Plumbing for 2-level solar?

[stever]

Many of you may remember I could have been happier with my solar heating installation. It now has 4 panels on a low roof plumbed in series with 10 panels on a high roof. The 4 lower panels (that ALL the water has to go through) cause quite a resitriction and either make the pump work very hard or makes the water flow too slow to get good heating (maybe 2 gpm/panel in the upper panels).

The solar installer swore that while the manufacturer recommended parallel plumbing for this situation that they have found that it works better in series. I think they were lazy/cheap.

Either way -- for this year I want to add 4 more east-facing panels (no more south roof available and I figure this will get the heating going in the morning when it's needed) to the lower circuit and have it re-plumbed to be in PARALLEL with the upper bank of panels.

The question is about the correct way to plumb parallel banks at different levels:

- split the two with a manual valve and adjust until the panels get the same flow.
or
- run the return for the lower bank all the way up to the upper bank and then tie in the upper bank there (instead of separate returns)

The solar install guy says the second method is best as the valve will add back pressure. I figure the elevated reutrn will add the same amount of back pressure -- the end effect is to cripple the lower bank enough to get the flows the same -- no mater how it's done.

Will the second method result in equal flows?

The solar install guy uses a temperature gun to check if the flows are the same between panels (cool idea).

Thanks!

Steve

 

[mas985]

Elevation has minimal effect on the flow rate through the panels since static head is offset on the other side. Flow rate through the panels is dependent on the dynamic head loss in the panels and the supply/return plumbing only. Draining will become inportant so just make sure that there are no water traps (i.e. local low spots) in the plumbing.

The best way to get equal flow through the panels is to first plumb all the panels in parallel which it sounds like you will be doing. Second is to equalize the head loss in the upper and lower panel plumbing. But it isn't all that important to get them exactly equal. Close enough is good enough. Sometimes you can design the plumbing such that the head loss is about the same even though the lengths may be different. Placement of vacuum release and attachment of the lower panels to the upper is important but here is what I would recommend:

On the main supply line, attach the lower panels to the main supply line with a branch tee (which helps equalize flow rate to upper panel) at a point which is slightly lower than the lower panels lowest point. On the return line, attach the lower panels to the main return line with a branch tee at a point which is slightly higher than highest point of the lower panels. It is not necessary to go all the way to the top of the upper panels. This design requires that the vacuum release is installed at a point which is higher than the point where the two return lines come together. I always recommend that the vacuum release is installed at the highest point of the upper panels for faster and more complete draining of the panels and pipe. But you want to make sure that all the plumbing drains downward from the point of the vacuum release to avoid trapped water.

The branch tee is a good way to equalize the head loss between upper and lower panels because the branch port of the tee has the equivalent head loss of about 12' of 2" pipe while the through port is only about 4' of head loss or a 8 ' delta. 8' feet may not exactly make up for the difference in distance between the upper and lower panels but it is probably good enough given the variables involved.

 

[stever]

Thank you Mark -- I will recommend this to the installer. I look forward to this working!

Steve