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Solaris 10-16-11 04:38 PM

Drainback Level
A drainback tank only needs to be a small distance below the bottom of the solar collector. The shortest distance is best. Not sure why you would go to all the effort to dig through your floor?

bennelson 10-16-11 08:07 PM

That part of your basement is weird.

I have no idea why anyone would have built it like that. There was no reason at all for that part to be higher. I thought for sure, there must have been a cistern or something under there! :confused:

Daox 10-17-11 11:44 AM


Originally Posted by Solaris (Post 16819)
A drainback tank only needs to be a small distance below the bottom of the solar collector. The shortest distance is best. Not sure why you would go to all the effort to dig through your floor?

The panels will be roughly 80 feet from the tank on a rack outside. That means I need 20 inches of vertical slope to have all the water drain back to the tank. The lower I can get the tank in the basement, the less fill I'll need outside to bury the pipes.

Daox 10-17-11 11:46 AM


Originally Posted by bennelson (Post 16827)
That part of your basement is weird.

I have no idea why anyone would have built it like that. There was no reason at all for that part to be higher. I thought for sure, there must have been a cistern or something under there! :confused:

I talked to my neighbor who used to own my house about 15 years ago. He said at one point there was a shallow well pump right in that area. I don't see why that would have any effect on the platform... But, thats what he was able to tell me.

dh1 10-17-11 05:10 PM

Would it be better to have a closed loop with anti freeze and heat exchanger instead of trying to make a drain back system?
If it freezes on you it could be a lot of work to repair.

Daox 10-19-11 10:54 AM

Drain back systems are cheaper, simpler and a little more efficient. So, unless I run into some other major issues I'll keep it as a drain back.

Daox 10-22-11 09:07 PM

While adding drain tile to the basement we also used the rented jack hammer to punch a hole in the wall to run pipe out to the solar panel rack, and take out a platform that had been made in the basement.

We also poured a new slab under where a raised platform used to be. This will give me a bit more room for the tank.

zick 10-23-11 11:01 AM

Keep up the good work!


Northernliving 03-30-12 09:01 PM

How is your tank project coming along? Please do keep us posted.

Daox 03-31-12 08:07 AM

There are no updates as of right now. However, I was thinking of this just yesterday. With spring here, I'll be starting to work on the whole solar system quite soon.

Daox 05-04-12 09:35 AM

1 Attachment(s)
Since doing the concrete work, I had to redesign the tank to fit the new area. It is now based off a 4'x6' sheet of plywood base. The new tank will hold around 500 gallons. Due to the shorter length it will loose a bit less heat, but not a ton. The main benefit is that I'll be able to insulate the tank to around R40-50. The combination of the two should bring the heat loss down from around 4k btu/day down to 2.4k btu/day. The new shorter design also reduces the maximum stress on the plywood down to a peak of 866 PSI which is about 1/5th the maximum rating of plywood (4500 psi). This should account for any imperfections in the wood, aging of the wood, any effects of thermal cycling, etc which might weaken the tank over time.

AC_Hacker 05-16-12 12:45 PM

1 Attachment(s)
Hey Daox,

I just came across this solar thermal storage tank kit. It has a fabric outer part and a water-proof fabric liner part...

The Softank Kit - American Solartechnics, LLC.

They sell you the liner parts all assembled, you build the tank...

What do you think?


Daox 05-16-12 03:34 PM

Not a bad idea. I like it and putting it up would probably be quite easy. Being cylindrical heat loss is also minimized which is also good. However, I would need 2.5 of them to get the capacity I desire and I'd rather not try to deal with two tanks.

AC_Hacker 05-16-12 05:57 PM


Originally Posted by Daox (Post 21959)
...I would need 2.5 of them to get the capacity I desire and I'd rather not try to deal with two tanks.

Did you check out the link?

They do have larger capacity solutions...


Mobile Master Tech 05-16-12 09:11 PM

Those softanks look great-I looked into them a bit. Like Daox said, you need a lot of tanks (or a lot of $ for a big one). If you have more than one tank, you can't take advantage of the simplicity of stratification. I doubt the advantages of multiple staged tanks outweigh the benefits of a single simple stratified tank-I suppose it would depend on the situation.

