Header slope in ganged evacuated tube collectors in a drainback system

[Art VanDelay]

I Don't See the Value of a VB in any Drainback Scenario

Naptown,

Sorry to all for getting way off topic.

You say you put VBs in all your drainback systems.

I can see that they would stay closed in a pressurized drainback loop, but I think they are just a bit of unnecessary added cost and leak potential.

Of course, in an atmospheric pressure drainback loop, a VB would suck air in and break the siphon in the collector loop. This is usually undesirable, mainly because it increases the required pumping power for a given flowrate.

If the loop is accidentally overfilled, air can always be reintroduced by simultaneously draining the tank and cracking open the pressure relief valve.

Gary at Build it Solar website has excellent drawings and installation instructions for atmospheric drainback system design. Almost every issue is addressed and explained.

[Naptown]

Naptown,

Sorry to all for getting way off topic.

You say you put VBs in all your drainback systems.

I can see that they would stay closed in a pressurized drainback loop, but I think they are just a bit of unnecessary added cost and leak potential.

Of course, in an atmospheric pressure drainback loop, a VB would suck air in and break the siphon in the collector loop. This is usually undesirable, mainly because it increases the required pumping power for a given flowrate.

If the loop is accidentally overfilled, air can always be reintroduced by simultaneously draining the tank and cracking open the pressure relief valve.

Gary at Build it Solar website has excellent drawings and installation instructions for atmospheric drainback system design. Almost every issue is addressed and explained.

It is extremely rare that we do a drain back system. Mostly closed loop glycol. For those very rare times i would install a drain back we have used VB's. In fact the only one I can remember that is a drain back is on a Chesapeake Bay foundation building that is used intermittently as a dormatory. It is a fairly large system with evacuated tubes so overheating was a prime concern and the reason we went with drain back.
You mentioned a steam back? system used with glycol. How do you keep the glycol that remains as the interior will not completely dry out from overheating and turning acid and eventually to gunk?

[LucMan]

No need for vacuum breakers if the drain back tank is properly sized and piping is sloped properly. min. 1/4" per ft
The air in the top of the tank will allow all the water to drain back to the tank.There must be enough air in the system so that the collectors and exposed pipes are empty when the system is not running. Vacuum breakers would add more complexity to the system and are prone to failure.
The headers should be piped reverse return as Naptowns pdf. shows but may also require using smaller piping from the main supply & return piping to the headers. For example 1" mains 3/4" or 1/2" branch lines depending on the flow rates required for your collectors, length of piping runs and header & piping head loss.

[Art VanDelay]

Steamback System Design

I think all the questions are addressed here:

http://www.heatinghelp.com/forum-thread/133064/Steamback-System-Design