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Pex Solar Water Preheat Project
I'm starting a drainback type solar preheat system on a rental property. I have the tank mostly built and that will be fed by a pex collector I have yet to build. Off to move the tank into the property's basement, I'll have pics up soon.
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Hi,
You are probably aware of this already, but you want a steep tilt and only single glazing on the PEX collector to avoid stagnation temperatures getting high enough to damage the PEX. This one is tilted at 70 degrees and still shows occasional stagnation temps as high as 230F, which is about a high as you ever want to go with PEX even in a non-pressurized system: $1000 Solar Water Heating System -- Performance Gary |
Thanks for the tip Gary, I plan on mounting it vertically to get around the stagnation issue and btw I must I'm a huge fan of everything you do on builditsolar.com.
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Any updates on this?
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Yes! I've been busy recently but I got my pictures finally downloaded onto my computer. Right now I'm probably 75% done on the Storage tank side and I'm generally following the $1000 design on builditsolar.com. I plan on having 166 sqft of collector area that drains back into a tank roughly 3.5ft x3.5ft x3.5ft of interior storage space that yields 322 gallons of storage capacity.
https://lh3.googleusercontent.com/-a...0301170641.jpg I started with 3 4'x8' sheets of 3/4" treated OSB as I wanted something strong, moisture resistant and economical. I cut them into 4'x4' pieces. The sidewalls rest flush on top of the edge of the bottom sheet. The wall widths are 3'11&1/4". This allows them to but up against the next one in a "circular" fashion (you'll see in later pics). https://lh3.googleusercontent.com/-x...0301204451.jpg I screwed on the top and bottom supports (from the tank inside of course ;) ) using treated 2"x4"s I have them overlap each other for easier final reassembly. One thing in retrospect I wish I had done is to directly attach the bottom support to the base sheet as the sheet would take the outward pressure at the bottom walls better. To address this issue I simply added another layer of 2x4s to reinforce it more. Also, Torx bits are your friend :thumbup: https://lh3.googleusercontent.com/-J...0310134957.jpg Here I glued and screwed the middle supports onto the walls. A good New Glarus Fat Squirrel beer makes things go much faster (or maybe it only seems lol ) I also used Gorrilla Glue brand woodglue, idk how much of a difference it really would make over regular woodglue, but I figured it couldn't hurt. |
https://lh6.googleusercontent.com/-1...0415215744.jpg
https://lh5.googleusercontent.com/-P...0416150124.jpg Disassembly to put 2 coats of sealing primer on each piece. https://lh6.googleusercontent.com/-m...0416174436.jpg Detail of how the base rests on 4" of XPS foamboard. https://lh3.googleusercontent.com/-u...0416174502.jpg Closeup of how the corners interlock. One issue I had was that after priming, it was harder to reassemble everthing cause the pieces liked to stick together. https://lh6.googleusercontent.com/-M...0416201423.jpg Final check before moving the unit. The inside looks so neat and clean after the paint job! |
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Moving the pieces and reassembling in the apartment basement. Space was tight and had to fit in a space between 2 walls, a dryer and a basement jack. Definitely had some tetris moments. https://lh3.googleusercontent.com/-k...0418151732.jpg Finally assembled! https://lh4.googleusercontent.com/-z...0430183533.jpg I put nail plates to reinforce the outside supports as an extra precaution. You can also see how the interior polyiso insulation fits on the walls. I put 2" of polyiso on the floor of the tank first. https://lh6.googleusercontent.com/-9...0430195944.jpg How the polyiso wall insulation is layered. |
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Tank with insulation. https://lh5.googleusercontent.com/-q...0505192001.jpg Cut the insulation flush with the top. I also made up and printed a label and a some safety warnings, laminated them in 2 layers of contact paper and stapled them up. https://lh4.googleusercontent.com/-R...0505202344.jpg Fitting the liner. I used a 15'x15', 45 mil EPDM sheet. Took some finagling to get it mostly fitted. So that's where I currently am in the project. I have to tackle the lid next and then probably the pex heat exchanger and all the necessary plumbing. |
Wow, 166 sqft. That sounds like it should be quite capable of providing nearly all the DHW unless there are many people in the building. What is this setup going into?
Nice work with the tank too. It looks like it JUST fits! Are you planning on adding any additional insulation on the outside? |
Currently there are only 3 people, but with changing renters that number could always easily change. The tank feeds into 2 hot water heaters (one gas one electric) that each have their own water supply. I debated putting extra insulation, and maybe I should have, but that's something I could do later with some spray foam and EPS. It might be tricky though with the tight fit haha.
