57 minutes ago, ThatGuy said:

So the motor, although less loaded, will still be getting hotter than if it had a "normal" water flow carrying the heat away

It is slightly complicated.  Whilst it is true that the pump part will get hotter as you decrease flow (you can actually boil the water if you close the outlet completely) the motor itself is air cooled.  As load increases current also does, and speed (and hence cooling) slightly decreases.  Now, if only the pump was water cooled....

2 hours ago, ThatGuy said:

I know right, if only?  but the cooling comes from the water

My turn to have a brain fart: I meant to say "If only the motor was water cooled".  The pump is of course water cooled, as long as there is at least some flow.

2 hours ago, ThatGuy said:

Arm-chair mechanicking aside... bottom line for @MKRandburg:  You'll need to just give it a bash and see how it goes

 

@MKRandburg To be perfectly safe, have a flow control valve in the main return pipe to the pool as well as in the pipe through the heater.  (I assume that the outlet from the heater drains directly into the pool)

Start up with both valves fully open.  Then, close valve for heater until water coming out of it is say 5-10C higher than pool water (in full sun).

Then, if necessary, you can close the main return to the pool a bit if the pump motor is running too hot (or if you do not get sufficient flow through the heater).

If you have a pressure gauge on the pump outlet you can use that to ensure that the pressure is the same (running through both pipes) as it is when the main return valve is fully open and the heater valve is fully closed.

 

19 hours ago, MKRandburg said:

Oh cool. So sending some water round black pipes (I'm thinking LDPE by the by) might save me a few pennies on power. hehe

19 hours ago, MKRandburg said:

  19 hours ago, Calvin said:

Bottom line: you pool's pump uses less power when the filter is dirty than when the filter is clean.

 

You guys are joking right?

53 minutes ago, DeepBass9 said:

You guys are joking right?

Afraid not.  I know that it is counter-intuitive, but it is absolutely the case.

If you find it difficult to believe, wire up a KillAWatt or similar meter to your pump and see for yourself.

On 2021/04/23 at 2:37 PM, Calvin said:

Afraid not.  I know that it is counter-intuitive, but it is absolutely the case.

If you find it difficult to believe, wire up a KillAWatt or similar meter to your pump and see for yourself.

The motor turns at a fixed rpm and drives an impeller. If there are no constrictions in the system, the impeller will have less resistance and will pump the water through the system will less work done. If the filter is blocked then more work needs to be done, and more power will be drawn. I have measured my pump which is rated 800w and it usually uses 450W but 650W with the creepy attached. The power consumption increases with the work done. As the filter blocks up it will take more work to force the water through the filter.

Conservation of energy, its the law so I hear. 

On 2021/04/23 at 12:45 PM, Calvin said:

My turn to have a brain fart: I meant to say "If only the motor was water cooled".  The pump is of course water cooled, as long as there is at least some flow.

Most pool pumps are air cooled, the water doesn't flow around the motor, only through the impeller which is driven with a shaft from the motor. 

Eventually the pump will burn out. 

I didn't mean to come across as sarcastic or whatever, so I hope no offence was taken.

The reality is the more work needs to be done, the more power is drawn. There are formulas to work out the friction losses in long pipes, and losses due to turbulent flow in valves and elbows. If you block up the outlet on a pump, the impeller will be doing no useful work, just working against friction and water turbulence and that energy will need to be dispersed as heat. So in a worst case your pump will overheat.

Exactly, so a blocked filter will increase load not decrease it, and a quick google to get some other opinions gives a variety of sources giving the same answer which is:

"Although sand and D.E. filters function more effectively as dirt accumulates, a dirty filter can increase the pump’s workload. In fact, the difference between a clean and dirty filter can nearly double the pump’s energy use."

As you add longer pipes, narrower pipes, more bends, more constrictions in the pipe you increase the effective head, reduce the flow rate and increase the load on the pump. This relationship is not linear and there is a peak efficiency where the flow rate is maximised. If you effectively create infinite head by closing the outlet valve, I would put my money on a burned out pump, but you are welcome to test that on your own pool pump.

 

Edited April 26, 20215 yr by DeepBass9

3 hours ago, ThatGuy said:

Note I don't practice thermodynamics daily, so if there's an expert working in the field who can correct me where I'm wrong, please do!

I have both studied and lectured thermodynamics at UCT, but unfortunately the field we are talking about here is in fact called Fluid Dynamics.🤣

But anyway, you are mostly correct.  Everything @DeepBass9 says is true for positive displacement pumps: if the filter gets dirty, the pump has to pump "harder" to force the water through.  The flow stays approximately constant, pressure goes up, hence power goes up.

