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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    I'm trying to make an assessment of our energy usage for DHW so wish to monitor the electricity consumption of our immersion heater which is currently used for all our DHW.

    The immersion is 4.5kW, 3 phase, 400V and has a single cable of about 12mm diameter permanently wired via a safety switch box.

    When I go looking for a monitor to do the job all I seem to find are units with 3 clamp on sensors each to be clamped on to a single phase normally at the consumer unit to measure the buildings total electricity use.

    Is it possible to have a monitor with a sensor which will clamp on to the cable with all three phases combined in it or will I have to remove the sheathing from a length of cable so that I can place 3 individual sensors on to each phase?

    Is there a better way?

    I don't want to spend a load of money on this and don't want the expense of having to call in an electrician just for this although at some point in the future I will have one here to do another job but that may be quite a while from now.
    • CommentTimeSep 9th 2015
    If the meter has a small flashing LED (most modern digital ones do), then you can use an opto-sensor that counts the pulses and can be calibrated to your meter.

    You can't use one clamp around the 3 phases as they are out of phase. Not sure what it would read, certainly not real power.

    How much do you want to spend and how much detail do you need?
    Thanks Steamy, I want to measure the electricity use of the immersion only not the entire household consumption so the opti-sensor is out. As I suspected from my searching I will have to split the cable going to the immersion to put a sensor on each phase.
    • CommentTimeSep 10th 2015 edited
    I find I can disaggregate most individual loads.
    With resistance heating it will be a known load and on for quite a while, shows up nicely.
    This is half hour mean data.
      Power Usage.jpg
    • CommentTimeSep 10th 2015
    Posted By: Chris P BaconI'm trying to make an assessment of our energy usage for DHW so wish to monitor the electricity consumption of our immersion heater which is currently used for all our DHW.

    Instead of measuring the current, you could perhaps measure the temperature of the immersion head, or failing that perhaps detect changes between heating and cooling of the water inside the tank. That might tell you when it is active so you can deduce how much power it is using.
    • CommentAuthorborpin
    • CommentTimeSep 10th 2015
    Try OpenEnergyMonitor - they do a Pi based system but the all in one will not do 3 phase. The EmonTX will with 3x CT clamps. Had one running for several years now.

    No, there is no way to determine the usage without splitting the wires. There are a few single phase plug through measurement devices out there now.
    • CommentTimeSep 10th 2015
    Actually, thinking some more, since this is a simple resistance heater, it would only be necessary to separate one phase, as the total power could be inferred from that.
    • CommentTimeSep 10th 2015
    It depends on how many thermostats it uses.
    Posted By: SteamyTeaIt depends on how many thermostats it uses.
    Looking at the manufacturer's website it describes it as a "two stage immersion" but doesn't detail exactly what that means.

    Our household consumption for June/July/August is down 24% compared to the same three months last year due mainly I think to insulating all the pipework to the tank and reducing the set temp. from 60ÂșC to 50ÂșC.

    Now I'm looking to see where I can best invest to reduce things further.

    We don't have a feed in tariff for PV here, there is a 20% grant on installation costs but even with that the calculator on our electricity supplier's website estimates a payback of 25 years on a 10kWp system at current prices, so we would need rampant inflation to make it a worthwhile investment.

    That leaves the possibilities of an air to water heat pump for both DHW and space heating or an air to air for just space heating with the possibility of solar thermal for summer DHW.

    My previous estimates gave a 10 year payback on an air to water, but I was really only guessing at what portion of our total household electricity consumption goes on DHW, so now I would really like know for sure. Although I just calculated that if all our electricity consumption went on DHW and heating the payback would be 5 years so I really doubt if it's going to drop much, perhaps 8 years at best.
    • CommentTimeSep 10th 2015 edited
    You could combine technologies.
    ASHP, some PV and some instantaneous in-line water heaters.

    Not sure of the rules for fitting PV in Sweden, but it is pretty basic kit really. Any half decent roofer could manage it. Any domestic electrician should also be capable of installing the inverter and any diverted.

    If you were in the UK I would post you one of my spare data loggers. I would think that you would get enough information and accuracy from an opto unit to find out what was going on (as long as you can drag off the data as a text file and plonk it into a spreadsheet).
    Thanks very much for the offer Steamy.

    The cost of the PV I have calculated at the lowest possible, buying the kit from Germany and installing myself, just allowing for cherry picker hire, I roofed the house myself.

    It's mainly down to your old mantra "energy costs are too low" but that may change in the medium term as a number of our nuclear plants are due to close and it is that that is spurring me on to look at reducing our consumption.

    For a variety of reasons I think that air to water is probably the best solution for our situation.

    It's mainly a question of whether to continue to wait as technology improves or if our DHW usage is high enough to justify installing now.

    First step is to take a look at the wiring over the weekend.
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