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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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      CommentAuthordjh
    • CommentTimeFeb 21st 2016
     
    I've finally got around to thinking about adding some monitoring to my thermal store. I've got a bunch of 1-wire temperature sensors (DS18B20) and a Raspberry Pi and some cable and now I just need to decide exactly what I want to do with them. I'd be interested in any thoughts people have, especially from anybody who already has a monitoring system like this!

    My initial thoughts are to have some sensors up the side of the tank at various levels, and some monitoring the DHW.

    But how many sensors on the tank? Every foot would give me five sensors up the side. And are there specific places where it is, or isn't worth having a sensor? For example, does a sensor at the very bottom of the tank tell you anything or should I only start at PHE return level or even bottom immersion level?

    And what is worth monitoring on the DHW output? Presumably input mains temperature and DHW out, but is there anything else to bear in mind?

    Cheers, Dave
    •  
      CommentAuthorSteamyTea
    • CommentTimeFeb 21st 2016 edited
     
    I currently monitor my mains water temperature, the difference between the initial temperature (after a few hours not being used) and after a minute with the tap fully on is quite different.
    Today the initial temperature was 13.6°C, after a minute, 7.4°C.
    You also need to sense the air temperature around the tank.
    Ideally you need a heat meter on the inlet and outlets or you end up just measure the temperature losses if you are not careful.

    I spent two weeks, measuring the starting conditions at the bottom, middle and top of tank, input and output pipe temperatures, shower head temperature and volume and worked out that my standing losses where close to 4 kWh/day.
    I then repeated it 10 minutes after a shower to see what had changed (gave it a bit of settlement time).
    I then insulated the cylinder more and got this down to 2 kWh/day.
    That is on about 1.3 kWh/day of DHW usage.
    To double check this, I left the DHW switched on when I went away for a few days, the energy loss was 2 kWh/day.

    Not sure if any of that helps, but I found it a difficult thing to measure easily but found out that leaving the cupboard door open cost me about 6p/day.

    If you are serious about it, there are some reasonably priced heat meters, if you consider 200 quid reasonable.
    You may be able to make your own with a flow meter and a temp sensor.
    • CommentAuthorPaul_B
    • CommentTimeFeb 21st 2016
     
    I have a 750L thermal store with a baffle at about 3/4 height. The thermal store has pockets where I have located temperature sensors that feed into my home automation system. I also have some pipe temperature sensors that I haven't got around to fitting. SteamyTea has just given me some motivation to measure the temperature as it leaves the piping to the top of the store, as it enters the store and on the showers.

    Here is the graphical output over a 24 hour period, it shows stratification and the top of the store keeps a fairly constant temperature at around 60 degrees, the lower levels giving up their heat to keep the top at temperature. The boiler comes on at 06:00 so stratification is lost.
      ThermalStore.jpg
    •  
      CommentAuthordjh
    • CommentTimeFeb 21st 2016 edited
     
    Posted By: SteamyTeaYou also need to sense the air temperature around the tank.
    Ideally you need a heat meter on the inlet and outlets or you end up just measure the temperature losses if you are not careful.

    What do these things tell me?

    I suppose I haven't thought enough about what it is I'm going to be measuring. Why am I doing it and how will it benefit me or anybody else?

    Given the price of a DS18B20 or even a Raspberry PI, then yes £200 does sound expensive. And putting in a flowmeter sounds like a lot of hassle.

    I suspect it's easier to measure losses by measuring the electrical inputs, which is the next stage.
    •  
      CommentAuthorSteamyTea
    • CommentTimeFeb 21st 2016 edited
     
    Do some manual tests first, then you get a feel for what is going on.
    I measured top, middle and bottom of store, water temperatures before and after use, and water volume used.

    One thing I found that was a bit perplexing at first was that the top of tank temperature dropped after usage, as one would expect, but could go to a higher temperature than the initial temperature.
    Once I started to notice this I kept a constant time between readings (10 minutes), this seemed to sort the problem. All I could put it down to was general circulation within the store while no tap was running.
    Sounds odd, but there will be a bit more cooling at the top of the tank than lower down, but given a few minutes to settle a 'plume' of warm water could rise up before stabilisation. Similar affect to hot water freezing faster than cold water, the Mpemba effect.

    I think you can get ultra-sonic flow meters that are easy to fit and not too expensive.
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