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Old 06-07-12, 10:02 AM   #31
Xringer
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I just skimmed this thread and found the workmanship of the tank very impressive!
The insulation job looks topnotch, so performance should be excellent.
I'll bet it's going to outlast all the other system parts.
Bottom line, pure Craftsmanship!

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Old 06-07-12, 03:18 PM   #32
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I realize that I may have missed something, but wouldn't it have been a better idea to have put the insulation on the outside of the box, rather than on the inside?

It looks to me that the spaces between the 2x4s would be a good place for the insulation. This would have resulted in a considerable increase in internal water volume and therefore heat storage.

-AC
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Old 06-07-12, 05:04 PM   #33
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I've a question about the pumping heads. Once the flow is established, won't the return pipe, the down leg, be full of water? If it is, won't there be a siphon effect, effectively reducing the pumping head to the friction losses only? So you need to have a pump capable of lifting the water the full head height to prime the system, for which a relatively low flow should be OK, but it only needs to be able to pump at full flow against a much lower head.

I realise that I am almost certainly wrong here, given all the gurus commenting here, but I'd really appreciate an explanation of where my logic has gone astray.
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Old 06-07-12, 05:17 PM   #34
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The return pipe will not be full of water on the way down (it'll drain faster than the pump can pump). Therefore it can't create any suction to assist in pulling the water up. Even if the pump could keep up, it would then also not create suction because the pump would still be pushing everything along.
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Old 06-07-12, 06:26 PM   #35
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Hi Daox, Water can only drain faster than the pump can supply if air is able to flow up the pipe against the water flow. Unless the return pipe is over-sized, I am a bit surprised that this would happen at 5gpm flow.

It's over 40 years since I last did any pipe network calculations and I wasn't much good at it then. Even so, I am pretty sure that your second sentence is not right. Assuming that the pump can establish the flow to the point where air ingress does not occur, the top of the circuit can be in suction even as the pump operates.

I surmise that the critical point would be to ensure that the pump must be capable of establishing enough flow to drive the air out of the system before the partial siphon effect can kick in. An alternative might be to have a flow restriction valve at the outlet, that is switched in for a short time after the pump starts, but this is getting complicated.
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Old 06-07-12, 06:49 PM   #36
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I should Google first. see here:

Pump and Pipe Sizing for a Solar Water or Space Heating System

Quoting from that page:

Step 2: Measure the Pump Vertical Head Requirement

For a drainback system, when the sun comes on the collectors, the controller will turn the pump on, and the pump must be able to pump water all the way up to the top of the collector to start the flow. In order to make sure the pump can do this, you must carefully measure the VERTICAL distance between the water level in the tank, and the top of the collector. If the top of the collector is 30 ft over and 11 ft up from the tank water level, the number you want is the 11 ft -- this is the vertical distance from water level to top of collector. Measure this carefully -- don't eyeball it. Note that you measure from the water surface level in the tank, not from the pump level.

This requirement at startup to pump water from the tank all the way to the top of the collector is a tough requirement for pumps, and it will likely be the main consideration in picking the pump to use.

For the my Solar Shed system, the vertical distance from the water level in the tank to the top of the collector is 11 feet 3 inches.

So, the pump must be capable of pushing water up at least this vertical distance. Some margin should be allowed about the vertical distance. In other words, don't choose a pump that just barely has enough static head to make the top of the collectors. People vary on how much margin should be added, but Alan R.ushforth, who has done a number of drainback systems and has had experience with pumps that don't quite get the flow started recommends that the pump have a static head capability that is 20 to 30% greater than the vertical distance from tank water level to the top of collector. I would use that recommendation.

So, for the Solar Shed, the minimum pump static head would (11.25ft)(1.25) = 14 ft of static (startup) head.

Once flow is well established, and the return line is running full, the only pressure that the pump has to overcome is the friction losses in the pipe -- that's what the next step is about.
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Old 06-07-12, 06:57 PM   #37
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Hi A C Hacker,

The problem of putting the insulation outside the wood is that the wood will be at elevated temperatures for very long periods. It is considered undesirable to have structural timber at more than 150F for very long.

http://thermotreatedwood.com/Researc...H%20+%20WI.pdf
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Old 06-07-12, 08:46 PM   #38
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Quote:
Originally Posted by Daox View Post
The return pipe will not be full of water on the way down (it'll drain faster than the pump can pump). Therefore it can't create any suction to assist in pulling the water up. Even if the pump could keep up, it would then also not create suction because the pump would still be pushing everything along.
Hi Tim,
On a drain back, my understanding is that the return line should run full, and if it does not something like a partially closed valve should be put in the return line near the tank to offer enough flow resistance to keep the return line full.
I have to do this on my Solar Shed system because the return line is short and direct -- without the valve it won't run full. In addition to increasing the pumping head requirement when its not running full, it also makes quite a bit of of noise.

Gary
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Old 06-07-12, 08:50 PM   #39
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Quote:
Originally Posted by AC_Hacker View Post
I realize that I may have missed something, but wouldn't it have been a better idea to have put the insulation on the outside of the box, rather than on the inside?

It looks to me that the spaces between the 2x4s would be a good place for the insulation. This would have resulted in a considerable increase in internal water volume and therefore heat storage.

-AC
Hi AC,
I think either inside or outside works. I like the inside because its easier to install and you get no thermal bridging.

You can also do both -- especially if you don't want to give up too much of your tank volume to insulation space.

Gary
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Old 06-08-12, 11:04 AM   #40
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Quote:
Originally Posted by GaryGary View Post
Hi AC,
I think either inside or outside works. I like the inside because its easier to install and you get no thermal bridging.

You can also do both -- especially if you don't want to give up too much of your tank volume to insulation space.

Gary
Gary,

Thanks for your reply. The data sheet volunteered by Snail:

http://thermotreatedwood.com/Researc...H%20+%20WI.pdf

...has some very important information that should be read by all.

-AC

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