Overall piping length in swh systems

[bernard]

There is one thing that I am curious of:
What are some rule of a thumb, or approximate overall piping lengths in residential solar water heating systems?

I know it may be difficult to answer this question, as it depends on a lot of factors: whether or not a swh system's storage is right next to the collector (ICS, thermosyphon), or located in the basement.
I also understand that it depends on the number of floors, position of a collector (on the roof, on the ground) and some other factors too.

But for example what are some approximate values of piping lengths for ground floor + one/two floor houses with collectors on the roof, and storage tank in the basement or ground floor?
70 feet? 150 feet?

Just to clarify: I am not interested in losses of energy due to piping. I am just interested in the overall piping length.

Thank you for the reply, and I apologize if it is difficult to answer this question.

[stevehull]

Piping length depends on several factors before you can make a decision on pipe diameter, pump size, etc.

How many collectors?
What is total fluid flow?

Steve

[bernard]

Thank you for the reply Steve.

I do not have a particular swh system in mind.
For example, let's say I have three collectors (each one 20 square feet of area (1.85 square meters)): 3 * 20 = 60 sq ft (5.58 sq m).

And the total flow is: 0.02 l/s/m2 * 5.58m2 = 0.12 l/s = 6.7l/m
6.7 liters/minute = 1.77 gallons/minute

[stevehull]

If you do a drain back system, you need a larger and more powerful pump compared to a closed loop. In the closed loop, you only have to overcome pipe flow resistance and not gravity (as with the drain back).

The flow rates you quote are small and with a closed loop a 1/20 HP small pump is all that is required. For this flow rate a pipe diameter of 1 cm (1/2 inch) would be adequate. If you were going to pump up three stories, I would go with a diameter of 1.5 cm.

Drain back systems require a pump that can handle not only the flow rate but the effect of gravity. Larger pumps are required, but a 1 HP pump can push water up many, many stories. Each 10 meters (~ 30 ft) = about 15 psi so a 300 ft pipe would have a pressure of about 150 psi in it. So you also need to make sure your pipe can handle high pressure of you are going up a significant number of stories.

Standard plumbing pipe, used in homes is typically rated for 200 psi, so it would work for many applications.


Does this help?

[bernard]

Dear Steve,

Thank you for the reply, it has been really informative.
But it seems there is a misunderstanding.

I am interested in the overall length of the pipes, not their diameters, and pressures they can withstand.
The length of the pipes from the collector to the storage, and from the storage to the collector. The ones from storage to the load (tap) can be excluded.

So what is the overall length of mentioned pipes in meters, feets?

[stevehull]

Pipe resistance (and thus the size of the pump) is HIGHLY dependent on the diameter (radius to the 4'th power). The length is only a minor contributor (unless you are talking of hundreds of meters long). The gravity head is the other issue and we talked of that in the last post.

In my roof top solar water heater, it is about 30 feet (~ 10 meters) above the storage tank and the total pipe run length was about 200 feet. It is three collectors just about the same size as you talked of. The water flow was set up for about 5 gallons per minute (20 L/min) as a drain back system with a 1/10'th HP pump. Has been working since 1992 - with one notable problem.

The only thing I would mention is that a section of copper pipe needs to be used if you are using a drain back. Here is why.

Assume the system shuts off for some reason in the daytime (power outage). The collector heats up to several hundred degrees (doesn't matter if C or F) in the hot sun. When the power is restored, the water in the drain-back supply is pumped up, goes into the collectors and immediately flashes into super heated stem - and it melts plastic pipe in the return.

Use a thick walled copper for the first dozen meters right off the solar panel on the return to storage if you are using a drain back. The copper has sufficient thermal mass to absorb the flash heat in the above situation.

Steve

[bernard]

Originally Posted by stevehull

... and the total pipe run length was about 200 feet.

Thank you Steve.

Is there some way of knowing how much total pipe run length will one need?
For example, depending on a number of stories, in case the storage tank is located at the lowest story, and collectors are on the roof.
Can it be said that each story adds 70 feet of pipe run length, or similar?

This is basically what I want to know. An approximate way.

[stevehull]

If you have a straight shot from the basement to the roof, then each story only adds ~ 5 meters or so for each run (10 meters for the input and output; two pipes). Also depends on how tall each story is!

There should be minimal pipe on the roof (maybe 2-3 meters).


Steve

[bernard]

Originally Posted by stevehull

If you have a straight shot from the basement to the roof, then each story only adds ~ 5 meters or so for each run (10 meters for the input and output; two pipes). Also depends on how tall each story is!

There should be minimal pipe on the roof (maybe 2-3 meters).

Thank you Steve.

So basically if one has a two story building with basement (basement, ground floor and first floor), where storage tank is located in the basement, collectors are on the roof and connected in a straight shot, a rough estimation of the pipes length would be:

3 stories * 10meters + 3meters (on roof) = 33 meters.
?

How come your total run length is double than that?
You do not have a straight shot connection?

[stevehull]

You need a supply pipe to the collector and then a return pipe from it. Thus, I doubled the point to point length.

Steve