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Green Building Bible, Fourth Edition
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    I have a 450 litre Heat Bank with a 3kw immersion heatertowards the top of the cylinder and a blank immersion plate in the middle of the cylinder. I am considering adding a PV immersion heater device and was wondering about my options.

    Initially I thought of swapping the immersion heater and blank plate so that more of the tank is heated by the immersion heater. However, I thought that in the spring and autumn months I may not have enough diverted energy to heat the whole tank, but may be able to heat the top of the tank for DHW purposes.

    This then got me wondering whether I could keep the top immersion as is, and install a second immersion (with a long element that went to towards the bottom of the tank) and wire them up in such a way that the top immersion received the diverted electricity first, turning off when the top of the tank reached the peak temperature, with the energy then going to the second immersion heater to heat the rest of the tank (if there is enough diverted PV left).

    This would give me the most flexible arrangement and I know the immersun device will operate more than one heating device (I have read that some people divert to oil filled radiators once the immersion heater has turned off).

    What do people on here think of this arrangement? Will it work, will the top immersion heater heat the whole tank anyway, should I just move the immersion to the middle of the tank?

    As always, thanks in advance for all responses!

    I've just read the installation instructions for the solar iBoost diverter and I now have a fourth option. I can connect the PV diversion to the lower immersion heater and have the 'boost' mains power going to the top immersion heater (the boost is used to heat water when there is no solar power and you don't want to fire up the gas boiler for hot water).

    Note: I've had the heatbank for 5 years and have never used the immersion heater...

    So the options are:

    1. Keep the immersion heater at the top of the tank and connect the PV diverter.
    2. Move the immersion heater to the middle of the tank and connect the PV diverter.
    3. Add a second immersion heater to the middle of the tank and connect both to immersun device for dual solar.
    4. Add a second immersion heater to the middle of the tank and connect to iBoost device to power lower immersion with solar PV and the upper immersion with mains electricity (if/when required).

    • CommentAuthorringi
    • CommentTimeMar 9th 2016
    Or fit a small pump that pumps from the top to the bottom of the tank when you have PV power and the current immersion heater at the top can not operate as the water is too hot at the top.
    • CommentAuthorgravelld
    • CommentTimeMar 9th 2016
    450l is a fairly big tank...

    A lot depends on that top immersion. Can you measure how much energy it takes to raise the top of the tank to 60C or whatever?

    Then what size PV array do you have?

    But in general your thinking is correct. I wish we had two immersions, the top one heated first then trip over to the bottom. We only have a top one and we export a lot to the neighbour's Agas as a result.
    not read full post quick thought

    check out the iboost+ its got 2 dump circuits
    • CommentTimeMar 9th 2016
    Immersions generally only heat the water above them, because the water that is heated as it comes into contact with the immersion then rises up the tank to the top, or wherever it finds water at the same temperature.

    So you should definitely try and put an immersion at the bottom of the tank if you can.

    We have two immersions. I have my diverter connected to the bottom one, and an E7 timer connected to the top one. The thermostat on the top one is set lower than the thermostat on the bottom one. So if the tank is cold overnight, E7 heats the top of it so we have enough for showers the following day. If there's PV during the day, it can heat the bottom of the tank and it can heat the whole tank hotter than the E7 makes it. So if there's enough PV during the day, the E7 isn't used during the following night because the top half of the tank is hotter than its thermostat setting.

    I never tried to work out whether there's a good strategy for switching loads, because I don't see any need. In summer, I just switch the E7 timer off altogether.

    As Ringi says, if you can't fit an immersion that heats the bottom of the tank, you can use a destratification pump to allow an immersion higher up to heat the whole tank.
    read this now , iboost+ has 2 dumps and boost on one so
    circuit 1 to top immersion
    circuit 2 to bottom ( will switch when C1 satisfied )
    boost would do C1 on demand I'd guess
    • CommentAuthorringi
    • CommentTimeMar 9th 2016
    Some pv diverter can automatically control a destratification pump.

    A long immersion heater inserted in the top of a tank, can get close to “top to bottom” heating due to the heated water floating to the top of the tank along the immersion heater. It is a shame these are not made with the immersion heater inside of a large copper pipe to direct the convection currents.

    Another option is to use an external immersion (also known as a Willis) that will give true “top to bottom” heating of the tank if you can setup a thermostatic controlled value that only let convection currents take place when the water is hot enough.

    https://openenergymonitor.org/emon/node/2422 has some other ideals
    I like the idea of the dual immersion set up , enough for a shower inthe winter and a bath in the summer
    unfortunately could do with being the other way round :-)
    Posted By: jamesingramread this now , iboost+ has 2 dumps and boost on one so
    circuit 1 to top immersion
    circuit 2 to bottom ( will switch when C1 satisfied )
    boost would do C1 on demand I'd guess

    Thanks for this! I had only looked at the information for the original iboost. After reading your post, I went to the manufacturers website and the setup I was proposing (two immersions, with the top one powering first) is exactly how the iboost+ has been configured to work.

    I did have concerns about whether I'd have sufficient solar excess to heat the whole tank, hence the option for just heating the top immersion, but this configuration is the perfect solution. With immersion heaters being so cheap to buy and relatively easy to install (bit of a pain draining the tank) it's a no-brainer to add the second immersion and wire up the iboost+ to both.

    Even if I only have excess for the second (lower) immersion heater for a few of the sunniest days of the year, the minimal expense of the extra immersion would soon be covered by the full tank of water.

    Thanks guys, this is such a great board for sounding out ideas.
    • CommentAuthorgravelld
    • CommentTimeMar 9th 2016
    Yes, for a new tank it's a no brainer. That said, if you wanted to run some numbers to satisfy your concerns a little tt's pretty easy to work out what you should be putting into the tank.
    • CommentAuthorgravelld
    • CommentTimeMar 9th 2016
    I noticed the old willis-renewables site has disappeared. But you can still purchase similar systems.

    Was this a company piggy backing on an old concept, or did they introduce the idea?
    The iBoost+ has been ordered and is on its way! :)
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