Immersion design issues for Solar PV Optimisation

[Jamieo]

HI Folks.
This is my first post so apologies if it's in the wrong location.

I recently installed a 3kw Solar system with an iboost diverter which diverts electrical power to the dual elements (3kw and 2kw) on the water cylinder when there is an excess of power.

I recently discovered that a top mounted dual immersion (sink and bath type) will not fully heat the tank of water EVER. This is because of the thermostats location. ( I installed a set of 4 externally mounted Thermocouples on the outside of the tank to verify my readings).
My tank is 40 in high. My dual immersion has a 36in and 11in elements and the single thermostat is 18 long. I believe the thermostat averages the temp along it's length rather than at the tip like a thermocouple. This is a standard enough immersion. (Model Tesla TIH 467) Obviously enough the thermostat is not reading the temperature at any point below 18 in which isn't even half way down the tank. The effect of all this is that the sink element will heat the top 11in plus a bit and then once the second longer element kicks in it will start to heat the water (which will rise) and cause the thermostat to "make" once the top 18in of the tank are at temp leaving the bottom half at almost cold !!!! What is that all about.!!!! So I've ended up diverting my excess power to the grid when I haven't even heated the full tank!!!
Previously I used to heat my tank from the gas central heating which used a coil located at the bottom of my cylinder so I always had a full tank of hot water.
Considering I have a teenage son who plays sport 5 days a week and a 11 yr old daughter who likes 30min showers I need every drop of free solar hot water I can get.!!!

So.......I've been considering my options.
btw I don't have battery storage on my PV system as that adds a lot to the cost and I wanted to have as short a payback as possible.

OPtion 1. I have asked TPFAY for a quote for a Dual element DUAL Thermostat. The 36in element should have a thermostat somewhere near the 34in mark at least and the 11in element should have a thermostat up around that height. Does anyone know why this isn't already done? I know these are mechanical style actuated so maybe there's something in the 18in that is limiting. Anyone know if there is a company already doing this. I haven't seen anyone with a thermostat more than 18in and that's the main problem. My iboost allows for two output channels (heater 1 and heater 2 ) so I can use the two seperate thermostats to control the two elements

Option 2. Fit a cylinder stat, externally mounted on the side of the tank down at the 34" mark, bypassing the thermostat in the head of the dual immersion. Is there certification issues for the electrican in bypassing the immersions thermostat and linking this one in instead.

Option 3 . Cut a new hole for an element mounted at the bottom of the tank. This might be a problem as I already have a coil inside at the bottom for the gas heating loop which will be in use in the winter months.

OPtion 4. Fit a Willis heater.....this will trickle heat the whole tank and provide circulation thus evening out the temp of the cylinder. I'd really prefer not to do this as it's more cost, and plumbing.

Option 5. Replace tank for dual immersion top and bottom element.


I've spoken with Tesla technical and they agree this happens with top mounted dual elements and see it as a design issue but don't have a fix.
I think there's a lot of solar installations currently NOT optimised for this exact reason and a new element design might solve the problem.

All input will be very greatly received
Thanks in advance
Jamie

 

[snowhead]

Dual element single stat immersions have been around for over fifty years that I know of.
They were never intended to be used as a dual stage heater, short followed by long.
They are either one or the other.

 

[Jamieo]

Dual element single stat immersions have been around for over fifty years that I know of.
They were never intended to be used as a dual stage heater, short followed by long.
They are either

Dual element single stat immersions have been around for over fifty years that I know of.
They were never intended to be used as a dual stage heater, short followed by long.
They are either one or the other.

 

[Jamieo]

Thanks Snowhead . Why a single stat though.....seems like a bad design or a money saving option. Can't make sense of it.

 

[Avo Mk8]

Jamieo
Do you really need the 'boost' function, or could you manage with a full tank of hot?
You could fit a single 36" element with a long thermostat instead of your current dual arrangement.
eg:

Tesla 36 inch Incoloy Immersion Heater & Thermostat TIH660

Tesla 3kW immersion heater fitted with a dual safety protection thermostat rated at 15 amp. The unit is fully approved and tested to EN60335-2-73. The thermostat is also independently approved by BEAB to EN60730-1.

www.cityplumbing.co.uk

 

[Jamieo]

Thanks Snowhead . Why a single stat though.....seems like a bad design or a money saving option. Can't make sense of it

