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Old 12-10-14, 05:50 PM   #31
solarhotairpanels
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Sunspot...

Just some food for thought.. and I don't want to get anyone mad at this forum for stating otherwise but... thru my own experience and Dr. Ben's Online design of a drainback system...

Oxygen entering copper pipes over time can damage internal pump parts without a doubt.. however we are talking years here for this type damage to occur as far as I've read.

It's very important to remember on a drainback system that a vacum lock does not occur inside the piping during drainback to the tank.

If when the pump shuts off the water is vacum locked, all of the water will not drain back to your storage tank.... thus the remaining water outside in the pipe or collector could FREEZE causing all kinds of issues.

So note.. my findings are that...
You have to allow some air to enter the pipe when the pump shuts down.

Many DIY drainback tank designs similar to yours has the collector return pipe enter the top of the tank but does not allow that pipe to touch the water when the tank is at it's most full level.

This way.. when pump shuts down and water begins to return, air is sucked into that pipe allowing the water to flow freely in BOTH directions. Down the return pipe and also back down the pipe thru the pump.

The other thing that can cause a problem is what I mentioned before to you is the check valve inside your collector pump.

If your pump is so equipped...That valve can stop water from returning back down the pipe THRU your circulator pump. The valve would have to be removed to allow free flow back thru the pump. Know what I mean?

Some pumps have the valve in there to keep the pumps primed with water for next startup...

Example: Let's say the collector pump was mounted ABOVE water level. (NOT RECOMENDED)
When the pump shuts down the pump would stay primed with water in order to pump that water UP at next startup.

In drainback systems the circulator pump should be mounted 'below storage tank water level' so they stay primed automatically with NO check valve installed or needed.

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Old 12-10-14, 08:05 PM   #32
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Originally Posted by SDMCF View Post
solarhotairpanels, It seems your system is very similar to mine. Copper pipe all the way, Resol controller etc. My system has a diverter valve to either heat the hot water or feed heat (via a heat exchanger) into the central heating return pipe. I have 5 x 20-tube collectors and my water storage totals 900 litres made up of one 600 litre tempering tank feeding into a 300 litre tank that is also electrically heated.

My system can stagnate if power fails so the pumps can't run. I found out the hard way that when this happens soldered joints are not good to have. Connecting to the panels I used compression fittings but I had a soldered joint about 1 metre after the final panel and that leaked after the system overheated.
I assume you are using lead free solder and I am curious what the max pressure is on your system.

There should still not be a reason for the solder to let loose during stagnation unless the pressure is too high.

That said, the OP will be using flat collectors so this should not be a problem.
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Old 12-10-14, 08:35 PM   #33
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Originally Posted by solarhotairpanels View Post
Sunspot...


Oxygen entering copper pipes over time can damage internal pump parts without a doubt.. however we are talking years here for this type damage to occur as far as I've read.

It's very important to remember on a drainback system that a vacum lock does not occur inside the piping during drainback to the tank.

If when the pump shuts off the water is vacum locked, all of the water will not drain back to your storage tank.... thus the remaining water outside in the pipe or collector could FREEZE causing all kinds of issues.

So note.. my findings are that...
You have to allow some air to enter the pipe when the pump shuts down.

Many DIY drainback tank designs similar to yours has the collector return pipe enter the top of the tank but does not allow that pipe to touch the water when the tank is at it's most full level.

This way.. when pump shuts down and water begins to return, air is sucked into that pipe allowing the water to flow freely in BOTH directions. Down the return pipe and also back down the pipe thru the pump.

The other thing that can cause a problem is what I mentioned before to you is the check valve inside your collector pump.

If your pump is so equipped...That valve can stop water from returning back down the pipe THRU your circulator pump. The valve would have to be removed to allow free flow back thru the pump. Know what I mean?

Some pumps have the valve in there to keep the pumps primed with water for next startup...

.
A couple of corrections here. There are very many well designed drainback systems (I am talking about 1000s of installs mainly in Europe) where there is no ability for external air to get into piping during drainback. They work very well, even better than those that allow air in.

When you have a sealed system, the water DOES drain back, but it does so a bit slower and this is not an issue if the piping is sized correctly. Also, the sealed air means that the O2 in the air adheres to the pipe wall and ferris components and you are left with mostly nitrogen in the piping which is preferable. Iron pump corrosion is almost non existent.

Pump check valves on normal circulators are not there for the reasons stated. They are there to prevent reverse flow in parallel circuits. Using a circulator in a non pressurized open system is a tiny part of the market.
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Old 12-10-14, 09:19 PM   #34
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Hello MikeSolar,

Thanks for jumping in here.