I figure in 5+ years when the water heaters need to be replaced, I could use cheaper smaller electric models and super insulate those :). I'm looking at a Swiftech MCP355 pump to lift the water around 16 ft up for both panels. It's a cheaper option around $70 USD but what do you think of the sizing and do you think it'll have enough head pressure to do the job? Looking at the PQ Curve I'm a little confused... MCP355 - Rouchon Industries Inc., dba Swiftech - PC Liquid Cooling Systems CPU Cooler VGA Water Block Heatsink Pump Radiator Heat Exchanger Kit |
I'd say its definitely not enough pump for this application. 16 feet is 4.87 meters. The DDC3.2 is the newer pump design so thats what you'll probably get. It can't even pump up to 4.8 meters. You could use two of them in series though. That would give you double the flow rate at the same head, so at 4.8 meters you should have somewhere around 300 L/hour or 1.32 gpm. Then you have to add in the friction losses from the piping so it'll be lower than that.
According to some guesstimating on my part that doesn't sound like nearly enough. I think my 200sqft array will need between 5 and 10 gpm. http://www.swiftech.com/images/mcp355-32.PNG |
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Here found some good info on the subject. Gary at builditsolar did some testing and here we have it:
Determining Solar Water Heating Collector Flow Rate http://ecorenovator.org/forum/attach...1&d=1336491328 So, lets say you go with .03. You'd need to be pumping 5 gpm. |
I still need to measure where the top of the tank waterline will be (I believe that subtracts from the head height?), but I believe I will need two pumps as the system is in two panels and could possibly be shaded at different times. The last thing I need is to be unnecessarily pumping how water through a cool collector. Panel #1 is 86 sqft and panel #2 is 80 sqft, giving flowrates of 2.6 for #1 and 2.4 for #2. Panel #1 has maybe 14-15ft of head and #2 around 12 ft of head.
On the Pump selection Wizard on the Taco pump website, I get results well inside pump's PQ curve. I take it there must be some way to limit the flow rate then? Like valves or a control on the pump itself? Also, I'm wondering if the head pressure changes after the water starts flowing through the collector causing some sort of suction effect? Would that in turn increase the flow rate? I'll get some definite measurements of the heat heights. |
The head height will be the top of the water line to the top of the collector outlet pipe.
You can limit the flow, but there really isn't a reason to. Find a pump that comes closest to meeting your flow requirements and just let it flow as much as it does. You could restrict the flow, but you'd still be using the same amount of power and with the reduced flow you'd just be loosing efficiency through the collector. The head pressure won't change as things start flowing. Basically you're spending energy to get the water to the highest height, then gravity takes over and takes care of getting the water back down to the tank. Unfortunately there is no suction effect. Also, I'm not sure how much distance is between your tank and collectors, but the pipe between them will add additional pressure to overcome due to friction. It probably won't be a ton, but its at least worth looking at. It will also determine what size pipe you go with. |
Thanks Daox, that simplifies things a lot. I just measured everything, and panel #1 has 13' 6" of head and panel #2 has 11' 6". As far as the distance goes, it's basically all vertical. The tank is on the basement side of the same wall the collectors are on. I'm thinking 3/4" pex to supply the collector right now...
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The Taco 008 Circulating pump looks like a promising candidate for the first panel. Max head of 15ft, 2.5 gpm @ 14.5ft, 3.5 gpm @ 14ft and 4.5 gpm @ 13.5ft.
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Yeah, the 008 sounds like a pretty good fit for the #1 panel. Looks like the 0010 might be a good choice for the 2nd panel, or another 008.
I threw this picture together real quick to show what we're looking at. Red is panel #1 and green is panel #2. Some rough calculations show that you'll loose another .7 ft of head due to friction losses. Gary has a nice write up here on how to calculate this. http://ecorenovator.org/forum/attach...1&d=1337090219 |
The Grundfos UP 15-18 B5 looks good for the second panel. Max head of 13.75ft. 2.25gpm @ 12.5ft, 3.25 gpm @ 11.5ft. Seems to be right in the sweet spot. $140 doesn't sound too bad either.
Both pumps pull under 1 amp so they could be relayed with a couple TinyDTC Temp Controllers. Anybody have any experience with them? Look like a bargain to me. |
Never used them and haven't heard feedback on them. They seem like nice little packages and a good list of features for the price as long as you don't care about having it all packaged in a fancy plastic box.
