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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?

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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
    
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
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3 Answers

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
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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.

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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
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