I use an old 6 gallon water heater for my drain back tank.
My systems is built using all 3/4 copper pipe up to and return from collector.
No check valves in my circulator pump.

During testing after the build my return water took forever to return all water to my drainback tank after my pump was shut down.

I had to add a T fitting to the return piping where it entered the drainback tank and stick a ball valve on the end of the T which I leave cracked open just a tiny bit.

With the ball valve cracked open my water returns from the collectors almost instantly.
You can hear it gushing into the tank.

I don't run glycol with my system... just water and dont' want to take any chances of it freezing so even though you have documented 1000's of them working fine in Europe with no air allowed into the system.. I have to stick with this current design until I see a design like you are talking about.

and honestly? I'd love to see how they're doing that if you could send me a layout of their design... I assure you, I'd well appreciate it.
no vacum lock? at all? that's amazing.

Just about every drainback system I've seen online and those seen in person prior to building my own system allows air into the return pipe or somewhere on the system in order for the water to drain back more quickly.
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Old 12-11-14, 12:53 AM   #35
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SDMCF

are you using evacuated tube collectors on your system?
Yes.
I have 4 x 20 tube collectors with twin wall tubes, and 1 x 20 tube collector with single wall, larger diameter, tubes. The larger larger diameter tubes were more expensive. I haven't been able to work out if they perform better - they are supposed to but I haven't confirmed that.
My collectors are plumbed in series with the larger diameter panel last in line. At first I plumbed the 4 collectors as 2 parallel pairs, but changed that. Having them in series greatly reduced the amount of external pipework which cut down on heat losses. In theory. Again, I haven't got any figures to confirm that, but logically it should be so.
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Old 12-11-14, 08:29 AM   #36
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I only have 2 - 20 tube collectors on my system.

One of these days when I get rich and famous like you I'm going to buy MORE!

you're system must be cooking with 5 sets.. holy mackeral!
Good for you Sir!

Sounds like a nice setup.

If you have pictures of how your collectors are mounted up there please shoot one over if you have time.
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Old 12-11-14, 06:08 PM   #37
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The double wall ones are almost certainly Chinese and the most popular single wall ones are made by Kingspan, Viessmann, Sunda and a couple of others. Also, there is a relatively new one on the market from Narva in germany which should last a long time and be quite efficient.

It will be no problem to have them all in series but the flow rate should be higher, per m2 of panel, so that the terminal temp is kept reasonable.
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Old 12-11-14, 08:33 PM   #38
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Quote:
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..My collectors are plumbed in series with the larger diameter panel last in line. At first I plumbed the 4 collectors as 2 parallel pairs, but changed that. Having them in series greatly reduced the amount of external pipework which cut down on heat losses. In theory...
I would think that running them all parallel would be more advantageous.

Mist obviously because the parallel piping would mean less fluid resistance, therefore less work for the pump.

But also, as the temperature difference (delta-T) between the fluid and the solar heated parts of the collector becomes smaller, efficiency also becomes smaller.

In a series setup, the first collector raises the temperature to some higher temp, "temp-1_out". Then the second collector begins with the higher temperature of temp-1_out, and with a smaller delta-T (and lower efficiency) raises the temp to "temp-2_out", etc.

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Old 12-11-14, 08:47 PM   #39
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It is not done mainly because of the piping issues. 5 panels in parallel will have inconsistent flow charactistics no mater how well it is piped and it must be piped in a reverse return fashion. This is made worse because on of the panels is of a different manufacture. Series connection, with a higher flow rate, will give a more trouble free system even though the final output temp could be higher than desired and the pumping wattage may be a bit higher.
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Old 12-12-14, 01:31 AM   #40
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I think all of my collectors are Chinese. I bought them all from the same source, at the same time, but that source was just an importer.

The parallel v series thing seems a compromise whichever way it goes. My pipework is all 22mm copper and the flow rate with the pump running at 100% is about 13.5 litres per minute. This seems OK.

When I had the 4 similar panels plumbed as 2 parallel pairs I tried to get the flow between the 2 runs as even as I could. Judging by the output temperature of the 2 runs I was fairly successful in that; not exact, but close enough.

I changed to a series set-up purely to reduce the amount of exterior pipework. I figured that with the pipe 100C or more above ambient the losses must be significant and I could more easily reduce that by reducing the pipe length than improving the insulation. I hope I made the right choice, but who knows?

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