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Nice. Okay, so it looks like being conservative and allowing even a 1ft head increase from friction losses, the Taco and Grundfos are close to or above the .03gpm/sqft range. 2.25gpm for panel #2 -> .028gpm/sqft and 3.5gpm for panel #1 -> .041gpm/sqft. And I expect the flow rates will be a little bit higher if only .7ft of head is added.
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Update:
Folding and securing the EPDM liner https://lh3.googleusercontent.com/-_...0516134027.jpg Putting on the composite decking boards over the liner. https://lh4.googleusercontent.com/-l...0516135828.jpg https://lh4.googleusercontent.com/-7...0516143335.jpg Assembling the tank lid. It's in 2 2ft by 4ft sections. I didn't have enough extra EPDM, so I used 4 mil plastic folded over itself in multiple layers and stapled down. It shouldn't reach the plastic's melting temp, and will see how it holds up over the years. https://lh3.googleusercontent.com/-Y...0517162021.jpg Lids in place, the nearest decking board isn't attached yet at pipping for the 2 pex Heat exchangers and 2 pump lines are yet to be put in. https://lh5.googleusercontent.com/-Z...0517162944.jpg https://lh3.googleusercontent.com/-K...0517170529.jpg I also ordered 2 300ft rolls of pex from pexuniverse and they're on the way. The next step will be integrating the storage tank into the existing plumbing :thumbup: |
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Stop the presses!! Before you buy any "traditional" circulators, be sure to check out the electronically commutated pumps built by ITT/Laing and sold under Laing, Bell & Gossett and others.
Their E10 composite housing pumps, often used as replacements for hottubs, have a similar max head as the Taco 014-21/22ft. They flow the same amount as the 014 to a little over 6gpm before falling off in max flow capability. The E10, however draws 50w at 2gpm/20ft of head and a max of 60w, at which it flows 6gpm at 20ft head. The 014 draws 178W! The E10's can be found on ebay for $130-180 depending on voltage and plumbing connections. The E5 draws 39w max, and produces 14ft of head. It flows 6.6 gpm at 9ft of head. The taco 008 produces 15ft of head and draws 91w! For lower head requirements, the e3-6 circulators are brass/potable water rated, have a max head of nearly 10 ft, outflow a Taco 007 to 3.5gpm while drawing max 28w. I'm using these in my open loop radiant floor system and love them. The 007 draws 82w! You could use 1 E10 pumping to both panels with an actuator valve and an inline ball valve for each, allowing you to finetune flowrates. You would then have 60w max draw instead of around 91w. Or, use 2 E5's for around 78w instead of around 182w. Corrosion of cast iron pumps is almost always what causes them to fail and also causes an insulating layer of rust deposits on your HX and collector surfaces, dropping efficiency. These Laing pumps are made of non-corrodable materials and are jamproof due to a single, pivoting spherical bearing: http://ecorenovator.org/forum/attach...1&d=1337628056 They also offer other models that may serve the need better than the examples I gave here. Grundfos has their ECM controlled Alpha pumps, but they are pretty expensive in stainless steel. Here is a pic of my SR728C controller ($135 shipped on aliexpress.com), an e3-4 circulator on the bottom right, and an e5 circulator on the top right: http://ecorenovator.org/forum/attach...1&d=1337628056 The SR728C has 5 temp inputs, 2 separate pump controls for multiple arrays, a 1500w additional output control, etc etc. My money is on these! Craig The MMT ps: LOVE the tank decals, menaus2!!!! You wanna sell some? |
Very interesting. I'm also working on selecting pumps for my solar hot water system. I'll definitely look into these other manufacturers. Thanks.
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Hi,
Here are the brochure and specs on the Laing E10: http://completewatersystems.com/wp-c...04/LTBR-21.pdf http://completewatersystems.com/wp-c...4/LTSS-301.pdf Looks like a lot of good features. One not so good thing is that the max temperature is listed as 140F. I've used the several of the Grundfos 15-58 three speed pumps with good results. Very reasonable price -- $70ish I think? The three speeds provide a lot of flexibility. I've not had any problem with using the cast iron case pumps -- have one that has been operating since 1996 and is still working fine -- this may be a function of water pH etc that the pump is running in. Gary |
The brass housing e1/e3, etc. use similar innards and are rated to 203F. The composite material used in the E5's and E10's is the same stuff car radiator tanks are made of. Those regularly see 230F temps on the inlet nipples and go many years before failure-I suspect Laing is just being cautious.
