In gathering quotes for a thermal store, I've been told by manufacturers that the store has to be kept at around 80-90 degrees. It seems to me that if it's got to be kept at that temperature, with only a 10 degree window of operation, I'm going to have to constantly be heating it up.
I had planned to heat the store with a multifuel stove with electric backup. If I'm forced to fire the stove more than once a day, then I might as well fit a mains pressure hot water tank, and al the associated protective gubbins.
Another point of concern, is that should I ever decide to fit a condensing oil boiler, it won't condense due to the high temperature return.
On Mon, 1 Dec 2008 01:42:54 -0800 (PST) someone who may be snipped-for-privacy@gmail.com wrote this:-
That rather depends on the size, design and what the store is being used for. They work quite happily at 50-60C for most things, though a small store operated at this temperature may only just fill a bath.
As far as I can tell it's a combination of the heat exchanger and flow rates that demands the store is maintained at such a high temperature. Perhaps I'm geting the wrong end of the stick, and to achieve reasonable storage the operating window is between 60 and 80-90 degrees, which seems more reasonable. I am asking for quote for the biggest stores manufacturers produce.
Apart from condensing boilers being problematic, the compulsory water softener isn't cheap!
I have a Thermal store based system for DHW and underfloor heating.
It works a treat .... loads of HW, and heating runs fine ...... happy to answ any Q's if you want to ping me a mail
Water in Body of store is circulated around boiler ... which helps with boiler efficiency (only long efficient burns) There are 2 heat exchangers ... a lower one for the low temp UFH circuit, and an upper one for the hotter DHW circuit
I also have this on a pumped return (timed) to provide instant DHW ... no need to run tap to get it hot - saves on water.
Only if it's not every well insulated... though of course heat loss does go up with the higher store temperature. 80-90 seems rather hot to me, I'd aim for more normal radiator circulating temperature of 60 to 70C letting it drop to 50C before firing up heat input automatically.
How often you need to top it up will depend on use and heat loss. The latter can be reduced with good insulation. The former is up to you and the weather. I might be making a bad assumption that the store will also be buffering heat from the stove to space heating.
So don't fit a condensing boiler. B-) Seriously I don't think the store needs to be maintained at such a high temperature. The design should be such to allow the boiler to run in condensing mode for most of a charge burn.
That seems a bit of a sweeping statement. The useable temperature depends on why you are using the store and what use you are making of the water in it.
I will be happy with DHW at 45C so I would expect to be able to get that from stored water at 50C so the operational range for that purpose could by 50-90C. And 50C should be hot enough for radiators in cool weather; probably not in cold weather. If you're using it for UFH it is usable at lower temperatures.
The store that I have, but not yet installed, is 2,500 litres. Its main function will be to enable the log boiler to run at maximum efficiency and minimum pollution and to prolong the boilers life.
Secondary function is to allow multiple heat sources.
The higher the temperature that you can get your store up to the more heat that you can store. E.g. 2,500l store heated throughout to 90C and with heat extracted to 40C will have stored about 145kwhr, from
70C to 40C it will only have stored 87kwhr.
Another important factor is to create and maintain stratification of the water inside the store.You can have the top part of the store at
70c while the lower part could be at 30C. This lets you get hot water out of the when the water temperature would be too low if it was thoroughly mixed.
With a solid fuel heat store you can create a stratified fill using a charging device like a Laddomat or Acaso. Maintaining the stratification whilst extracting heat needs very careful design of the system to ensure that the top hot water is not pumped staright back into the bottom of the store.
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Navitron forums and green building forums have lots of informed dicscussion about this sort of thing and are well worth searching.
Not necessarily. Heat flow _over_an_area_ will do so, but if a hotter thermal store also allows you to reduce its size, then you're likely to save more from the reduced surface area than from the extra temperature difference (2/3rd power). We're dealing with conduction here (flux linear to temperature difference) rather than radiation (fourth power). A smaller thermal store may also allow you the budget for a better insulated thermal store.
The big question is what you're heating it with and how this fits with acceptable input temperatures. Heating intermittently with a good hot feed from a woodburner favours small & hot, running it from solar needs to cope with whatever your panels can deliver, at the limits of your autumn working season. Lower acceptable temperatures mean usable solar gain for a longer period, although modelling this is tricky, as is choosing the best design point.
Eh? Condensing does not mean combi! For a thermal store you just need a boiler, condensing by preferance so you extract the most energy from the fuel you burn. The water in it is the primary water, that of the store and CH circuit.
Don't fit a combi or a thermal store then. A hot water storage cylinder will do the same job a lot cheaper unless you really need mains pressure hot water. You only need to keep it as hot as the water you want so it can save energy compared to a thermal store. It can deliver its stored energy faster too. Its also a lot easier to add a solar panel to if they are cheap enough.
Albion cylinders and McDonald Engineers both specify that their thermal stores are maintained at temperatures that far exceed the return temperatures needed by condensing boilers.
ITYM the heat exchanger needs to have variable flow so that the water leaving the exchanger is the same temp as the bottom of the store so it can be pumped back in at the bottom.
On Mon, 1 Dec 2008 02:23:58 -0800 (PST) someone who may be snipped-for-privacy@gmail.com wrote this:-
I see that Dave Liquorice has given a similar range of operating temperatures to the ones I gave.
This goes back to my first sentence. So far you have not provided enough information to say much more on the subject.
Whether a water softener is desirable or not depends on the water at a location, not the way it is heated. In an area with hard water a water softener is only one option.
My Nu-Heat system is like that, apart from the "long efficient burns" bit, which is in the manual but not in reality. One day I'll get around to fixing the controller so it isn't switching purely on the boiler thermostat with the pump running full time.....
Looking into it a bit more, I'm forming the opinion that thermal stores don't offer the benefits that the manufacturers pretend. Take the supposed benefit of utilising alternative heat sources. It's perfectly possible to combine my wood burner with an oil boiler on a fully pressurised system with the added benefit that the boiler will actually condense. Loads of hot water - no need for a water softener - that will actually stratify in the tank.
The notion of buffering the heat from a wood burner doesn't stack up either. A mains pressure hot water tank that is actually depleted will store just as much energy as an equivalent thermal store.
pressurised HW cyliners will also "technically" incur annual offcial inspection charges (possible home insurance probs if no evidential documentation?) and they have to be installed by a 'registered' installer supplying offical certification.
Thernal stores (as long as vented) don't need either - plus you can work on them on DIY basis if you so wish - as long as they are not made of stainless steel - apparently they cannot be modified once built if in SS as opposed to copper....
Most don't, because most manufacturers are selling snake oil barrels. To see an advantage you need some of:
Long storage times between uses (i.e. long enough that an unstratified HW cylinder will lose useful temperature).
Lowish acceptable input temperatures, such as from solar. You can drive a HWC with a woodburner, but it's damned hard to do it with solar (UK, much of the year)
Avoiding the costs, recurring costs and hassle of pressurised.
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