Some plumbing questions for a DIY heater

[JWBrandon]

Pardon me if this isn't the right folder, but as the questions I have are related to the plumbing for the home-made heater box that I put together, so this seemed the appropriate place.

The heater I made uses about 200' of black poly (sprinkler) piping in a coil. I use 1-1/2" PVC for all of the existing plumbing, though I sometimes wished I had taken the time to find a local supplier for flexible PVC, that's beside the point. To get the water from the pool to the heater, without having to purchase a separate pump, I added a "wye" connector coming down from the output side of the filter. I used a bushing female thread to attach a brass hose bibb. From there it's a simple matter of garden hose (wanted the flexibility and not having to worry as much about kinks) to travel ~30' to the box. Then the poly coil, then back out of the box in another garden hose which attaches to a bibb, in another bushing, in another "wye" just before the ball valve that controls flow to the pool's return jet.

Having used this setup for a few days now, and having done some experimentation, I have come to one conclusion, and devised one theory that I'd like some checking on.

It occurs to me that the heated water may not be returning to the pool with the plumbing assembly I have cooked up, or at least not regularly. With both bibbs turned full on, sometimes I can feel the outer rings of the coil (the supply side) are noticeably cooler than the middle rings (the return side), which tells me that cool water is flowing in and out of the assembly... some days it seems like there's very little difference between the two. If I disconnect the hose from the return side of the piping and drop it straight in to the pool (I don't worry much about siphoning since the box sites above the water line) it seems to me as if it will heat the pool significantly better. As I haven't had 3-4 back to back days with the same weather, this last observation could be subjective, but the one about the different temperature of the coil portions I believe is not. Is it possible that there isn't enough pressure returning to the second "wye" adapter to allow the hot water to rejoin the flow from the filter? Do I have any clue what I'm talking about?

The second question I have is this: It occurs to me that the effect of the heat from the surface of the pipe being transferred to the water flowing through it during the day when the sun is shining also means that at night when the air is cooler than the pool that some of the heat is being lost through this system in the evening. Even on an overcast day it's possible that I will shed more heat than I would gain. This means that I have to pay attention and go turn a valve when I want the water pushed through the system. What I envision, and I believe is possible if I wanted to get in to designing something myself (which I don't), would be a two-thermostat valve system that checks the temperature of the water on the supply (pool) side and if the temperature is below a certain point (I don't really want the pool warmer than 84 degrees, though my wife would disagree) opens, the other side of this valve, or maybe a second valve connected to the first, whichever, would check the temperature of the water from the heater, and if it's above a certain point (we'll just say 90 degrees for argument's sake, there would obviously be some fine tuning necessary), opens. In this manner if the pool is colder than I want it to be, and the water from the heater is warmer than the water in the pool, then the water gets pushed through the heater. If either of these conditions aren't true, then the water doesn't go. A quick search for thermostatic valves though only showed me items in the $300 and up range which I'm guessing are for commercial/industrial applications. Is there a "cheap" thermostatic valve available that would serve for what I'm describing?

Thanks in advance for responses, and I have no problem being told I'm stupid as long as there's some helpful commentary to go along with it!

 

[Bama Rambler]

While it's been proven that bought systems are much more efficient than homemade systems we won't get into that. Since you've already built the system let's make it as good as we can.

Unless the hose spigots are boiler drains or full port ball valve spigot, they're restricting the flow a lot. I'd start by changing those out. Also the hoses should be 3/4".
Is there a valve between the inlet and outlet hoses so that you can adjust the flow going through the heater?

 

[jblizzle]

Welcome to TFP!!!

I am a little unclear about the setup. Do you have 1 hose supplying the poly and then the water has to travel through 200' before it returns to the pool? If so, that is pretty inefficient and you would be better off make each coil into a parallel run ... although then you would be better off supplying it with something much larger than a garden hose.

Solar is more efficient at higher flow rates, so the more water you can get moving through the system, the better heating you will get.

Can you add a few pictures of your setup?

 

[JWBrandon]

Some pictures are attached.

Just to clarify, when I built it I was using information I had gleaned from several DIY sites. While I understand there probably weren't any engineers that specialized in thermo or fluid dynamics posting these howtos, a lot of them did agree on several key points. One of them was that the slower the water moved, the hotter it would get. A friend of mine, who happens to be an engineer at a nuclear power facility, already set me straight on that part... so I understand that there are some flaws. I'll answer the questions asked, because you all are obviously trying to be helpful, and I appreciate it... but my bigger questions are about the return fitting (you can see in the attached pictures it's not the same type of "wye" adapter as the supply, not sure how I missed it before) and the water returning through to the system post-heater, and then about the thermostatic valves.

Purchased systems probably are better, they also cost somewhere in the $200-300 range. This cost me around $50 since all I had to buy was the poly, the fittings and some cheap spray-on PVC cement (I'm redoing the system with flexible PVC next year, so I wasn't too worried about the quality of the cement bonds.) The pool thermometer that I have stops at 140 degrees F. I placed it in the bucket that I used to measure flow rate the other day (4-5 gallons per minute, in case I didn't say it before), the water coming out of the heater at around 11AM on a sunny day with 85 degree air temperature topped out the thermometer. The bucket was steaming, and I actually scalded myself (complete with red skin) when I was disconnecting the return garden hose from the pool, so the heater works in principle. I might have gained more efficiency paying 4-6 times as much, but since the energy I'm using is free... no harm, no foul, right? (For reference, see above... I know that I'd be better off with a lot more water moving and not scalding myself on it )

The spigots are not boiler or full-port, but those are cheap enough that I can certainly replace them, thank you. I can switch out to 3/4" hoses, but with the Poly being 1/2" isn't my flow restricted by that, not the 5/8" garden hoses? To be completely honest, buying 2 new 50' garden hoses will probably double what I spent putting the thing together in the first place... would it really be worth it?