In any case, since pumps are typically on the supply/upstream side of the heat collector and stratification in the tank makes for lower supply temps to the pump, I think it is unlikely the pumps would ever see sustained temps much above the 140F rating. Even if used in a radiant heating setup where the pump sees the hottest water, not many are running hotter than 140F supply water. Many setups do fine with cast iron, but if there is a problem, once you realize it you already have that insulating coating of crud on inner surfaces. Addding together the benefits of quiet running (you can drown out the sound of my e1'S and e3's with a whisper), the power saving, and the avoidance of corrosion possibility, it was a no-brainer for me. Craig |
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I also was thinking about the possibility of using a single pump with a 3 way actuator, but I think the valve could prevent proper drainback. Programming some kind of delay between the two sounds over complicated, and an extra pump cant be much more than the actuating valve anyways. Instead maybe an E10 for the 13.5ft head panel and an E5 for the 11.5? I like the idea of using a couple inline ball valves to adjust the flow rates. |
Forgot yours was a drainback system-easier to just use 2 pumps and 1 each of the e5's and e10's should work great. Ball valves are the easiest way to throttle flowrate. If you are using separate pumps anyway, unless you can take advantage of stratification (difficult in a shallow tank), you may as well just let them run full bore and get better efficiency out of the collectors.
Another thought-some of these Laing designs are also available in variable speed like the potable e1/e3's like I have. This would allow you to finetune flowrates by dropping power consumption, saving energy. You may want to kick around and see if they offer other variants suiting your needs that come with a speed control. The only downside (a small one compared against the benefits) to these pumps is since they are electronically commutated, they probably won't work on a PWM variable speed control. My SR728C can control 2 arrays simultaneously and one of the pump outputs can be set normally or vary pump pulsewidth to maintain a specific deltaT across the collector array as solar energy waxes and wanes. |
Hi,
It looks like your collector area is 166 sf and your vertical distance from tank water level to top of collector is 16 ft? If so, I'll make the case for something like a Grundfos 15-58, and you can compare it to the Laing pumps. The Grundfos 15-58 looks like a good match for your startup head requirement of (16 ft)(1.2) = 19 ft. And if you go with about 0.04 gpm per sf of collector, you need 6.6 gpm. At 6.6 gpm, the 15-58 delivers about 13 ft of head, which is probably enough to overcome pipe friction (which is all you need to do after startup). You can work through this sizing procedure to make sure: Pump and Pipe Sizing for a Solar Water or Space Heating System This is something you want to do for any pump you are considering. The pump curve for the 15-58 is on this page: New Page 1 The 15-58 may actually do more than the 0.04 gpm/sf, which will give you somewhat greater efficiency -- all good. The 15-58 is $75 depending on where you buy it. On high speed power consumption is 85 watts. The pump is very quiet -- I have to touch mine to see if its running. Its good up to 230F. These cast iron circulators are built like tanks and have the reputation for very long lives. I don't mean to sound like a salesman for Grundfos -- Taco and others make similar pumps that would likely work as well. I have trouble with the argument for varying pump speed to save pump power under lower sun conditions for these reasons: When your 166 sf of collector is in full sun and assuming it achieves a standard sort of efficiency, it will be producing about 24000 BTU/hr or about 7035 watts. This is from: Solar Collector Efficiency Calculator If you look at this page: Determining Solar Water Heating Collector Flow Rate The drop in collector output in going from 0.04 gpm/sf down to (say) 0.02 gpm/sf is about 3%, or 210 watts. So, to me, one message is that you want to be very careful in dropping pump flow rate on the idea that you will end up saving pumping power -- the drop in collector output for such a change is likely to be a lot more than the saving in pump power. Granted, the example above is done with full sun, and the drop in collector output would be less than 210 watts in part sun, but is it really going to be less than the 20 watts or so that you save by ramping the pump speed back? To me it, the variable speed pumps seem like a lot of complication and expense to maybe end up not saving anything? I'd be glad to be wrong about this if someone has good numbers to show a real saving under real operating conditions. The one advantage I do see to variable speed pumps for drain backs is that they can prevent the kind of short cycling that sometimes occurs under low sun conditions when the collector can get hot enough to trigger the controller to start the pump, but cannot collect enough heat to keep the controller from turning the pump off as the collector cools when the flow starts. But, this is not normally a problem worth worrying about much, and a simple 2 speed pump would be plenty to handle this. I'm not saying this is as slam dunk decision for either Laing or the Grundfos (or similar) pumps -- I can see pluses and minuses to both. To me it comes down to is the power saving worth the extra cost and the risk associated with the low 140F operating temperature limit? --- Very nice job on the tank! Gary |
Sweet!
