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Daox 06-16-11 03:28 PM

Designing & building a solar hot water tank
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With the solar panel racks designed to my liking, I am moving on to designing the tank for the solar hot water setup. I think I'm actually going to build the water tank first as I can use it as a temperting tank for my domestic hot water until I get the solar panels installed and working.

The tank will be made of wood, and hold around 400 (updated to 650 now, see later posts) gallons of water. It will be built similar to Gary's DIY solar hot water tank. The main differences are that mine is going to be smaller, it lined with a different material, and insulated mostly with cellulose instead of foam board. These changes are mostly to decrease cost. The smaller tank size matches my 200 sqft of collector with a rule of thumb that you need 2 gallons per sqft of collector. I'll go through the heat gain calculations in another post to see exactly why more than 400 gallons of water is not needed and might actually be bad for the system depending on how you intend to use it.

Anyway, I got it somewhat modeled up over lunch today. As you can see, its very similar to Gary's tank. I will be doing some FEA to see if the plywood thickness can be reduced. This could save a decent amount of money.

Daox 06-20-11 01:56 PM

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I ran a few simulations over lunch today. The weight of just over 400 gallons of water is roughly 3,600 lbs. That is more weight than most cars, so its nothing to play around with.

The one thing I struggled with was getting the water weight forces to come out exactly how they would in the real world. The bottom of the tank is obviously going to see the full 3,600 lb force of the water, but what about the sides? At the bottom they'll see the most pressure, but at the top it will be less. So, I'm still searching for a formula that will help me figure that out. For now, I used 1 psi as my baseline. The base of the tank is 3,400 sq/in, so roughly 1 psi. I used this for the sides as well. Here is what I got:

This is with 15/32" thick plywood. Try to ignore the colors as they don't truely tell you the whole story. The graph on the right shows the peak stress and the scaling.

This is with 19/32" thick plywood. Again make sure to reference the graph as the scale varies.

Now with 23/32".

And once again with 3/4" thick plywood.

As you can see, the 3/4" plywood showed quite a bit more strength than the other two thicknesses. I'm not quite sure why the huge reduction in stress. But, its not really consequential. Plywood has a tensile strength of 4500 psi. So, even the 15/32" can hold up to the stress of the water (assuming my simulated pressure isn't way off). I'll guess I'll run a few more simulations with even thinner plywood. However, there is a point of diminishing returns I'm sure. The 19/32 4x8 sheet only costs $3 more than the 15/32 sheet, so there isn't huge cost savings to be had.

Daox 06-20-11 02:19 PM

So, I talked with an engineer at work and he cleared up the numbers I should be using and the forces involved. I was using 1psi where I should have been using 1.3 psi. So, the stresses should go up about 30%. I'll rerun the simulations with the new numbers to make sure this is the case. I'll also incorperate the correct loading on the sides of the tank.

RobertSmalls 06-20-11 10:15 PM

Interesting. I just figured tanks were something you had to buy, but it looks like Gary has pulled it off.

I would caution that wood, ceramics, and other materials with brittle failure modes don't always behave the way the FEA models suggest. Minor cracks and porosities that would have little effect on a ductile material will make the strength of brittle materials more unpredictable. Stress concentrations, such as at fasteners, will be significant.

I wonder... could you use steel hoops (or fiberglass) for their tensile strength, to allow you to reduce the amount of plywood required?

My intuition tells me a square tank would be better than rectangular, in terms of both cost and heat retention.

Btw, what software are you using?

Daox 06-21-11 08:11 AM

Yep, Gary has a few different ideas on DIY tank designs on his site thankfully. However, I believe his is the cheapest and easiest to build and insulate. A 120 gallon tank retail for $1000+ and I'd need a few of them! My current cost estimate for the tank and liner (no plumbing) is about $250 in materials.

I agree, a square tank would be more thermally efficient. The currently modeled tank can hold about 650 gallons of water. Since I only need 400 I will be shortening it up a bit length wise. However, I still have a rectangular footprint to put the tank in, so that will also be determining the shape of it to some extent.

The tank (and the rack) is modeled in SolidWorks and analysed with CosmosWorks.

Daox 06-21-11 01:08 PM

I redid my calculations on the tank sizing and decided that going with a larger tank will actually be worth it. The end result is the lower temperature increases collector output enough to be worth the reduced tank temperature.

Here is a quote from another post where I explained the calculation:


Thats all quite true AC Hacker. It made me rethink things (as well as S-F's comments). So, I ran my own numbers on using a 400 gallon vs 650 gallon tank. There are a few effects to consider.

