Drain-back system and evacuated tubes

[solarhotairpanels]

I built a drainback system using 2 evacuated tube collectors. 20 tubes per/collector.
My system has been working flawlessly for over 8 years now..

No antifreeze in system
No pressure in system at all
No stagnation issues whatsoever even on the hottest days of summer.

I think one of the biggest keys in a drainback system is the size copper used for drainback. I use 3/4 copper. 1/2 inch copper to me just doesn't allow the water to drain back quick enough.
I use 3/4 to and from collectors and thru my whole system.

My system cycles even on cloudy days in the winter.. and some days even when it's snowing which is crazy but evacuated tube collectors are just fantastic.

The only tweak I made to my drainback piping after installation was I added a T fitting to the drainback line right where the hot water was entering back into the drainback tank on TOP. I installed a ball valve to the T fitting and cracked it open so I could allow air to enter the pipe during drainback mode. I also added a pressure relief valve at the highest point of the evacuated tube collector. This valve allows a tiny bit of air to enter the system during drainback mode so that a vacum is not created, thus slowing the water from draining.

by doing that the returning hot water returned 20 times quicker then before.

My system is a bit different in that my solar heated water returns to my 6 gal drainback tank (use to be 6 gal hot water heater) then travels into a coil located inside my 40 gal insulated storage tank so the heated coil is surrounded by domestic hot water.

So my pre heated 70 to 120 degree storage tank water feeds my Tankless hot water heater.

My goal was to STOP feeding my tankless hot water heater 48 degree ground water.
My tankless has to use more energy to heat ground water from 48 up to 110 degrees for use with tap water / showers etc. (domestic hot water use)

By having my domestic hot water preheated via solar I am saving MILLIONS. (ok ok not millions but I'm saving, rest assured)

That's about it..

My drainback tank is located in my basement next to my storage tank and every drop of water running thru the collectors drains right back without any problems very quickly once the pump shuts off.

My 2 evacuated tube collectors are mounted side by side and tilted for quick draining.

I have PDF files of my system if anyone wants to see them just shoot me an email.

Nice to see everyone involved with solar. Every little bit helps.

(To the gentleman from Hong Kong...Parabolic's pointing at evacuated tube collectors? Holy moley look out... and don't be afraid of evacuated tube collectors draining. They drain faster then flat panel hot water collectors guaranteed)

Take care

[solarhotairpanels]

Originally Posted by jeff5may

Drain back is not generally recommended with evacuated tube collectors. Mainly two reasons:
First, stagnation temperature can literally melt down gasket and sealing materials. When the sun shines brightly and there's no fluid to absorb the heat, temperatures in the collector can skyrocket to over 150 degC. If there's any liquid pooled in the collector, it expands to steam, which causes more things to cook and melt.

The other big problem is cold shocking. When the collector is under full sun, and fluid pumping stops, you get the previous paragraph. Stagnation and steam generation and pressure and yadda yadda yadda. When the fluid starts pumping again, all of the hot collector plumbing gets a sudden blast of cold fluid. At first, your manifold acts as a large coffee maker boiler, boiling and spewing shots of hot water with steam downstream. If the pump is strong, it pushes the hot froth down the pipe, overcomes the boiling action, then the boiling froth is replaced by lower temperature fluid. This heat shocks the whole plumbing all the way to a mixing vessel (if there is one).

Jeff how are you doing?

I've had my drain back system in place for over 7 years now and no issues at all even with stag temps going off the wall..
Just wanted to chime in on your thoughts

2 - 20 tube evacuated tube collectors
1 - 6 gallon used electric hot water heater for drain back tank with site glass (electric disconnected
1 - 40 gallon used Superstor storage tank with heat coil in bottom connected to collector piping
1 - Grundfo's circulator pump
Resol solar controller... allow me to slow pump / water speed down which works beautiful. // uses less watts at lower speed and water picks heat quickly running thru evacuated tube collector manifolds.

Couldn't be happier..
Hardly no maintainence at all other then flushing system with vinagar now and then to remove rusty water from solar loop and now and then I have to add water to the loop because it evaporates over time.

[jeff5may]

Sorry, got busy living. As correctly speculated, the main concern relates to loss of power/pump events. Some e-tube collector models are designed and tested to withstand the 150 degC stagnation routinely throughout life, some aren't. A memory from another drainback survivor and his journey yields the following driven points:

- double wall tubes last longer than single wall and are easier to replace

- A large diameter brazed header pipe and metal pipe at least 10 feet on each end of the collector, with at least compression fittings holding stuff together is a recipe for success.

Needless to say, dude melted, popped, swelled and otherwise broke a decent amount of equipment developing said rules.

When a boiling event occurs, you can hear it through the plumbing. Sounds a lot like a coffee maker gurgling. Depending on the plumbing and how it's supported, the pipes may reverberate and clang against whatever they can.

[solarhotairpanels]

my tubes are double wall.. holding up great so far.
Pipes to and from collector are all 3/4 copper.

When water is returning to my small drainback tank in the basement you can hear the purculation occuring like a coffee pot during summer but no damage to anything yet. Systems been terrific.

Purchased my evacuated tubes thru www.houseneeds.com
Went to their website the other day and noticed they don't sell evac tubes anymore from what I can tell. I wrote to them cuz wanted to buy a couple more but so far no one has gotten back to me.

Thanks for your response.
Have a Happy New Year