The most efficient storage shape, considering volume inside divided by surface area, is the sphere. The next best is the cylinder, like the softanks. The closest easy to fabricate shape is the cube. Heat loss is a function of surface area, temperature differential, and the heat conductivity of the whole package. The heat loss that really matters, though, is how much you lose vs. how much you can store, assuming you have a heat source large enough to bother with storing anyway. A 4x4x4' tank holds 478 gal and has 96 square feet of surface area, for 0.2sf/gal. An 8x8x8' tank holds 3,830 gal and has 384sf of surface area, for 0.1sf/gal. A 16x16x16' tank (sorry, I couldn't convince Charmaine to let me build it!!!) holds 30,640 gallons and has 1536sf of surface area, for 0.05sf/gal-1/4 of the loss area per volume! So if the purpose is to store heat when you can get it to use when you can't, and any heat you lose through the tank does you some good anyway in 6-9 of the 12 months (meaning the tank is within the building envelope), it makes sense to build as large a tank as is practical so your loss area compared to storage capability is as favorable as possible. Multiple 60-120 gal round tanks? Don't even get me started!!

Mobile Master Tech 05-16-12 10:56 PM

Since it is tough to install anything that won't fit through a doorway, most tanks have been small, collapsible like the softanks, or built of dimensional lumber. Plywood being sold in 4x8' sheets plus the limitations of sheet lining material and concerns about bulging due to water pressure has meant that most homebuilt tanks have been approximately a 4 foot cube or a 4x8' tank 4' high, which is not enough to acheive effective stratification.

Although I personally will build a tank with 7x7' outer dimensions due to space limitations and to keep as close to a cube as possible, this plan will work for up to 87.5x87.5" square by 69" high water dimensions, assuming you don't want to get too far away from a cube shape, use 8' long dimensional lumber for the support perimeter, have 3" headroom from the top of the water to the top of the tank, and have it fit and be maintainable in a room or basement with an 8' ceiling. This will hold 2295 gal and have 0.12sf/gal surface area.

To build the base, use a 2x4 laid flat to join two 4x8' sheets of 3/4" plywood to form an 8x8' square. Scale down all dimensions to make a smaller footprint tank. From the underside, screw down (with lots of weather resistant screws) 4 2x4's in a butt joint pattern to make a band perimeter all the way around the outside edge. The joining 2x4 will be on the bottom. Use construction adhesive anywhere things join together. Place this on top of 2 sheets of ISO (polyisocyanurate foamboard insulation) in your choice of thickness (I recommend 3"), with a cutout for the 2x4 joiner on the bottom made with a router or whatever means you see fit. Even though 2300 gallons of water and the tank will weigh nearly 20,000 lbs (wow!), you have 9,216 square inches to spread this over, for a little over 2psi. Most polyiso is rated for 16-25 psi at 10% compression. You probably would worry more about ground settling due to the weight.

Start assembling on or very close to the tank's final position because it will be hard to move when done. Screw 4 4x8 plywood sheets horizontally to the inside of the base band and to each other. Now you have 8x8 by 4' high tank walls. Screw 3 more bands of 2x4's on edge horizontally around the sides with the first 2 8" on center then the other 16" on center (you could keep doing 8" on center if you doubt the extra supports described later will do the job, but there is no need for a solid stack of 2x4's as seen on one build at, but this time overlap the end of one 2x4 over the other so the 2x4's are staggered and leave the ends loose. Now screw a band of 4x4s with 2 butt joints so they are at the same perimeter height around the top edge of your plywood box so that half is screwed to your existing box and the other half forms a support for 2'x8' horizontal plywoood pieces making the sides 6' high. Put those in, screw them to the 4x4's and continue to the top with more horizontal bands at no more than 16" on center. The top band of 2x4's needs to be butt jointed flush with the top of the plywood sides so the top can be screwed to it all the way around. Make the top the same way as the bottom, except frame in a removable hatch with flat 2x4's in one corner for a manhole and with the joining 2x4 on top instead of on bottom. Dont cut the corner out of the plywood-move away from the edges a few inches. The top and bottom need to be intact at the edges and supported from one side to the other at the manhole opening so they can be strong in tension to take the strain of the supports that prevent bulging.