First off, this change lowers the tank temperature. The reduced tank temperature reduces the heat loss of the tank due to the water being cooler. However, the tank is also physically larger, so it has more surface area to loose heat from. In the end, the larger tank does loose more heat, but the effect is very minimal. All in all, the 650 gallon tank looses ~300 btu per day more than the 400 gallon tank. This is nothing when you consider even in December (worst month for solar), my average solar heat input is ~79,000 btu.

Second, the tank change also increases the collector efficiency. This is because cycling hot water through a collector makes it picks up less energy. The panel also stays cooler and looses less heat to the environment. This is where most of the benefit comes from.

Lastly, going to a larger tank supplys you with cooler water. In my case, going from 400 to 650 gallons will likely lower the average tank temperature about 10 degrees (from 115 to 105F), and lower the peak temperature from 140F to 120F. 105F is just barely warm enough to shower, so some additional energy may be needed for DHW. However, 105F is fine for hydronic heating. Considering that space heating is by far the larger load and energy hog, I should primarily design to its needs. In addition for 6 months of the year I'll be collecting more heat than will be used, so hot water won't be an issue then.

All in all, going from a 400 to 650 gallon tank will allow me to collect about 6.7% more heat from the panels. I just need to be aware that I'll be dealing wih a lower temperature water and design accordinly for it.

Daox 06-21-11 01:28 PM

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So, I just finished up running the next simulation with the 650 gallon tank and 15/32 plywood and things look pretty good. A max stress of just under 3000 psi puts us in pretty good shape. I didn't bother correcting the load distribution on the sidewalls. If its good now, it'll only be better with less loading on it.

Here is the stress distribution. I'll probably add some corner supports like Gary did with his, especially since I'll be using thinner plywood. Without something solid to screw into, it'll be hard to make a strong joint.

strider3700 06-21-11 02:38 PM

I'm curious what are the dimensions of the tank you're thinking about?

Daox 06-21-11 03:32 PM

The base of the tank is a 4x8 sheet of plywood. There is 2x4 framing around it and the side sheets are 48" tall. The dimensions of the inside of the tank are 40"x88"x48". If filled to the brim, it'll hold about 730 gallons. With the fill level 6" below the top, you get 640 gallons.

AC_Hacker 06-21-11 03:58 PM


Originally Posted by Daox (Post 14182)
...Here is the stress distribution...

The stress distribution looks the way I would expect it to look if the box had a lid and was filled with a compressed gas...

I would expect that an open box filled with water would show some indication of greater stress toward the bottom of the sides and end, and a reduction in stress to zero toward the top of the box.


Daox 06-21-11 04:05 PM

Yes, you are correct. However, to keep things simple for me, and to build in a bit of safety factor, I kept the load uniform on the sidewalls. So, the same amount of stress that is on the bottom is also on the top. Its a bit of overkill, but thats not a bad thing and it simplified things for me.

I still have a few more iterations I want to run. I might take the time to make a full nice and proper model.

Daox 06-21-11 06:19 PM

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I ran a few more iterations since the last posting. I'm really not sure how well wood ages strength wise, so I was looking for a little added assurance. I'd like the tank to last as long as everything else, so I'm looking at may years of service. Also, if I happen to get a small leak and the wood gets wet... There are many things that could compromise the tank, so a little extra beef isn't bad.

Anyway, I tweaked my pressure setting for a deeper tank which I forgot to do, and I also added some additional bracing. By adding a couple more 2x4s it is really looking good ($10 of wood maybe). The only high stress point is right under the bar that goes across the tank in the middle. Tomorrow I'll try running a simulation with correct loading on the sidewall and see how that effects things. It should reduce the stress on the cross bar. Until then, this is the current design using 15/32 plywood sides/bottom.

Daox 06-22-11 09:13 AM

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This morning I ran one more simulation. I still couldn't get a nice gradual pressure change over the height of the sidewalls. So, instead I split them into three different sections. I applied 1.7 psi to the bottom, 1.1 psi to the middle, and .56 psi to the top. You can see the divided sections here.

I then ran the simulation and here is what resulted, 1800 max psi. This is roughly 1/3rd what the plywood can take so I should have plenty of safety factor.

The end result of all of this design work is that I'll be saving around $55 in materials cost. Considering my total estimated cost for the tank is now around $300 (I haven't figured out all the insulation costs yet), that is a substancial percentage of the cost of the tank.

S-F 06-22-11 12:30 PM

Just find some recycled polyiso. I just got 80 sheets @ 4' x 8' x 2.25" for $14 a sheet delivered.

AC_Hacker 06-22-11 12:48 PM


Originally Posted by Daox (Post 14199)
This morning... I split them into three different sections...

Looks like a fair approximation...


Daox 06-22-11 01:19 PM

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I emailed Gary to get his input on the tank design. He recommended not exceeding 1800 psi. So, I reran my simulation with 19/32 plywood (an extra $9), and the stress dropped to just under 1400. I'll probably go with that just because a little extra won't hurt. $9 for peace of mind is more than worth it.