I have one 50' garden hose leaving the return pipe from the filter, this attaches to the box which contains just under 200' of 1/2" Poly, then another 50' garden hose should reconnect to the pool return pipe near the jet... or so I thought. Right now the end of the hose is sticking in the pool when we're not swimming in it as, as I said, it seemed to me that I can feel the difference in termp in the pool water after a few hours with that, where having it connected to the return seemed to make no difference. (This was one of my core questions, is there something wrong with the return fitting, or do I need a different style of "wye" fitting there?)

I believe one of Bama's suggestions will improve flow some, I was kind of wondering why I didn't seem like I was getting 1/2" Poly worth of water out given how much the pump can put out (there's enough pressure coming out of the jet that I literally can't force my hand against it all the way when the pump is running.) I'll consider doing something different with next year's budget, though... perhaps running 1" supply out and back and using wider Poly for the heater surface and running in parallel. It's just not going to happen this year unfortunately, especially since I'm hoping to spend the money on some sort of automatic valve system.

Anyway, assuming I replace the spigots/bibbs with the full-port ball valves, how can I make sure the water is actually coming back in to the pool via the return pipe and not have to hang the hose over the side?

And any clue about thermostatic valves or is that not something that comes up for pools very often?

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[JasonLion]

One of them was that the slower the water moved, the hotter it would get.

While technically true, this is extremely misleading.

The slower the water moves, the hotter it gets, and the less efficient the heating system becomes. It is far far better to heat a lot of water just a little than to heat a small amount of water a great deal. The overall efficiency of a solar heating system is maximized when the flow rate is high and the temperature gain is minimized. The higher the heat gain, the lower the solar transfer efficiency and the more heat you lose to the environment along the way.

 

[JWBrandon]

Right, that's the part my friend clued me in to. Makes perfect sense when you stop listening to the internet and think for a second. The Poly that I'm using is scorching hot during the day, too, which also tells me there's a lot of heat not getting absorbed... also likely due to not enough water moving.

 

[Bama Rambler]

I'd at very least chang out the spigots for something full port. If you look at the flow path through those spigots, you see that they're tiny.
I'd also shorten the hoses to and from the heater as much as possible. Reducing flow restrictions as much as possible will help the overall system. Changing to larger hoses would help but I understand not wanting to spend more. Shortening the ones you have will help some.
You also need a valve somewhere between the inlet and outlet ports so you can close it partially so you can force an increase in the flow through the coils.

 

[JWBrandon]

Okay, so go with full port.. going to make that stop on the way home today. The shorter hoses I can do with hose repair kits and a knife, easy enough.

As far as the valve placement goes, I think i know what you mean, but to be clear: I want to place a valve somewhere in the 1-1/2" return pipe between the two spigots? Does it matter whether it's closer to the supply spigot or return spigot?

 

[Soupy]

You could consider cutting the heater coil in half and running two supply/return hoses in parallel to give yourself better flow through the heater. The valve need to be between the hose spigots so it blocks water flow in the main return pipe to divert it through the hose. I doubt it matters whether you put the diverter valve near supply or return. At the moment, it looks like you could use the valve that's right at the return for this purpose so long as you leave the return hose hanging over the side of the pool.

 

[Richard320]

I tried something like that, since my black garden hose got so dang hot on the patio. I just left the return end hang into the pool. Absolutely no backpressure that way, although it wasn't handsome by any means. Didn;t work well, either. The black hose is now drip irrigating my vegetable garden.

 

[JWBrandon]

okay, chopped off 15' of extra hose and replaced the spigots with full port quarter turn shutoffs. It looks like I can go about 30-40% closed on the return ball valve with the hose stuck in the pool before the pitch on the pump starts to change. I didn't take the time to measure, but there's easily 2-3x the amount of water moving through there now. I will let it run tomorrow and see what happens. Thanks for the tips, I will probably do a new homebrew model next year but with all this in mind from the beginning.

 

[jblizzle]

What you just did likely will improve the heating at least 2-3 times.

If you duplicate what you made a few times and ran the water through them in parallel you could get a lot more heating, but would need to feed them with a full size pipe. Still not sure that is cheaper than buying a few panels off Ebay.

 

[Bama Rambler]

Do post back and let us know your perception of how it's working. Also, check the pipes and see if they're cooler to the touch than in the past.

 

[mrmacdad12]

To have Automation for On/Off settings. try looking into a electronically actuated valve (maybe air suspension style) you would need a thermostat, relay, power source and electronic valve to have automation. Since your already using solar for your heat source why not use it as a power source too. 12volts is all you need!

http://www.ebay.com/itm/1-Electric-...025?pt=LH_DefaultDomain_0&hash=item43ba2844a9 1" Electric Valve

 

[JWBrandon]

Okay, needed a day with some sun to report results. Today was 90 degrees outside at some points, and I had direct sunlight on the heater box again for the first time since Monday. I can verify that the box and the output temperature of it are much lower than what they were before, but you can still feel the temperature differential between it and the surrounding water. And, there's a lot more water flowing out of it than there was before, I stopped filling buckets at 45 seconds and I was almost done with my 3rd 3 gallon bucket full, so... All in all I would say thank you for the advice, I know it's putting more heat in to the pool now. I'm also fairly confident that with the increased flow I can reconnect it to the pool plumbing at least on days where I know we're going to be in and out of it a lot so it's still doing something and not in the way.