Jamieo
Do you really need the 'boost' function, or could you manage with a full tank of hot?
You could fit a single 36" element with a long thermostat instead of your current dual arrangement.
eg:

Tesla 36 inch Incoloy Immersion Heater & Thermostat TIH660

Tesla 3kW immersion heater fitted with a dual safety protection thermostat rated at 15 amp. The unit is fully approved and tested to EN60335-2-73. The thermostat is also independently approved by BEAB to EN60730-1.

www.cityplumbing.co.uk

Hi Avo Mk8.
That's a really good idea....and relatively inexpensive. Definitely worth a try.
Cheers mate

 

[Avo Mk8]

Hi Avo Mk8.
That's a really good idea....and relatively inexpensive. Definitely worth a try.
Cheers mate

Hi Jamieo
We've got a 200L tank with a 'boost' immersion in the side half way down, and the main immersion at the bottom. I have a 'timed boost' button for the upper, and a wi-fi switch for the lower (see below - not without its problems, but working well enough now)
We find we don't need the boost immersion. If ever worried we might run out of hot water (which is hardly ever) can just turn on the bottom immersion with the phone app, wherever you are (or press the button on the switch if at home!).
Not suggesting you need this, but I just thought since we now don't use 'boost', you might get away without that function. Also with your solar system, remote control of this sort probably not relevant.

WiFi Controlled 16A 7 Day Programmable Timeswitch | Timeguard (NTTWIFI)

Good luck with resolving it ?

 

[Jamieo]

Hi Jamieo
We've got a 200L tank with a 'boost' immersion in the side half way down, and the main immersion at the bottom. I have a 'timed boost' button for the upper, and a wi-fi switch for the lower (see below - not without its problems, but working well enough now)
We find we don't need the boost immersion. If ever worried we might run out of hot water (which is hardly ever) can just turn on the bottom immersion with the phone app, wherever you are (or press the button on the switch if at home!).
Not suggesting you need this, but I just thought since we now don't use 'boost', you might get away without that function. Also with your solar system, remote control of this sort probably not relevant.

WiFi Controlled 16A 7 Day Programmable Timeswitch | Timeguard (NTTWIFI)

Good luck with resolving it

Yes , the more I think about it I will get rid of the dual immersion. I'm currently analysing how running the long immersion only works. I also noticed something else bizarre........my 36" and 11" dual elements have an 11" stat fitted TS 225. I removed the stat to see if I could fit an 18" stat but the sleeve it fits into is also only 11" !!!! No wonder I'm not getting the bottom half of the 40" tank heated at all. The element cover suggests it's a TIH 467 Tesla 36", 11" elements (measure out at 20ohms and 30 ohms) and 18" stat but I won't know for sure until I drain down and remove. Maybe someone fitted the wrong element cover , wrong label or maybe the wrong thermostat sleeve was fitted only allowing an 11" stat.
I'll let you all know what happens but one thing is for sure.......there is plenty optimisation that can be done after the Solar installers are gone and careful analysis of before and after might show up problems like this.
Solar installers should really NOT install iboost type diverters to dual top mounted elements but I have a feeling ( and one of the installers actually told me) the are a lot of cowboys doing these installs and really it needs some careful optimisation to get you best return.
Thanks again

 

[Avo Mk8]

Jamieo, as you intimated, immersion heater thermostats only seem to come in 11" and 18" versions.
That means the Tesla 36" heater would have a thermostat tube only half as long as the element. The illustration on their website I linked to shows the sensor nearly as long as the element, so I believe doesn't represent what you'd actually get with a 36" immersion. It would still be an improvement, but maybe not enough of one!

It looks like a little de-stratification pump may be the only way of getting the tank completely hot from top to bottom (and with this, you wouldn't need to change the immersion! ?). But maybe too much hassle.

 

[Jamieo]

Jamieo, as you intimated, immersion heater thermostats only seem to come in 11" and 18" versions.
That means the Tesla 36" heater would have a thermostat tube only half as long as the element. The illustration on their website I linked to shows the sensor nearly as long as the element, so I believe doesn't represent what you'd actually get with a 36" immersion. It would still be an improvement, but maybe not enough of one!

It looks like a little de-stratification pump may be the only way of getting the tank completely hot from top to bottom (and with this, you wouldn't need to change the immersion! ?). But maybe too much hassle.

