nodes are the sink element.
I cannot be sure but I believe the High limit stat is a separate unit from the control stat and it all seems to fit in quite snugly. The unit came prewired and I simply wired it up to the sink/bath switch through an appropriate junction box.

I had a thought concerning the heating elements. It is not uncommon for them to short out and fail but I have never experienced a failure with a new immersion heater. I assume the internal wiring is a continual coil running from one end of the element to the other. Could there be a possibility of a manufacturing defect such that part of the wiring in the bath element is not consistent through the entire length: The resistance measures at the value it should and there is continuity but this would not reveal such a defect. Has anyone ever experienced this?

Can you just select the Bath element and check that you have 230V between it and earth (brown, wire 2 in my photo) , then ensure no volts between terminals 3&4 on the control stat, if you are getting 230V turn the setting to max, say 70C, you should hear a click and then get 0 volts between 3&4 to confirm that the stat is closed, you should now have a circuit with the current flowing through the bath element only.
You also said the the bath element is 2.85kw which should give a resistance of 18.56ohms, you get a resistance of 20ohms which is element power of 2.645kw, however, I don't think that is your problem.

Can you post a photo of the immersion wiring with trhe cover removed?.
Also a photo of the wiring connections/terminals on the switch.

 

[shackec]

Thank you Johntheo5. I'm tied up tomorrow but I will do the tests you suggest on Friday morning. I will also try to get a photograph but the cyiinder is in a cupboard under a sloping roof; all a bit tight.
I am recording a voltage supply of 230V; but this was not a problem on the old immersion heater. I suppose different elements might respond differently to a given voltage. However, can't see this being a source of the problem? regards. Charles

 

[Johntheo5]

You could test it "easier" if you run off ~ 40 litres (immersion switched off), then switch in the bath element, it should then, IMO heat ~ 28ins of water from the top of the cylinder to the bottom of the bath element as the water is all being heated uniformly at around 100 watts per inch of the heating element, the problem probably is, as suggested, that the single stat is only the length of the shorter bath element so if the top 14ins of the cylinder is hot then switching in the bath element will have little or no effect unless starting from a uniformly cold cylinder, if you carry out this test, at least it should show that there is nothing wrong with the immersion itself, its a bit strange though because I have seen a few dual immersions left permanently on Bath and havn't heard of any complaints of shortage of HW, its hardly the case that the whole cylinder of HW is used every time.

 

[Johntheo5]

Any update on your problem?.