OK Not sure how well these charts will show up but here goes.
This is data from the last couple of weeks
It shows the potential of my store to produce hot water at 50 degrees. it takes account of the temperature of layers of water in the store ( temp taken in seven vertical places) as well as rolling average minimum of the cold water inlet and a rolling average maximum of the thermostatic mixer, nominally set to 50C. I call this processing "the calorimeter"
The vertical scale is in 100s of litres and horizontal is elapsed time.
When the chart reads zero, this just means that the mixer will fall out of regulation and the output water will be cooler than 50 not that it will instantly be too cold to use.
The one problem most people have is not understanding the principle of the store. NO WATER EVER LEAVES THE STORE OR ENTERS IT!! The store is simply a reservoir of energy and could be made of concrete or cotton wool if it had suitable thermal storage properties. It happens to be full of water plus copious amounts of corrosion inhibitor
Ok looking at the chart you can see the general trend up and down daily and the disastrous fall over the last few days when the sun has been hiding.
This chart is a zoom in over a couple of typical days when the big yellow ball is out there.
The gradual upward trends are when the system is getting heat from the sun, the calorimeter reading rises at around 60 litres per hour maximum and often a bit less than that. 60 Litres is enough for a modest bath.
Showers a lot less. On a good sunny day we could support 2-3 showers per hour whilst the sun is out.
The short rapid falls are when we run the taps to draw off water. The big things to note are the steady decline overnight falling typically 6 litres per hour.
The store is lagged with 75mm of foam. 50mm is standard but when they were making the store for me I asked for an extra 25mm. It is not a case of adding more and more insulation and the loss will stop.
So insulation will only slow the rate of loss down. Most of the loss I experience is conduction along the pipes. Even if I were to fit non conductive pipes - the water in them would have convection currents to take heat away.
Not that even when we have long days in UK the periods of no or minimal solar input are still long many many hours losing 6litres per hour.
So possibly have at best 8 hours production, 16 hours or more of losses let alone using any water!
Here are the results of the last few days.
The inexorable slide to zero when the sun don't shine!
Modest consumption, little or no solar input and those hours and hours of losses.
The spike just right of centre is when I cracked and let the boiler spend some money :shock:
All three of us were going out for the evening and needed showers and I did not want to incur the wrath of the family - they are generally tolerant of my system experimentation but do kick up if they get a cold shower for some reason :lol:
The boiler still throttled back was adding about 150 litres per hour to the store.
Preliminary conclusions.
Storing hot water in a tank however well lagged is a mugs game. The losses are too high. Most older houses still do it. Many newer houses are using combi boilers which do have supply volume limitation but near instant water heating makes far far more sense. In a large property with a need to supply simultaneous DHW, multiple combis or other local "instant" water heaters are the way to go.
IF like me you can get DHW free most of the time, then storing is OK because what you lose did not cost you anything. Generally speaking we are pretty self sufficient in DHW from mid March to mid October
When my system switches to winter mode, the control system treats any solar input as a bonus. First thing in the morning it top up to about 50 litres and keep replenished to that level say up to 10am.
Then it will check again around 5 pm and top up to 50 litres for evening meal food prep and washing up. These should be enough to leave a bit for the odd evening shower even in the depths of winter. Remember that because the input water temperature is monitored and included in the calorimeter calculations, this is automatically compensated for.
I would aim to have little or no paid for energy in the store overnight for maximum economy.
Hope this is of interest.
Bob
I think I pay about $2000 in power cost per year, mostly electrical. (It would be more without the wood stove for extra winter heat.) Hot water costs are minimal with only two people taking showers. My house is larger but newer than yours, and decently insulated. I would probably need 3-4kw of PV panels to reach a break even point. 