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Basic physics any experts?


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I want to run some pipes downhill from the pool into a field which is a few metres below the level of the pool. This is part of an experimental solar heating system. Now if the pipe is continuous and full of water is there any reason why the pool pump should not be able to push water through this pipe with no reduction in flow rate just because the pipe runs downhill and back up again. The pipe is 100mm and experiments have shown that a four metre length painted black and a flow rate of 23 litres a minute will elevate the water temperature by 0.26 degrees Celsius so I am expecting a very significant rise with 40 to 50 such pipes in full sun. I seem to have forgotten my basic physics. Would someone be kind enough to remind me.........John not Jackie
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Hi Jackie,

If the collector is lower than the pool, won't the heat will rise to the pool, without the need for a pump?

Another thought, would not the cost of all of the pipework and the pump be more expensive than a evacuated tube solar panel? This type of panel would give you heat even on a cloudy day.


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The fact that the pipes go down and then up again should not change the flow through the pump, as a syphon depends on the relative heights of the two ends.   But maybe a great length of pipework would cause increased resistance.

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Many thanks for your comments folks. It looks

like the water should flow ok through my proposed pipe system down the

hill and back up.

I am using 100mm pipes to get a reasonable

surface area, ¼ to 1/3 the circumference x the length, to expose to the

sun and because 100mm pipes seem to be cheaper than smaller sizes at

the local Bricos. I also think the problems of drag should be less for

a larger diameter pipe. No one can see our field except us so

appearance is not a problem. To get to the field a short length of the

pipes will be underground so I will wrap them in polystyrene mousse to

reduce heat loss. The pump will be the pool pump and the Desjoyaux

system has two outlets into the pool and it is the lesser one of these

that will drive water through the pipes so I will

not rely on  the thermosyphon effect. I like the evacuated tube idea

but too high tech and metal components are a no no with a salt water

pool. What I want is a low tech effective system at, of course, low


I have found that pipe orientation to the sun, all tests

were done with pipes laying flat on the ground, does not make as much

difference as you might expect. You would think that broadside on would

be best as the sun would be at 90 degrees to some part of the pipe's

surface at the solar peak. In fact pipes laid long ways on to the sun

seem to perform almost as well now though I suspect there would be a

greater difference in Spring and Autumn. I have used Sika resin for

paving slabs and pathways with some black concrete coloring agent added

to give a black durable paint. The initial shine soon wears off giving

a nice matt finish. I have used the same paint on the solar pavement,

which replaced my open trough collector with good effect though the

solar pavement is only two thirds as effective as the open trough. The

open trough is worthy of further experiment but I would use matt black

tiles next time as no paint I tried could stand the combination of salt

and chlorine in the pool water. Not a problem with black painted

plastic pipes of course.

I must say I wonder if I will bother to get this project underway for this summer considering the grotty weather at the moment!

Again many thanks for your replies, all good low tech science and keeps me off the streets!......................John not Jackie

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We, as you can see, are in the Aude near Carcassonne. In the winter we often get very sub zero temps. What will you do when you get the same? You will have lots of ice cubes, peut etra?

I'm not trying to put a freeze (sorry) on your project, but it would be a terrible shock to find most of your pool in your field?

Even if you kept the pump running every time freezing temps were forcast there are always times when they get it wrong? That's a lot of pipe work to protect.

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Hi again,

Metal components are not a problem for a salt water pool with a stainless steel heat exchanger. Before you go to all that trouble and possible expense (unless you already have all of the tubing), check out the evacuated tubes. Also have a look at Navitron's Forum. Lots of like minded people there. It's got to be worth a bit of research, especially as you are not sure as to whether you will complete it this summer. The tubes will give you more heat for a longer period of the year and will take up less room. You will benefit by having a longer period to enjoy your pool. You can also install the system yourself.


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Yes thanks for the gen and the evacuated tube system looks really good and I might go that route for house hot water preheating in the future. The great thing about my low tech system is that it is cheap compared with commercial products. Bricomarche charge 5.4 euros for a 4m 100mm pipe just now and some suppliers are cheaper still so although my system may not be as efficient or attractive on size as a commercial product,  it is cheap, my Scottish ancestry showing here, and a few extra pipes will compensate for the lower output.

The external pipe system can easily be drained in winter so freezing is not a problem by the way. Anti-syphon device fitted, a small diameter hole in the feed pipe before it leaves the pool. We don't lower the level of the pool  in winter other than to pump out water when the level gets too high because of rain.We do run the pump at night on cheap rate when the temps drop below zero to protect the pump and there is no build up of green stuff of any quantity maybe because of the salt and the fact that we clean the pool from time to time in winter. I have only seen the pool surface freeze once in the last 7 years and 75000 litres has quite a huge thermal capacity. The last pump carried on working for 6 years before the bearings gave up the ghost. I guess about a quarter of the water gets changed each year because of rain and we only have to add about 4 bags of salt in the spring.....................J

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Yes I would think so. No leaks in all the many joints made so far. I have no idea what the pressure is and it is only a percentage of the pump output and a small percentage at that. The pipe prices are standard for 100mm pipes. They are cheaper because they are most frequently used for waste water etc I guess. Prices for plastic pipe do vary so maybe price is linked to the cost of oil. E. Leclerc seem to be the most expensive around here and Emeraude the cheapest that I know of. I have yet to try Point P.....J
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