TimJFowler 06-22-11 05:44 PM

How are you planning on fastening the wood and sealing it?

Daox 06-22-11 05:58 PM

The wood frame will be screwed and glued together.

To line the tank, I'll be using a paint on liner material called permaflex. It says it holds up to 250F temperatures. It is also less expensive than a pond liner like Gary used on his tank. We'll see, but I think it might also be easier to install as well. One person who submitted his project to builditsolar is using it. I've tried to contact him to verify that its all working well for him, but haven't gotten a reply.

greif 06-23-11 08:06 PM

Paint on Waterproof Liner For Solar Heat Storage Tanks

I highly suggest to use 2 x 6 supports and run a few more around the preimeter like I did, maybe every foot up as that is alot of pressure on those walls, I have 1.25 thick walls and as you can see I have lots of support and still I get about 3/4" bowing about 2' up, I use 2 x 4 but wish I would have did 2x6, I will be adding more this summer.

Gary Reif

Daox 06-24-11 09:53 AM

Thanks for the info Gary, I really appreciate it!

As I recall your tank is very tall. The height of the water directly effects the pressure at the bottom and thus the gradient of pressure that the wall sees, the bottom seeing the most. Since my tank is so much shorter, it shouldn't need the extra reinforcement that a taller tank would. According to all the engineering data I can find, this should tank should hold up fine. Will it bow? I'm sure it will, but wood bends a lot before it breaks.

Moving forward, I have to get measurements of the exact envelope that I'm working in. I'll be designing another frame around the tank to hold the cellulose. It will be a very low cost design being framed in 2x4s or 2x2s and covered in construction plastic as much as possible. I'll then blow cellulose into this framing and that will give me my inexpensive insulation.

TimJFowler 06-24-11 05:38 PM

Permaflex looks like a promising tank liner. I'm also interested in hearing about some 'real-world' experience with it.


Originally Posted by Daox (Post 14229)
Moving forward, I have to get measurements of the exact envelope that I'm working in. I'll be designing another frame around the tank to hold the cellulose. It will be a very low cost design being framed in 2x4s or 2x2s and covered in construction plastic as much as possible. I'll then blow cellulose into this framing and that will give me my inexpensive insulation.

Why not go with XPS or other non-hygroscopic foam for the insulation? You wouldn't need to build a 2nd frame to hold the insulation and you avoid possible issues with water condensation in the cellulose.

Also, how will you be insulating the top and bottom of the tank?

Thanks for the write-up. I like seeing someone else's thought processes on a project like this.


strider3700 06-25-11 05:04 PM

any of the foams are going to be much more expensive for the same r value. also depending on the temperature you're going to run the tank at you may need to go polyiso as the others melt at a lot closer to solar hot water temps.

Since my tank will be in ground foam was the only option. I think I could get away with any of the foamboards but I found used polyiso on for real cheap and bought a bunch.

Daox 06-27-11 09:08 AM

Strider is exactly right. The cellulose is quite a bit cheaper than rigid foam, and low cost is the reason why I'm using it. The frame will costvery little and be quite simple.

Directly under the 4x8 sheet of plywood for the tank, I'll be using multiple sheets of rigid foam. I know the first layer will be polyiso to take the heat, after that I'll go with whatever is cheaper which will probably be polystyrene.

The top will be similar to the bottom with at least one piece of rigid foam. I'm not exactly sure how the rest will be. I'm thinking I might make a frame and fill it with cellulose too, but we'll see.

strider3700 06-29-11 12:23 AM

can polystyrene handle the weight of the tank? I'm not sure but isn't that the white stuff with little beads? It compresses really easy compared to the pink or blue stuff which we put under slabs all the time.

Daox 06-29-11 08:34 AM

They're actually both polystyrene. Expanded polystryene (eps) is the beady stuff, extruded polystryrene (xps) is the blue/pink foam. I'd be using xps. I forget the R value of EPS, but XPS is a fair amount better.

Solaris 07-07-11 03:22 PM

Have you considered using a freezer as the container?

Used freezers are a great (free) metal box with a nice white finished look, have decent insulation to start, and a hinged sealing lid.

The freezer can be enhanced by lining the inside with 1" rigid foam, and a one-piece EDPM liner, glued up above the water line, and a floating pool-blanket to reduce air convection loss.

Size can easily be in the 150-200 gallon range, which is fine for solar preheat. If your floor is cement, you could stack two, and have passive thermo-syphon heat the top tank.

There are liquid transport "totes" that hold 500 gallons, have a metal cage frame with a platic liner, and can be picked up cheap.