I just skimmed this thread and found the workmanship of the tank very impressive!
The insulation job looks topnotch, so performance should be excellent. I'll bet it's going to outlast all the other system parts. Bottom line, pure Craftsmanship! |
I realize that I may have missed something, but wouldn't it have been a better idea to have put the insulation on the outside of the box, rather than on the inside?
It looks to me that the spaces between the 2x4s would be a good place for the insulation. This would have resulted in a considerable increase in internal water volume and therefore heat storage. -AC |
I've a question about the pumping heads. Once the flow is established, won't the return pipe, the down leg, be full of water? If it is, won't there be a siphon effect, effectively reducing the pumping head to the friction losses only? So you need to have a pump capable of lifting the water the full head height to prime the system, for which a relatively low flow should be OK, but it only needs to be able to pump at full flow against a much lower head.
I realise that I am almost certainly wrong here, given all the gurus commenting here, but I'd really appreciate an explanation of where my logic has gone astray. |
The return pipe will not be full of water on the way down (it'll drain faster than the pump can pump). Therefore it can't create any suction to assist in pulling the water up. Even if the pump could keep up, it would then also not create suction because the pump would still be pushing everything along.
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Hi Daox, Water can only drain faster than the pump can supply if air is able to flow up the pipe against the water flow. Unless the return pipe is over-sized, I am a bit surprised that this would happen at 5gpm flow.
It's over 40 years since I last did any pipe network calculations and I wasn't much good at it then. Even so, I am pretty sure that your second sentence is not right. Assuming that the pump can establish the flow to the point where air ingress does not occur, the top of the circuit can be in suction even as the pump operates. I surmise that the critical point would be to ensure that the pump must be capable of establishing enough flow to drive the air out of the system before the partial siphon effect can kick in. An alternative might be to have a flow restriction valve at the outlet, that is switched in for a short time after the pump starts, but this is getting complicated. |
I should Google first. see here:
Pump and Pipe Sizing for a Solar Water or Space Heating System Quoting from that page: Step 2: Measure the Pump Vertical Head Requirement For a drainback system, when the sun comes on the collectors, the controller will turn the pump on, and the pump must be able to pump water all the way up to the top of the collector to start the flow. In order to make sure the pump can do this, you must carefully measure the VERTICAL distance between the water level in the tank, and the top of the collector. If the top of the collector is 30 ft over and 11 ft up from the tank water level, the number you want is the 11 ft -- this is the vertical distance from water level to top of collector. Measure this carefully -- don't eyeball it. Note that you measure from the water surface level in the tank, not from the pump level. This requirement at startup to pump water from the tank all the way to the top of the collector is a tough requirement for pumps, and it will likely be the main consideration in picking the pump to use. For the my Solar Shed system, the vertical distance from the water level in the tank to the top of the collector is 11 feet 3 inches. So, the pump must be capable of pushing water up at least this vertical distance. Some margin should be allowed about the vertical distance. In other words, don't choose a pump that just barely has enough static head to make the top of the collectors. People vary on how much margin should be added, but Alan R.ushforth, who has done a number of drainback systems and has had experience with pumps that don't quite get the flow started recommends that the pump have a static head capability that is 20 to 30% greater than the vertical distance from tank water level to the top of collector. I would use that recommendation. So, for the Solar Shed, the minimum pump static head would (11.25ft)(1.25) = 14 ft of static (startup) head. Once flow is well established, and the return line is running full, the only pressure that the pump has to overcome is the friction losses in the pipe -- that's what the next step is about. |
Hi A C Hacker,
The problem of putting the insulation outside the wood is that the wood will be at elevated temperatures for very long periods. It is considered undesirable to have structural timber at more than 150F for very long. http://thermotreatedwood.com/Researc...H%20+%20WI.pdf |
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On a drain back, my understanding is that the return line should run full, and if it does not something like a partially closed valve should be put in the return line near the tank to offer enough flow resistance to keep the return line full. I have to do this on my Solar Shed system because the return line is short and direct -- without the valve it won't run full. In addition to increasing the pumping head requirement when its not running full, it also makes quite a bit of of noise. Gary |
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I think either inside or outside works. I like the inside because its easier to install and you get no thermal bridging. You can also do both -- especially if you don't want to give up too much of your tank volume to insulation space. Gary |
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Thanks for your reply. The data sheet volunteered by Snail: http://thermotreatedwood.com/Researc...H%20+%20WI.pdf ...has some very important information that should be read by all. -AC |
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