Probably just showing my ignorance here but this one's been puzzling me and I'm sure someone will know....
I was discussing thermal stores (for DHW) with a heating man. Being a bit 'green' he suggested getting one with a solar coil. It sounded plausible, even though I don't have any immediate ambitions for solar.
But....How would feeding solar into a thermal store give an advantage? We use DHW in the mornings and evenings. When it's used, the boiler cuts in and heats the tank again. Pretty much the same thing would happen with a store. So, apart from the long days in the middle of summer, the solar wouldn't contribute at that point.
On the other hand, when the solar was 'working' the tank would already be hot.
Am I missing something here - it seems to me that solar heat could contribute in a preheater? (i.e. a second store, which would be expensive if it was a proper thermal store).
diont piss around with solar ubnless you make it yourself. My FIL got conned into spending thousands for a rig that saves him less than £100 quid a year.
He finally got draughtproofing on the doors and let me set his thermostats and timer correctly, which saved him far far more.
its straightforward to retrofit a sidewinder coil if one is ever needed, so if you're not going solar I'm not seeing any point adding an extra solar coil
You can buy cylinder with two coils, one solar and one for the CH. (Solar is the lower one). You can also buy retro-fit addtional cooils. In Summer you will easilybe able to heat your water for free and also save by leaving the CH off. Don't worry about how hot, I can almost boil the water in my tank with the solar thermal array in Summer. Even in Winter energy is gathered but it will need the CH/immersion heater to top up the temperature.
But you need to determine how much hot water you use (and cost) and do the sums as regards capital cost.
But even if it makes no sense now it will in five years with rising energy costs.
A thermal store is layered. The solar coil would be at the bottom where it is not heated via a boiler, so any solar gain is stored in the cylinder. A store can say be set to 70C for the boiler, but if the solar panel is giving out real hot water the store water can go to 90C. Overheat protection is needed. So you gain and the boiler does not cut in.
What CH do you have or having? Rads? UFH? Thermal stores give mains presure DHW.
What are you planning on having doing? That will give more scope to advise you better.
As "summer" is coming to an end last night I did some sums on the data o= f our weekly oil comsuption that goes back to May 2000. Split the year int= o two "seasons" (May - Oct and Nov - Apr) and averaged the weekly consumption for each season. The solar/woodburner system came into use Nov 2011. Average summer consumption of oil May 2000 to Oct 2011 was
1.92cm/week, May 2012 to Oct 2012 (projected) is 1.43cm/week. So that's =
0.48cm/week saving. A cm of oil in the tank is about 25l so at 60p/l tha= t is about =A3180 saving. The wood burner saved us about 38cm of oil over = the "winter" or > =A3550. Though we did spend about =A3400 on logs, half of = the last load we still have though.
Agreed, stopping drafts and a decent control system (programable stat) will give faster return. Mind you about 5% of capital overall in the first year isn't to shabby in my book. Especially when you consider how =
Well ish. I don't think you have quite got your head around how a thermal store works. The water in the store is not the water that comes out your hot tap, it is the primary water and you have socking great tank of it (ours is 300l). The store temperature could get close to 100C but ours normally runs around 80C at the mid point. DHW is heated on demand by passing through a coil in the top half of the tank, there is a thermostatic mixer valve on the output to limit the DHW temp to safe levels.
The solar input heats the whole tank as much as it can, the boiler input only heats the lower half and stops when the mid point of the store is at
70C, the upper half of the tank will be hotter due to convection/stratification. Our boiler doesn't cut in to heat the store until the mid point drops below 65C and stops at 70C.
So until you have cooled the mid point of the store to
A thermal store should be very large. The boiler should only heat the top layer. Solar should heat the whole tank.
The solar controller should inhibit the boiler if the solar panels are at working temperature. Hence in the example you give the boiler should not fire unless it is a cloudy day.
A thermal store should also take heat from all possible sources including log burners.
We have one, it has halved fuel use but it is a system that I designed, installed and commissioned so it cost a fraction of the price of having one fitted.
Insulation, insulation and insulation. Then draught proof. The cheapest and most effective way.
A friend does not have insulation under the slab of his house. He insulated the foundation walls of his house by putting underground foam insulation against the walls. Then gravel against the insulation to allow better drainage away from the walls. The drainage needed attention anyhow, the reason for digging, and he fitted a French drain to assist as well. He noticed the difference in the first winter. His bills were less but the comfort conditions were greatly improved with no cold feeling. The cold bridge from the slab to the surrounding cold earth was stopped.