Daox 07-07-11 04:09 PM

The only concern I have with a freezer is the plastics they use in them. I'm not sure what they use and how they'd hold up to very hot water. Same goes for the totes.

bennelson 07-11-11 09:17 AM


Originally Posted by Solaris (Post 14440)

There are liquid transport "totes" that hold 500 gallons, have a metal cage frame with a platic liner, and can be picked up cheap.

Trouble with those is that they don't fit through doors!

I had sort of the same issue working on my home graywater laundry to toilet system - I was limited by the accessway to my crawlspace.

With a "Built" water container, parts can be carried down through narrow basement stairs and assembled in place.

zick 09-22-11 10:19 AM

How's this coming along?

Daox 09-22-11 10:38 AM

I haven't had any time to work on it. :)

I did measure out the area that it'll be going and I think its going to have to be a bit smaller than I had hoped. We'll see though.

Solaris 09-22-11 01:38 PM

I can supply a 360 gallon, 540 gallon, or larger solar mass storage tanks that you can get through a 24" door, or down stairs, that is designed for potable hot water, and they are less expensive than standard DHW tanks. I was getting lots of calls for retrofit solar heating using 10-12 collectors, that required at least 500 gallons, but the tank had to get inside the building. Our big German solar tanks do not often go into retrofit building.

Daox 09-22-11 01:54 PM

PM me with some additional info and we can see if it'll work out.

strider3700 09-22-11 02:07 PM

roughly what does one of those cost? Very slick solution if you've got that kind of space in the basement.

Solaris 09-22-11 04:50 PM

Yes, these Fiberglass Reinforced Plastic "FRP" tanks are made for potable hot water up to 180F, are food grade inside like a sink, and have stood up water-tight for 20 years in Canada.

Cost for 360 USG tank is $1198, and for the 540 USG is $1798 (2045 liter / 450 Imp. gal.)

Heat loss is not generally understood in solar. German scientists show with 10m2 collectors, insulated pipe heat loss cancels 2 m2 and tank heat loss another 2.5 m2, which is nearly half the heat input. By comparison, uninsulated pipes is triple the figure, making the system lose 8.5 of 10 heat units.

That is far less than adding up a row of 4.5 120 gallon tanks, and has 83% less side and top surface area to radiate heat loss, more like 100% taking into account heat loss with manifold pipes on multiple tanks. (540 USG tank vol=72 ft3, so 120 USG = 4.5 tanks to make 72 ft3, with side and top area of 156.7 ft2 vs. 85.6 ft2 = 1.83:1 )

They ship "cupped" to reduce space and expense. Each section is a manageable 54 pounds.

We have an enginneer drawing at the bottom of this page:
Solaris Manufacturing - Products

Daox 10-15-11 11:20 AM

I started preparing the spot where the hot water tank is going to go. To do this, I needed to bust out a concrete pad that was in the basement. This will get the tank an additional 12-13" lower and help the drainback system work better.

This is the location of the tank and the pad that is in the way.

I started smashing the poured center of the concrete pad.

Quickly enough the bricks on the outside became loose and I was able to pull them away.

Daox 10-15-11 01:09 PM

I continued on removing the concrete to find that the floor does not extend under the raised platform. So, it looks like I'm going to have to pour a floor in where ever I chip out. Not a big deal, but it needs to be done.

However, this brings up another issue I have with the house, and that is a damp basement. For a while now I've been wanting to fix the problem. The best solution would be to entirely remove the floor of the house, dig down a bit, lay down some rock and add drain tile around the edges going to the sump croc, and then repour the floor. That is a whole heck of a lot of work. So, I may just chip around the exterior of the basement, lay down drain tile and repour that area... more work, great!

Anyway, here is a pic. Its hard to tell, but the shovel is below the grade of the floor.

strider3700 10-15-11 01:17 PM

It looks like there is no vapor barrier under the cement also? That would help some with the moisture. If you're going that far you might as well add insulation under there instead though.

Daox 10-15-11 01:20 PM

Thanks for the suggestions, I just started a new thread about the water issues.

dh1 10-15-11 02:06 PM

Have you considered using a galvanized livestock water tank instead of building one?
I know if you build one you can make it any size you want but they come in different sizes, you could also use 2 or 3 of them.
This one holds 300gal>Oval Galvanized Stock Tank, 3 ft. W x 8 ft. L x 2 ft. H, 300 gal. Capacity - 2177285 | Tractor Supply Company

Not sure if this is usable for what you want, just tossing around ideas.

Daox 10-15-11 02:29 PM

Thats not a bad idea. However, I do have a limited space and with the wood tank I can build it to fit my exact needs. I'm thinking I might have to go with a shorter but wider tank to get the water level low enough for the drain back on top of chipping the platform out.

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