Once all 6 sides are assembled tightly, screw the butt joints and lap joints for all the horizontal banding together. Now, use chainlink fencing end posts (the nearly 3" diameter ones that would take a truck to bend) and lag bolt them with modified chainlink clamps or whatever you choose made into a "u-strap" to all horizontal bands. I would suggest 2 posts per side, but one may be enough if you are making a smaller tank. Use 2x4 spacers to support a joining 2x4 perpendicularly across the top of the 2x4 that joined the 2 sheets of plywood forming an "x" or tic-tac-toe grid of supports across the top, depending on whether you have 1 or 2 supports per side. Now the top and bottom are strong diaphragms that will act in tension against any bulging of the sides that are transmitted to them via the chainlink posts.

Follow Sani-tred's instructions for sealing a water tank(they use this on municipal waste water tanks) with their Permaflex/LRB system and don't skimp! Be sure to coat everything inside, don't paint yourself into a corner, ensure you have plenty of ventilation piped in through a duct in the manhole, and that all screws are countersunk enough to not cause a screwpop through the Permaflex. Check for splinters, debris, etc that might compromise the thickness of the Permaflex membrane.

Add a 1" to 1.5" CPVC pickup pipe, depending on your system, that goes within 2" of the bottom, preferably with pipe insulation on it and a reducer on the end that will act like a pickup bell. Any pipe that goes from the top of the tank to the bottom can conduct heat from the top to the bottom and dilute your stratification. Add a 1" to 1.5" return that expands to at least 3" sanitary tees through a mesh screen or somesuch to slow the return flow down. If you have 3 sanitary tees around 1-1.5' apart, the water has 3 choices to enter at its own density. If it is really hot, denser/cooler water lower in the tank will prevent it from flowing down to the lower exits. If it is cooler than the hottest water at the tank, it will continue to sink down to the lower levels. Remember that an open system like this needs a circulator pump with enough head to get the water to the top of the system in case it loses it's prime. If this goes to solar collectors, you need to have a UPS backup for your controller & collector pump so warm water can be moved to the collectors if the power goes out in freezing temps. Alternatively, you could use an antifreeze solution and a separate heat exchanger in the bottom of the tank, but you will lose a lot of your stratification ability. This would be better insurance in extreme cold climates, but not much of an issue in Georgia. Antifreeze increases pumping power due to the viscosity increase and decreases the thermal conductivity, but 20% propylene glycol won't cause too much problems, will provide freeze protection down around 20F, and provide burst damage protection everywhere but where Santa lives.

Add another pickup to get hot water from the top (such as for underground heat injection that I will describe in another post) and a heat exchanger for your working fluid in the upper 1 foot of the tank (DHW in my case for both the house plumbing and radiant flooring), a floating swimming pool thermal blanket on top of the water once full (I will use my reverse osmosis system to fill so there is no significant mineral content) and you are good to go inside. Make these penetrations through the top, not the sides. Insulate the outside as you see fit. I recommend 3" of polyiso cut to fit inside the banding, then another 3" routed to go over the tops of the banding with joints staggered to a different location than the first run and extending to the floor. If the insulation is at least 4" thick, there will be no exposed thermal bridges except the contact points between the horizontal bands and the chainlink posts. If needed, you could build up an insulation "box" around the posts as well. Studies I have seen show there is very little benefit to insulating thicker than 6". The bottom is thinner, but also will be a lower temperature and eventually will warm the ground beneath it so the deltaT will be low.

Thanks to all on ecorenovator, builditsolar and many other places that helped me formulate my ideas. I will detail my own construction once I get started. I've got the Permaflex already!


Mobile Master Tech 05-16-12 11:05 PM

Daox, are you sure about the 866 psi? A 6ft column of water exerts about 2.5 psi.


AC_Hacker 05-17-12 10:59 AM


Originally Posted by Mobile Master Tech (Post 21970)
Daox, are you sure about the 866 psi? A 6ft column of water exerts about 2.5 psi.


Going by THIS formula...

P(static fluid) = ρgh

ρ = fluid density
g = acceleration of gravity
h = depth of fluid
ρ for water = 62.30 lb/foot cubed = 0.0360532407407407 lb/in cubed

g = 32.2 ft/sec squared = 2.683 in/sec squared

h = 6 ft = 72 in

P(6' static water column) = 0.0360532407407407 * 2.683 * 72

P = 6.965 lb/in squared

... or there abouts.