I get the basic principle (store heat exchanger etc) - I guess I hadn't realised how much of a temperature gradient there is in the store, which now makes it all make sense(!) It seems the essence of the issue is getting the controls right.
The primary purpose of the store is to get DHW up to the top floor of the house and do away with the airing cupboard in the bathroom, then to provide (potentially) a heat source for UFH in some other parts of the house, without having a hot pressurised bomb with all the inspection issues etc. Of course, mains pressure DHW would also allow most of the taps and fittings now available to be used (!)
As it will be sited the other side of a wall which carries ~ 30m2 of glazed 'lean-to' roof, a solar feed seems possible. It's south facing, probably not at the ideal angle etc but sometimes you have to be pragmatic. At present the roof has a hardboard cover under it as the previous owner said it got too hot, which indicates solar water might work. Clearly it will have to be custom built but then I would DIY anyway. The roof might require some judicious strengthening to carry the load, depending on weight, but the existing glazing could act as the font of the collector(s). Ultimately, the whole structure will be rebuilt and, if I found it worked out, I would incorporate the solar into the build.
I'm not really able to do much about it right now as there are far more pressing jobs to do. What I'm really thinking of is a little 'future proofing' the store against getting the time to experiment with it later: The extra coil would simplify trying it all out when I get the chance.
I'm yet to be really convinced that it will be a real benefit but (in common with many people here) I like to 'experiment' a bit and who knows, it might even pay back...one day(!)
Meanwhile, I've no doubt I shall be boring everyone with questions about the thermal store when I set it up (!)
I couldn't agree more: Insulation and draught proofing are very much on the agenda and on-going. I'd be a bit wary about the foam against the walls though: While it sounds attractive from a heat POV, a Victorian house with shallow foundations could be compromised by digging around the perimeter to any significant depth, then only having foam to hold it up!
Cheap, almost free, heat into the vastly expensive thermal store you've pre= sumably already committed to buying. If you're already going for a wooodst= ove(?) fired thermal store, then this solar extra makes sense. (ALthough th= e external heat exchanger approach is a good one too.)
OTOH, it's (IMHO) a bad idea to add a thermal store to a solar system.
Heat ongoing energy demands are 90% for space heating, 10% hot water. Sola= r capital costs are 50% to achieve space heating, 50% (or more) to add on h= ot water and a thermal store. So unless you're opening a Spanish laundry, s= olar heating is (IMHO) a great way to space-heat in UK autumn and spring, b= ut an over-expensive way to heat your hot water.
A thermal store is =A32k (or equivalent in your time). I can do a whole hea= ted floor system for that.
Large but matched to the size of the solar panels. Too big a store won't get particulary hot even on the sunniest days, to small and it'll overheat and shutdown.
No the boiler needs to fire if the store gets too cool (from drawing off DHW or CH) as the input from the solar thermal panels is not enough to maintain the minimum required store temperature and you will "run out of hot water". Not to mention what happens in the winter on a bright day and the solar panels are working and so is the CH...
The boiler is controlled to maintain the lowest acceptable store temp. Just checked the backup oil boiler here comes on when the store mid point temp is 65C. That works most of the time but not for the coldest days of winter as the rads are too cool to provide enough heat. If that happens I up the settings to 65/70.
Looks like we have used about 1200l less oil in the last year, normal use is around 4,000l/year so about 1/3 saving. But spent a bit on logs...
I haven't got a sensor right at the top of our store (yet, the 1-wire stuff arrived today, F knows how I'm going to solder the SO16 chip...) but the sensor about 3/4 up sits around 75C the bottom of the tank is more often often around 55C, unless the solar has been having a good go then the whole tank can be up at 80C+
Very much so. Position of sensors relative to tappings and what they are controlling etc. Not to mention the values at which things switch on/off
Agree, I don't really expect to get the capital back but it's nice going out the oil tank each week and not seeing the level drop as fast... B-)
You don't need to heat the floor, it's already warm - large thermal mass ov= er good insulation.
So what's your thermal store? I have no idea how you even begin to do onef= or that kind of budget. Even hardcore DIY, taking my panels and other buy-i= n costs out of that small a budget doesn't leave me enough to build one.
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