Can Evac Tubes Gravity Feed?

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Can Evac Tubes Gravity Feed?

Postby Shebee » Wed Feb 13, 2013 3:10 pm

Please help!
We are building a new home off-grid and want to set up our hotwater system to be heated by our woodfire heater and solar ONLY, no gas or elec.
The hotwater storage tank will be located in the roof space of the house and the wetback will thermosyphon to it. We want to position our solar collector lower than the HWS tank to do the same thing, so not requiring a pump to circulate the water.
We are in Victoria and I understand that Evac tubes are more efficient in colder/cloudier whether. I would like to use these but have been told that evac tubes wont thermosyphon in a split system.
Reading other posts here, particularly those responded to by Gordon, say that it can be done. I am finding it difficult to get information and that most companies are only interested in selling their products and systems, rather than genuinely designing a system that best suits the customer.

Is it possible to do this? How far below the HWS tank can the collector be placed. Is there a rise per metre equation that I can use? Obviously the steeper the rise the better, but as a minimum what would you want it to be? Is a 4-5 metre run from the collector to the HWS tank too much?

Lastly, if we are using a tank on a stand to provide water to the house, how high above the HWS tank would it need to be to provide reasonable pressure to the house? Some calculations I have seen say 10-15m tower to get the pressure needed, which seems unreasonable high but I don't really understand it all.

Any information or referrals to people with information are VERY greatly appreciated! :)
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Re: Can Evac Tubes Gravity Feed?

Postby Gordon-Loomberah » Wed Feb 13, 2013 4:00 pm

Hi Sheena, welcome to the Energy Matters Forums :)

I dont run any gas or electric heating of my hot water, and I still haven't gotten around to connecting the wetback yet either :oops: but with careful management I dont run out of hot water. It is easily doable, you just need to have a sufficiently sized system, and keep an eye out on the weather conditions and forecast so you can plan your hot water use.

You may have seen a pic of my hydronics array posted on the forums a year or 2 ago- the water circulates between the 24-24-30tube collectors via thermosyphon with 20mm horizontal-vertical-horizontal pipes between the three of them- you do not have to have continuous rise, so long as the total rise is enough to keep it working. However, if you are talking about 4-5m pipes for the thermosyphon, I think you should consider not using a split system, at least to minimise losses. If the tubes are directly coupled into the tank like mine, then it will be a bit more efficient, and there is no worry about loss of thermosyphon or freezing the long pipes in winter. The wetback stove pipes can connect directly to the storage tank, which must be open vented. Putting the tank on the roof gives you more pressure at the hot water tap too, if running direct tank to shower gravity feed. If you wanted to take advantage of the full available pressure from a 10 or 15m high house pressure tank, then you would need a system with a heat exchanger- often just a huge coil of pipe within the tank, so that the house pressure is isolated from the hot water storage tank, which must be open vented due to the wetback connection.
If you do use a 4 or 5 metre length pipe on a split system, I'd recommend you use 25mm pipe- the larger pipe will allow the thermosyphon to operate more effectively. I think you'd probably want at least half to a metre rise, and yes, a steeper angle is better. You'd need to insulate it very well too. Use UV resistant insulation, or otherwise just the regular pipe insulation and run it inside some thin walled rural poly pipe works very well IME.

I run my tank 25m above the house through 40mm poly, which gives a big flow when required without much pressure drop- it is worth going for large diameter pipe, don't waste your time with 25mm or smaller- it will become annoying when the shower pressure drops off if someone turns on another tap ;)
15m is a reasonable pressure head for the house, but 10m will still work- the flow out a garden hose wont be very spectacular though... and forget about trying to run 2 hoses if you need to fend off a fire.

Evacuated tubes are a lot more efficient than flat plate collectors, and work in surprisingly overcast conditions, but what sort of collector you use is not really important as far as getting a thermosyphon to work, other than hotter collector output works best, and that's what you get from evac. tubes. Layout and pipe size are what you need to get right.
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Re: Can Evac Tubes Gravity Feed?

