# Forum Home Renovation Solar Electrical Systems  Solar HWS electric backup

## davewastech

Does anybody know the layout inside a solar HWS tank that allows for example a 200L booster volume for a total 325L tank volume? Is it like one 200L section up the top of the tank for boosting? Something to limit the flow between top and bottom? I can't find details on the web. 
My system is a Rheem Solar Loline. 
Thanks for explaining,
Davewastech

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## Bloss

> Does anybody know the layout inside a solar HWS tank that allows for example a 200L booster volume for a total 325L tank volume? Is it like one 200L section up the top of the tank for boosting? Something to limit the flow between top and bottom? I can't find details on the web. 
> My system is a Rheem Solar Loline. 
> Thanks for explaining,
> Davewastech

  A manual is herehttp://www.rheem.com.au/Assets/1294/1/Rheemsolarlolineelectric511series.pdf  but does not have a cutaway - see the brochure here to see http://www.rheem.com.au/Assets/1307/...hure_final.pdf . Not sure why you want to know, but in any case the answer is simple - the location of the booster element is what controls the boost volume. 
In a regular resistance element HWS tank with a single element it will be close to the bottom of the tank - around 200 mm from the bottom (but around 100mm from the bottom of the cylinder itself). Any 2nd booster element will be around 2/3rd up and in most solar systems the booster is at that higher point (sometimes a little lower and of course a single element at that position not two). Not these will be different for a horizontal tank, but the principles remain the same and are based on the hotter water rising. 
It does not need a physical barrier - the water circulation combined with the booster/ thermostat location(s) (the booster has it's own thermostat control) restricts the boost to the particular volume.

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## Smurf

It's the old "heat rises" principle at work. 
The booster element will by its very nature only heat the water at and above the element. It will not heat the water below at all. 
So it's simply a matter of the manufacturer putting the element in the right location to result in heating part of the tank only. Eg put it at the bottom (as with a conventional electric HWS) and it will heat the whole tank. Put it half way up and it will heat the top half only. Etc. 
Note that you can't move the location of the element once the tank has been manufactured. Well, not without ending up ruining the tank (assuming it's mains pressure vitreous enamel) anyway. You can however replace it if it fails - it's just a matter of pulling the old one out and putting the new one back in the same hole. But you're stuck with whatever location the manufacturer decided upon at the design stage.

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## davewastech

Thanks Bloss and Smurf, 
It was just curiosity as to how it worked. I suppose I'm a bit surprised that the cold water at the bottom doesn't mix to some extent with the hot water at the top, particularly when the householder turns on a couple of hot water taps making water gush in through the cold water supply pipe entering the tank and causing turbulence. Perhaps it's designed in some way to minimise this? 
Cheers,
Davewastech

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## Bloss

> Thanks Bloss and Smurf, 
> It was just curiosity as to how it worked. I suppose I'm a bit surprised that the cold water at the bottom doesn't mix to some extent with the hot water at the top, particularly when the householder turns on a couple of hot water taps making water gush in through the cold water supply pipe entering the tank and causing turbulence. Perhaps it's designed in some way to minimise this? 
> Cheers,
> Davewastech

  Think of the physics - a 200litre tank means there is 200kgs of water and that is heated so under pressure too and the cold water inlet will be 12mm or 19mm inlet. Most inlets nowadays will also be pressure limited too, but in any case the release of hot water at the top with one or more taps coming on will not make much impact at the inlet end. 
Only the same volume flowing out will flow in - no more and no less and at no more than the same rate.Your tap will be flowing at maybe 6L/min if full on and most aren't full on so it will be less. Heading in at the bottom of that water it will meet the weight as well as the friction/ resistance of the water too. If you have ever put your hand in front of a hose out of water then put the hose into a swimming pool or a bucket full of water and put your hand in front again you can easily feel the reduction in force. 
As to the heating - as smurf said the heat rises and although convection currents form they are strongest from the heat source up - there will be some transfer between the boost/ hotter zone to the lower colder zone, but not much. There will be convection currents flowing from the bottom of the tank to the top when no boost is on and when boost is on there will be a split with a lower convection flow and a higher one with some transfer at the junction of the two currents (just as on Earth with ocean flows - much more complex, but at heart not dissimilar).

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## Smurf

A bit off topic but it's the same reason why a room space heater belongs on the floor, not on the ceiling. 
Since air is less dense than water, you can get around this with a decent fan (and it works well enough with a split system A/C to not be an issue) but still, you always want to either put the heater on the floor or direct the air vent down if it's on the wall. 
You'd be amazed how often I've seen reverse cycle A/C especially, with the air blowing toward the ceiling in heating mode. Or people who put the air to the floor when using the air-conditioner in the car - should be through the top vents for cooling, bottom for heating.  :Smilie:

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## GraemeCook

> A bit off topic but it's the same reason why a room space heater belongs on the floor, not on the ceiling. 
> You'd be amazed how often I've seen reverse cycle A/C especially, with the air blowing toward the ceiling in heating mode.

  Spot on, Smurf.   Its amazing that almost all commercial premises are heated via ceiling vents.   Easy to run ducts through the ceiling cavity but ..... 
Dave, adding to what Bloss and Smurf have said.   If you read the literature on solar hot water (or solar assisted) then you will find references to tank stratification.   Its what they are talking about. 
Fair Winds  
Graeme

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