A centrifugal pump on the other hand is essentially a constant pressure device. The static head (pressure at 0 flow rate) is proportional to the square of tangential speed of the outer perimeter of the impeller.  As pressure differential across the filter is increased (dirty filter) LESS pressure is available to overcome the pump's internal resistance and piping losses.  This reduces flow rate, hence less work is done, hence less power is used.  If flow is completely blocked you will simply spin the same water around, wasting a bit of energy on frictional losses that will heat the trapped water, possibly to boiling point. 

A very good analogy of a centrifugal pump pumping through a filter is a battery discharging through a variable resistance.  The static head is the equivalent of open circuit voltage. As you crank up the variable resistance (filter gets dirty) current/flow decreases and less power is used. 

 

Edited April 26, 20215 yr by Calvin

So how does that explain more power used with more load on the suction side, with a creepy? And also just about every pool website says, dirty filters use more power. 

21 minutes ago, DeepBass9 said:

So how does that explain more power used with more load on the suction side, with a creepy?

Really good question.  I will wire up my pool pump through a power meter tomorrow and check if I can repeat that.  Will report back.

 

22 minutes ago, DeepBass9 said:

And also just about every pool website says, dirty filters use more power. 

That one is easier.  As I said earlier, most people understand positive displacement pumps intuitively, and centrifugal pumps not at all.  Perhaps they write what they believe should be true.  Untruths has been known to flourish on the WWW....

16 hours ago, Calvin said:

18 hours ago, DeepBass9 said:

So how does that explain more power used with more load on the suction side, with a creepy?

Really good question.  I will wire up my pool pump through a power meter tomorrow and check if I can repeat that.  Will report back.

So, this morning I re-wired my pool motor so that the pump runs through the plug (meant for the chlorinator) so that I can run it through my Kill-A-Watt meter.

What I found was:

• Maximum flow: (no creepy, multiport valve on bypass) 855W
• Filter flow (no creepy, multiport valve on filter) 830W
• Zero flow (multiport valve on closed): 635W

This is pretty much what I expected - theory confirmed by experimental result. Q.E.D.

To answer "how does that explain more power used with more load on the suction side, with a creepy?" I then attached the creepy.

• Normal flow (with creepy, multiport valve on filter): 800W

This is also in line with my expectations: attaching the creepy reduced power consumption from 830W to 800W.  Not dramatic, but certainly no increase.  I have no idea how @DeepBass9 got his result of significantly increased power - it violates the principles of fluid dynamics and is (to me anyway) not replicable.

I am aware that, in this day of "alternative facts" I might be accused of being selective with my own facts.  Anybody who is near Stellenbosch is welcome to contact me and come and witness for themselves - I will leave the wiring as is for a few days.  Easier - try it for yourself.

2 hours ago, Calvin said:

So, this morning I re-wired my pool motor so that the pump runs through the plug (meant for the chlorinator) so that I can run it through my Kill-A-Watt meter.

What I found was:

• Maximum flow: (no creepy, multiport valve on bypass) 855W
• Filter flow (no creepy, multiport valve on filter) 830W
• Zero flow (multiport valve on closed): 635W

This is pretty much what I expected - theory confirmed by experimental result. Q.E.D.

To answer "how does that explain more power used with more load on the suction side, with a creepy?" I then attached the creepy.

• Normal flow (with creepy, multiport valve on filter): 800W

This is also in line with my expectations: attaching the creepy reduced power consumption from 830W to 800W.  Not dramatic, but certainly no increase.  I have no idea how @DeepBass9 got his result of significantly increased power - it violates the principles of fluid dynamics and is (to me anyway) not replicable.

I am aware that, in this day of "alternative facts" I might be accused of being selective with my own facts.  Anybody who is near Stellenbosch is welcome to contact me and come and witness for themselves - I will leave the wiring as is for a few days.  Easier - try it for yourself.

I really thought you were wrong but a clamp meter agree with you. Thanks you for the information

1 hour ago, Chris Louw said:

I really thought you were wrong but a clamp meter agree with you. Thanks you for the information

You're welcome - nice to find someone with an open mind.

Many years ago I tried the plastic pool heater panels (I had 4 of them in Spain). They were, in my case, pretty much a failure - too little heat gain for the pool.

In the end I abandoned them and installed a home made air to water heat pump. That was about 10 years ago. It's still working now - my pool is at 30 degrees as I write 🙂

Details of the build are here Pool heater heatpump