Mobile Master Tech 05-17-12 12:45 PM

I think we've got an unit conversion error in that calculation. One foot of gauge water pressure (or head), disregarding absolute pressure since the atmosphere is working on all surfaces, is 0.433psi. 6*0.433=2.598psi. Since you got me thinking, I pulled out my dual pressure differential manometer and stuck it in a bottle of water to be sure-I didn't bother with many decimals but 12" of air in the tube below the surface read 12" of water column and 0.43psi.

Daox 05-17-12 02:14 PM

Well, my tank is 4 ft tall. So 4/6 * 2.6 = 1.7. That is the pressure I'm using at the bottom of the tank.

strider3700 05-17-12 02:38 PM

Mobile, how large of a collector bank are you planning to heat 2300 gallons and what is the intended end use of the heat? Getting 2300 gallons up to DHW temps is going to take a huge collector.

Mobile Master Tech 05-18-12 09:19 AM

Daox, that sounds about right to me. You will have more to worry about with bowing of the supports than the burst strength of the plywood. Your tank isn't too big so you won't have that much bowing. Have you thought about going bigger vertically instead of horizontally to gain stratification?

This will be for DHW and all space heating combined. I installed an open loop radiant hydronic floor heating system (details on the radiant heat thread) for comfort, quietness, efficiency, resale value and the ability to use a multitude of heat sources.

I have now had the system in for over a year so I have details on energy usage. My family of 5 used 805 therms of natural gas in 12 months, with 150 therms peak usage in Jan. Hot water (and cooking!) used only 7-15 therms each month during the summer-not enough demand to be worth the bother and expense to size solar for, and only a drop in the bucket compared to the needs during the winter months when you have the least solar available. This is why I say having a large ability to store useful amounts of energy when you can get it so it is available when you need it is important.

Our hot water is from an Eternal hybrid gas heater. It's 98% efficient most times but more like 90% when return water temps are hotter because the floors are on a long time. Taking 10% off of 150 therms for efficiency losses, allowing a 20% reduction in demand from further improvements in the building envelope and adding a 10% margin because last year was milder than usual, I feel that 120 therms(12 million BTU), is the January usage I should size for. That works out to about 400,000 btu/day. However, mother nature sends us several days in a row where weather is harsher causing demand to be greatest while availability from ASHP and solar is at its' lowest. That 2300 gallon tank seems huge until you realize that the 1.6 million useable BTU it can store between 110F and 195F is 4 days of average January demand at 100% fraction.

This brings in the next benefit. Having that big 4 day reserve evens out demand so each heat source can be sized for average demand instead of peak demand, reducing cost due to downsizing & improving efficiency due to infrequent cycling. I plan on having 4 heat sources: a 1 ton (12,000 BTU) hacked groundsource heat pump (thanks to AC's inspiration on the GSHP thread) 2 solar collectors with 30 evacuated tubes each, a desuperheater on my 22 SEER inverter air conditioner, and a small electric water heater to make up the last few degrees in case the hot water output drops below 130F.

Those solar collectors together will probably output 30,000 btu/day for a January total of 900,000 BTU. This is assuming 70% of the time they will operate under the SRCC's D-mildy cloudy rating and 30% of the time under C-Clear. Decades worth of government observations show that the average cloud cover in NE Georgia is 63-65% from Dec to Mar and the percent of totally clear days is only 14-15%.

The GSHP will be switched on by the solar controller whenever the top of tank temp gets below 135F. If it needs to run 24/7 it will give around 288,000 btu/day, for 8,640,000 btu/mo. Combined, solar and GSHP can give 318,000 BTU/day. This might not even be half of the demand on peak cold and cloudy days. But with a total of 9,540,000 BTU/mo to average over the peaks and valleys, that's an 80% alternative heat fraction under the harshest conditions all by themselves, thanks to the peak-shaving storage capacity of the large tank.

An AC desuperheater produces around 2-3000 BTU/hr per ton of air conditioning. Since we have the downstairs A/C dismantled for basement renovations, the "1.75 ton" upstairs air conditioner runs about 20 hrs a day when it is about 90F. It has gone over 90 several times already this year, so our A/C load is at least 35 “ton-hours” per day for at least 4 months out of the year. I therefore expect to get over 100,000 BTU/day from the desuperheater. The solar collectors will also be putting out around 90,000 btu/day during the summer. In both cases, they will be blasting out the heat during the day and almost nothing at night.