Postby Cherokee Solar » Wed Feb 13, 2013 8:19 pm

Hi Shebee,

I run a system for my hot water similar to what you are proposing. I'm unsure where you are but I'm in the central highlands in Victoria around the Macedon ranges.

As an observation, both solar hot water and the wet back on the hot water system work really well. However, there are days during Spring, Autumn and Summer when it is both warm and cloudy. On such days you will not want to run the wood heater and the solar hot water will not produce much heat. :shock:

The simplest and cheapest solution is to not worry about hot water on those days and enjoy the tepid water experience. It isn't that much of a drama. ;)

On the other hand if you require hot water on those days it is very cheap to install a Rheem instant on LPG water heater. Truly a 45kg LPG bottle will last you a long, long time on such a system...

The system here has a 400L header tank in the ceiling (make sure it sits on hardwood joists of at least 140 x 45 at 200 centres and in a galvanised drip tray which is plumbed to either outside the house or under the timber floor).

The wet back thermo-syphons to the header tank

The solar panels (I use 2 x flat panels) connect to the header tank via a controller and a pump.

I strongly recommend having an electrical switch installed somewhere inside the house on a wall connected to the power-point that controls the pump and controller so that you can turn it all off without having to climb into the roof cavity. ;)

The pump itself from the header tank to the panels is an extra complication, but it uses only 1Ah at 25v and is almost totally quiet (Grundfos pump designed to handle high temperatures). The controller works by having a temperature sensor on the header tank and also on the panels.

If the panels were gravity fed, then what is to stop the hot water going into the panels at night and you losing all of your stored hot water to the atmosphere? :o

Just for your interest, the water for the hot water system you'll see at the taps is not taken directly from the header tank itself, but from water that travels through an 18m copper coil that sits within the header tank itself.

You'll also find that because the wet back thermo syphon is unregulated (and should not be, otherwise it will pop!), the water will be much hotter with the wood fire than the solar which is regulated to prevent over heating the header tank. The header tanks are usually open ventilated so you'll know when there is too much heat in it (usually after running the wood heater for 2 days in a row) as you can hear it bubbling away.

Good luck!

Chris
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Re: Can Evac Tubes Gravity Feed?

Postby Cherokee Solar » Wed Feb 13, 2013 8:24 pm

Hi again,

Gordon is spot on too. If you are not going to put in an LPG or electric backup then in Victoria you may well need more than just two panels as the winters can get reasonably cool. I can have days here where the temperature will start at 1 degree and rise to 3 degrees before going back to 1 degree again at night. 8-)

Chris
Off grid solar + hot water. Heavily insulated + owner built flamezone house BAL-FZ. 300 mixed fruit trees + herbs + flowers + vegetables. Bees + heritage chickens. High up in the mountains north of Melbourne. http://ferngladefarm.blogspot.com.au/
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Re: Can Evac Tubes Gravity Feed?

Postby Shebee » Sun Feb 17, 2013 10:56 am

Gordon, thank you so much for your reply. I do need to have a think about the best design/placement and components to use. Do you know if there is such a thing as an evac tube close coupled tank, that can take the wetback and be used under pressure? (so the solar and wetback would have to run through coils I think?)
I haven't seen anything like this so far on my journey to get enough info about all of that but that doesn't mean that such a thing does not exist :)
Thank you Chris for describing your system we are near Ballarat and do get frosts and cold nights too.
We will probably still go with a pressure pumped system (hopefully with a large capacity pressure tank as described by Collyn Rivers to reduce cycling and power usage) but otherwise are looking to eliminate pumps from the system as we are off grid and will not have a very large solar power system.

It really is mind bending trying to prioritise components and work out what will function most effectively and be most financially viable given our situation. Certainly communicating with others who have knowledge and experience is proving to be highly valuable :D

Thanks again for the replies. No doubt there will be more queries to come ;)
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Re: Can Evac Tubes Gravity Feed?

Postby Cherokee Solar » Sun Feb 17, 2013 11:31 am

Shebee wrote:We will probably still go with a pressure pumped system (hopefully with a large capacity pressure tank as described by Collyn Rivers to reduce cycling and power usage) but otherwise are looking to eliminate pumps from the system as we are off grid and will not have a very large solar power system.


