Hi Steve,"(i) The saving on gas is impressive but do you actually use all that hot water?
Yes we do, demand for hot water is high in our household with four adults at home. “(ii) Are you saying that the immersion is just below its thermostatic cutoff all the time just because of the pumping system? I'm not sure I understand how this could happen if you are producing 20+ kWh/day (I've had 4 days at that level this month).”
I wrote a detailed post about the pumping solution last year (Here)
post date 03/07/2014
If like me you have Gas central heating with a hot water only option, then a 20 to 30 blast in a morning will heat you a full tank up to at least 55 degrees. In our household this would be sufficient for the morning rush, though it would more than likely need another blast in the evening for pot washing, and the evenings water based activities.
When I had Solar PV installed and discovered proportional immersion controllers, what I wanted to achieve was to have enough hot water at all times as I did with Gas water heating, without having to fire up the boiler once or twice a day for 30 minutes. I knew how much Gas was used in summer (when the heating was off) which was around 21kw per day. Though, at that time I used to have the water heating on for much longer than 30 minutes in a Morning and evening.
With an immersion heater like mine (top mounted) how much water you can heat and the Kw’s used to heat that water, depends on a number of variables. These are the diameter of your tank, if your immersion is top mounting, the length of your Immersion heater, and the starting temperature of the water. My tank is only 18" wide but is 2 meters tall, holds over 220 liters, and my immersion is 36 inches in length.
Originally (forget pumping solutions for now) if my tank was cold (circa 18C) it took about 7-8kw of solar generation above base load for the Immersion to reach cut off point, which is set to 65C. When cut off is reached only the top third of the tank was hot, the other 2 thirds of the tank was still cold (I have four temperature displays mounted up the length of the tank). Since this is only enough water for a single bath at best, there was not enough hot water for morning, never mind trying to satisfy demands all day long. Therefore, I embarked on a quest to design a solution that would provide as much hot water as was needed, without having to use any Gas if possible.
It soon became clear that a number of issues needed to be addressed to achieve this. First and foremost I had to produce more hot water than was just was contained in the top third of the tank. This is how the pumping solution came about. This took a while to get right and I had to go through testing a number of thermostats and configurations over quite a few months to get the system working correctly. It’s pretty easy if you don't watch out, to pump all your hot water from the top to of the tank to the bottom, and end up with a luke warm tank overall, at which point the sun goes in. Of course if this happens you have not lost the heat, but you do have to fire up the boiler to get the tank upto a useable temperature.
The other main issue to consider is that peak hot water demand can be early morning, when PV generation is low at best, or not possible due to the time of year. Therefore you need to carry over some hot water from the previous day, or modify your bathing times. To achieve this you need a very well insulated tank and piping. My tank is 20 years old plain copper with no foam insulation, with just a standard red tank jacket, and with no pipes insulated in the airing cupboard. To give you an idea if at midnight the temperature at the top off the tank was 55C by 8am the next morning it would be under 40C which is not hot enough for bathing without using the entire tank. Therefore last autumn I embarked on a major insulation project of the tank and pipes, which has reduced the 8 hour overnight loss to about 3C which is a lot better.
Therefore, in a morning most days there is usually sufficient hot water to meet demand. After which much of the tank may be at ambient temperature, with just some heat remaining at the top. Throughout the day (assuming the sun is shinning) the tank recharges slowly as the day progresses, 65C hot water is pumped from the top to the bottom of the tank, never letting the top of the tank go below 62C. As I explained above, a third of a tank takes about 7kw of generation, the other 2 thirds take another 14 kw, so even on a 20kw day i do not export. On only two occasions in July last year when I was out, when generation was over 28kw did I export a little.
Even in winter we never have to put the water heating on every day in the morning or evening, maybe twice or sometimes three times a week for 30 minutes only once a day. Over the last fortnight in Sheffield where I live, the generation has been very good and no Gas water heating has taken place at all. All our water heating has been met by Solar Generation.
Of course in winter it is more difficult to achieve heating sufficient hot water than in summer, though there are plenty of days that reach 7kw or above. I suggest that if your system is tripping out after 2-3kw you need to knock of the Gas heating, consider a pumping solution, and in winter bath later in the day to suit the daily generation pattern. At our house I showed my adult son and daughter the four temperature gauges on the tank and explained what temperature they needed to read to get a warm bath or shower. There was moaning to begin with in winter, but now I hear them opening the airing cupboard door to check if the water is hot enough, and if it isn't they just wait a while and bath later in the day.
For those of you reading this I don't want you to think that my objective is to export nothing and to heat hot water at the expense of all else. In fact we do not actually ever think about the proportional controller and the pumping solution, it just works automatically, and 9 times out of 10 there is sufficient hot water all the time, if not we burn some Gas. We schedule our washing and ironing for sunnier days, and in the summer when it is hot,
we are able to run our air conditioners also, and still generate sufficient hot water. We just get on with life and save some cash along the way.