Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here


Pumping water to a height in a closed loop circuit

Pumping water to a height in a closed loop circuit

Pumping water to a height in a closed loop circuit

I have a tank of water with a variable speed pump to pump water to solar collectors on the roof. The return pipe from the collectors comes back to the tank creating a closed flow loop. The roof is 15m above the tank. I am expecting the flow to run through the system if the pump just compensates the pressure drop due to friction in the pipe (because the fluid circuit is a closed loop). Whereas this doesn't happen. If the pump slows down and generates a head below a certain threshold the flow in the return pipe from the collectors separates. I am guessing that at the top of the circuit the pressure drops below the vapor pressure of water. Can anyone clarify on this and let me know how to run water at low flow rates in such cases?

RE: Pumping water to a height in a closed loop circuit


you've answered your own question. The system pressure ( usually maintained by some sort of expansion tank or bladder) needs to be at a pressure where the top of the system is at a positive pressure compared to atmosphere, so > 0.1 barg.

The easiest way for you is to have a tank located at the high point and keep it topped up to a level above the highest bit of piping.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Pumping water to a height in a closed loop circuit

You need more pressure at the bottom to pump up. The pressure at the bottom must be at least equal to the 15 m water head + friction loss + residual pressure at the top.

This was discussed recently:


RE: Pumping water to a height in a closed loop circuit

If you need to reduce the flowrate you will require some method of restricting the flowrate by imposing more head on the system - not by reducing pump speed which reduces head as well as flow. Remeber you still need sufficient head to overcome the head being imposed by the system, - - - friction losses plus the additional imposed head.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Pumping water to a height in a closed loop circuit

I second LittleInch. With the tank at the high point, you'll be able to use your VFD to its full extent. With the tank low, you need to add friction to the downpipe to keep it full, which defeats the purpose of the VFD.

RE: Pumping water to a height in a closed loop circuit

you are heating a water tank. I imagine it is heavy. you certainly dont want to put that on the roof. you only need a small reservoir at the top to maintain positive pressure. A hydro pneumatic bladder tank would work.


RE: Pumping water to a height in a closed loop circuit

Nothing wrong with having a water tank on the roof- they're relatively common for solar systems (at least in Oz).

I've got a 300L tank mounted horizontally on my roof and there were no changes/structural reinforcing done when it was installed.

Question to the OP- can you pressurised the ground based tank? If it can handle pressure- close it up and run a small line up to a header tank to provide the required system pressure. Top up from the header tank. May require further thought if your area is subject to freezing temperatures.

As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"

RE: Pumping water to a height in a closed loop circuit

LittleInch is right. You need the system to remain liquid full and under a small positive pressure at the top of the system. When you do so, you no longer need a pump capable of generating 15m of head, period, nor is there any risk of generating a vacuum unless you open a drain valve. Your pump will then need only to overcome frictional loss AND to overcome the density difference (to drive hot water on the roof back down to the basement, displacing denser cold water). If you could put the heating at the bottom, the density difference would be assisting your pump i.e. acting as a thermosiphon, but because the sun shines on the roof you are fighting the density difference with your pump.

If you live in a mainly heating climate, your main storage tank needs to be inside your house, not on the roof. The expansion tank on the roof is only needed to ensure the system remains liquid full without having thermal expansion problems so it should be small. If you're in a mainly cooling climate, the opposite is true.

RE: Pumping water to a height in a closed loop circuit

Good responses everyone, particularly moltenmetal, This is giving a better understanding.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


White Paper - The Evolving Landscape of Commercial Battery-Powered Trucks
What’s driving the evolving landscape of truck electrification? What are the barriers, motivators and strategies for accelerating the electric transition? What insights and resources are available for today’s design engineers working to achieve industry disruption and evolution? For answers to these and other pertinent questions, read this white paper. Download Now
eBook - Rethink Your PLM
A lot has changed since the 90s. You don't surf the Web using dial-up anymore, so why are you still using a legacy PLM solution that's blocking your ability to innovate? To develop and launch products today, you need a flexible, cloud-based PLM, not a solution that's stuck in the past. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close