Hi,

Yeah a pressure pump is the way to go. I use a 20L pressure tank too to prevent cycling and it is very good. The pump itself is a Grundfos pump and it only uses about 22A for a 24v system, but it operates for only short periods of time (eg. a couple of minutes to half an hour at most - ie. watering the garden) so overall the usage is very low and a small solar system can handle it easy.

The pump between the header tank and the solar panels is a constant pressure pump rated to handle very hot water and has 3 settings. It uses only about 1A to 3A depending on the flow rate setting and I have it set on the 1A setting (ie. slow). This is so little power that you won't even notice it. During the depths of winter it will not operate at all, especially if you have a wood box heating the water instead.

Something to think about.

Chris
Off grid solar + hot water. Heavily insulated + owner built flamezone house BAL-FZ. 300 mixed fruit trees + herbs + flowers + vegetables. Bees + heritage chickens. High up in the mountains north of Melbourne. http://ferngladefarm.blogspot.com.au/
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Re: Can Evac Tubes Gravity Feed?

Postby Shebee » Sun Feb 17, 2013 11:43 am

Hi Chris, a pump drawing 22 amps is a huge power user for us and it will be the largest single power user in our home. Even though it doesnt run constantly the start up power each time it comes on uses more power than we can afford which is why we are looking to install a 500 litre pressure tank, with a draw off of approx half that before the pump has to kick in. In theory (and hopefully in practice :) ) the pump gets to run a few times a day, avoiding all the high-energy-use start ups everytime someone has a drink, washes their hands or flushes the toilet, which saves us power and wear on the pump.

If anyone has experience or knowledge of this type of set up I would be very interested to talk to them.

Thank you for the info about the circulation pump. I expected it to be more energy hungry than it is.

So much to learn :shock: !

Thanks again!
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Re: Can Evac Tubes Gravity Feed?

Postby jimbo » Sun Feb 17, 2013 9:15 pm

I know 22 Amps sounds a lot but I have a very similar Grundfos pump and its great. We can have multiple taps on and there is no pressure drop. The only time the pump is on for extended periods is during showers and watering the garden and even then the power use isn't much. I have photos of the CFA using my garden hose to put out fires last month all powered by my home pressure pump.

I have 3 flat panels on my roof that feed into a 270L storage tank under the house. In summer it's ample however as mentioned there are times when the house is warm enough to not need a heater but not enough sun for hot water. I would strongly recommend gas backup. We have a 45kg bottle for cooking and back up hot water and so far it has lasted 6 months and is not half used yet. Remember it is one thing to live off grid and another to do it comfortably.
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Re: Can Evac Tubes Gravity Feed?

Postby Gordon-Loomberah » Mon Feb 18, 2013 9:53 am

Shebee wrote: Do you know if there is such a thing as an evac tube close coupled tank, that can take the wetback and be used under pressure? (so the solar and wetback would have to run through coils I think?)


Yes they do exist, the water under pressure is in the big coil of pipe submerged in the open vented tank that is heated by evacuated tubes and the wetback.

The 500l pressure tank you are talking about, do you mean on a stand/up a hill for pressure due to gravity, or one that's connected to a pump with a diaphragm, using air for the pressure- which I think is what some people have assumed in replies?

I pump my water up 25m to a 5000l tank, using a relay driver to only pump when the batteries reach full charge (actually at 28V, just a few minutes before balancing voltage is reached at 29.3V for my Lithium batteries). If the weather is miserable drizzly overcast for a few days and the battery voltage doesn't get high enough, the pump doesnt come on.
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Re: Can Evac Tubes Gravity Feed?

Postby Shebee » Mon Feb 18, 2013 8:22 pm

Thanks Jimbo, perhaps I will re-think the gas back-up, it can't can't hurt to have the extra insurance :) .

Gordon, the 500 litre tank is the pump connected, diaphram type. I think it might be a better option for me than a tall tank stand. I'm afraid I dont do heights and if the tank on a stand needed maintenance it isn't something I could do myself :oops: . Unfortuanately we don't have a hill handy :) so the pressure/diaphram tank should help with power use from the pump.

Thanks again :D
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