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I have jimmied up a pool heater from an idea on YouTube, but just expanded it. I have a water line of 2" PVC, piped down to 1/2" copper, so it can run through a coil, then increased back to 2" PVC, and re-connected with the original pool hose of 2".

Will this keep the constant pressure that it starts with, or will I lose water pressure. It seems to have lost pressure, trying to get back into the 2" inch hose. Any suggestions?

enter image description here

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Going from a 2" pipe to a 1/2" pipe you're loosing almost 95% of your cross section. There's definitely going to be flow restriction. Not to mention the increased length from running the water through a heat exchanger coil or whatever you have set up. – Henry Jackson May 22 '13 at 15:04
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By the way, do not confuse pressure with flow rate or velocity. You say that the water appears to be "losing pressure" as it comes out of the 1/2" pipe, but in reality that expansion doesn't really cause any restriction. Much of the flow loss occurs where the water enters the 1/2" section, and then the long length of small pipe causes additional flow losses. – Henry Jackson May 24 '13 at 4:35

I believe the pressure loss depends on the flow rate. There will be some loss of pressure.

To avoid it you need 2" copper pipe or multiple parallel 1/2" pipes (probably more than 16).

enter image description here enter image description here

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I have a feeling your design looks like this:

enter image description here

You should do this instead:

enter image description here

If you make 16 parallel copper circuits, that would have an equivalent cross-section or a 2 inch pipe. However, I would probably do more like 20 circuits because all of those bends are going to introduce additional pumping/pressure losses and the extra circuits will make up for those losses.

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I tried to post a pic there of what i have...do you think if I just run a 1/2 inch hose from the end of the copper to the pool, that would solve the problem? (eliminate the back to 2 inch conversion?) – Darren May 22 '13 at 16:52
    
No. The problem isn't that you are restricted after the copper, but that the copper itself is the restriction. – longneck May 22 '13 at 17:07
    
Actually, a sudden enlargement in the pipe does cause some restriction, counterintuitively. But by far most of the restriction is going to come from the 2" -> 1/2" change and the long length of 1/2" pipe. – Henry Jackson May 24 '13 at 4:34

If you can't add parallel paths through the heater as @redgrittybrick suggests, add a bypass with 2" pipe and a throttling valve. The valve will let you balance overall flow vs heat gain.

enter image description here

The bypass arrangement will lessen the strain on your pump seals and motor.

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Is that a bar napkin I see? – Jason May 22 '13 at 18:57
    
oh no, too early for the bar – HerrBag May 22 '13 at 19:02

Generally water pressure must risen when you adopt from larger size to smaller diameter pipe.This is depend on length continued ,if is short adopt 1' or 2'" then you double the pressure and power spray ,but if go longer length and distribute to many branches then pressure will fail .in this case you need to create a lope return pipe in some section to the main 3/4 pipe .This indication of pipe system is pretty similar with radiator system known as octopus .The second drawing above is almost describe the state for equal pressure in the net distribution .The other method is to make differential between each object distance such as first extension far 3' feet then next 2'9" then next 2' and so on this will give you almost same pressure in all the extension ,but very lower than the original pressure .Other method is to use pressure tank in upper level ,which you need to connect both main and return from tank in to the system in order to have equal pressure as main pipe . '

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Welcome to Stack Exchange. This really doesn't make any sense; if you feel it's important, you should spend some time editing and cleaning it up. – Daniel Griscom Jun 10 at 2:39
    
Sorry, I find this answer hard to understand. Please take the time to edit it and make it easier to understand. Maybe adding some diagrams could help clarify your suggestions? – RedGrittyBrick Jun 10 at 13:30

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