Yes, I saw those pics and some are a bit misleading I think. Probably stock photos. The recirc pump would involve cost, plumbing and some electricity use and I'm trying to keep the cost low and payback as short as possible. I'll be changing to a single element later this week I'm nearly sure. Just a few more test runs, and a bit of sunshine would be nice!
Cheers for all your considered input.

 

[baldelectrician]

Assuming there is a separate over limit thermostat in the immerser heater you could install a cylinder stat on the side controlled by a contactor for each element

Most new immersers have a separate button style over limit stat

 

[Avo Mk8]

Assuming there is a separate over limit thermostat in the immerser heater you could install a cylinder stat on the side controlled by a contactor for each element

Problem here is this Tesla immersion has the safety cutout in the replaceable rod stat, not built in to the element head.

 

[Jamieo]

Assuming there is a separate over limit thermostat in the immerser heater you could install a cylinder stat on the side controlled by a contactor for each element

Most new immersers have a separate button style over limit stat

Thanks baldelectrician....
The installation electrician suggested just that when I told him of my problems but he felt he might have problems certifying this. I think if you bypassed the element manufacturers stat that might be problematic in legal terms anyway.

I've also just found that Backer make a dual immersion dual thermostat unit 30" and 16" elements at 3kw each with thermostats controlling each element.
I'll either fit a new single long element or that Backer dual unit. My initial tests using only the 36" element on partially sunny days are leading me back to the dual immersion idea. It partially heats (35 degree) a lot of water which is just a bit too cool for a shower but I could have a very long, cooler shower, rather than fully heating (65 degrees) approx 15 litres of water .....note all those values are very approx.
It's to be expected due to element length but may be a problem as the solar power drops away heading out of summer months. Still much to consider to optimise this year round.
Even though my tank is 40" tall and I currently have a 36" long element (and 11" also) it's a possibility that a 30" long element would be better than 36" as it would deliver the 3kw max power into a slightly less amount of water and so might have more success brining it up to an acceptable shower temp.
I need to analyse more before deciding on the best solution for my situation, and I still might not get it absolutely right but it will be an improvement on what's there right now.
Solar PV installers would be well served by a best fit program to help them decide what best suits each installation. In Ireland, at present, it seems to be just put the panels up, fit the diverter to whatever elements are there and forget about optimisation. Poor show really.

 

[brianmoooore]

Option 6. Fit an Essex flange on the side of the cylinder, near the bottom, fitted with a standard immersion heater, used solely for the solar and leave the existing immersions as they are

Mechanical Immersion Heater Flange 2-1/4" BSP

This mechanical flange adapts a copper cylinder to accept an immersion heater element

www.stevensonplumbing.co.uk

 

[Jamieo]

Option 6. Fit an Essex flange on the side of the cylinder, near the bottom, fitted with a standard immersion heater, used solely for the solar and leave the existing immersions as they are

Option 6. Fit an Essex flange on the side of the cylinder, near the bottom, fitted with a standard immersion heater, used solely for the solar and leave the existing immersions as they are

Mechanical Immersion Heater Flange 2-1/4" BSP

This mechanical flange adapts a copper cylinder to accept an immersion heater element

www.stevensonplumbing.co.uk

That's a good idea too......lots of options to consider now. Cheers

 

[FiachMH]

That's a good idea too......lots of options to consider now. Cheers

Hi Jamieo, I found this post of yours while searching online for a solution to our solar PV / eddi / immersion heater problems. I'm in Dublin and from what I can tell, not being electrician or anything, we have the same set-up as you in our house (Solar PV powering an immersion dual element heater in a 40inch tall water tank etc), and there have been problems.

Specifically our immersion and convertor keep cutting out when we try use the convertor (eddi) to boost / heat the water from the grid on cloudy days/mornings/evenings, or when the eddi tries to heat the smaller element in the dual immersion (either from the gird or from the solar).

The only way the water heats now is when the sun is out and it's powering the larger element. as soon as it tries to switch to the other, the convertor cuts out. We never get a full tank of hot water and the eddi has to be reset each time. Crazy stuff.

It took us a while to realise it was the immersion at fault. The solar installers have been of very little help when contacted. We learnt to reset the immersion, but the button is stiff and unresponsive on ours (elemex) and it seemd to work only once or twice that way anyway.

I very interested to know, seeing as it's a year later. how you got on with your set up, what you decided on in the end. Did you get a new immersion element? if so does it work well, would you reccomend it?

Thanks for making this post.