I am going to retrofit ground loops under my basement so I can inject that heat, timed by depth to return about 6 months after it was put in (I will start a thread with details on that one). Hopefully this will raise the average ground temp under the entire house from 62F to 72-73F, hot enough to add heat in winter while still being lower than the desired temp in summer. There will be a hotter core centered around 15’ below the slab that warmed during the summer. That heat will make its way to the surface during the winter. Because you have to stay at least 15' from the edges of the foundation to make sure your heat doesn't leak out the sides and you can only go so deep without groundwater carrying it away, there is a limit to how fast you can put it in the ground. Again, the big tank lets the underslab exchangers hum away, 24/7 if needed, while smoothing out the temp swings.

Since our Summer demand is approx 1.2 million btu/mo, and we’re getting 5.7 million btu/mo at least 4 months out of the year, that leaves a surplus of at least 4.5 million btu/mo. If we get even half that energy back either directly through the basement slab adding heat to the house while reducing demand (the basement will now be a net-positive heat source instead of a net-negative load sitting in a 62F heat sink) or by increasing the efficiency of the GSHP field surrounding the house, we now are capable of greater than 100% heat fraction for the whole year except Jan, and it will probably be darn near 100% then-all because of a large tank allowing multiple smaller systems to work together!

There’s not a huge difference in the component count/cost and effort required for a system that can provide 100% of your needs vs one that is noticeably smaller, especially weighed against the actual savings. I could build a basic solar DHW system that can cover about $120/year of my energy expenses before being oversized for peak supply periods. Instead, I could go to a little more trouble so I can drop my gas company and their monthly minimum charges while covering almost $2000/yr of my current expenses and become more self reliant. Big tank + little system-bad idea! But if you build a system big enough to be useful, greater storage capacity is important. I will be doing some more research on phase change materials to get more storage density in a given tank also.


Daox 05-18-12 12:42 PM

I could go a bit higher with the tank, but really not that much at all. My basement only has about a 6.5' ceiling. Once I get 6" or so of foam under this tank and another 6+ inches above it for insulation we're looking at not a ton of overhead room.

Mobile Master Tech 05-18-12 01:33 PM


Originally Posted by Daox (Post 21995)
I could go a bit higher with the tank, but really not that much at all. My basement only has about a 6.5' ceiling. Once I get 6" or so of foam under this tank and another 6+ inches above it for insulation we're looking at not a ton of overhead room.

You're right-you still need to have space to work in the tank and get things in and out it. For anyone with the headroom, there are many benefits.

Check out ebay seller "insulationdepot", they also have their own website. They recycle building materials from building demolition, particularly foamboard, and have many warehouses across the US with reasonable shipping. Their inventory is low now but should build up soon, as the demolition season starts in the spring and runs through fall. Their inventory dries up in winter, but I have seen a 4x8" pallet of 3" polyiso 92" high for $250 and less when it is mostly 4x4' pieces and smaller-perfect for fitting between joists/tank supports.

S-F 05-18-12 09:00 PM

Insulation depot is a company I have dealt with before. They are all over the place and just so happen to have a location near me. But they ship anywhere. They ship by freight so the cost doesn't vary as greatly from one location to another like with UPS or some such.

Daox 11-11-12 07:16 AM

4 Attachment(s)
We started building the solar tank yesterday. My in laws are here for the weekend and we're getting a lot done on the office and the solar system.

I ended up just going with 3/4" plywood because that is what they had and I really don't want this thing failing over time. I don't know how much wood weakens over 20-30 years but I want it to last that long.

The base plywood piece is 4'x6' with a 2x4 frame glued and screwed from the plywood side. The tank is a full 4' tall. We precut all the pieces before taking it into the basement to make assembly easier once we were down there.

This is the insulation that is going under the tank. I miscalculated / overbought insulation for it. Oh well, it is now insulated to R57 on the bottom instead of R45. I have 8 inches of polyiso and one inch of polystyrene on the bottom to block any moisture.

Mikesolar 11-14-12 05:59 AM

2 Attachment(s)
Here are pics of the insides of a Rotex tank. I got the demo tank which has a big hole cut out for trade shows but the innards were good.

This tank has a large HX which is big enough to do DHW in a conserving household, a coil for the solar which is at the bottom and one for a backup boiler, plus maybe one more. It was to go into a tank like yours but I messed the tank up so it needs some adapting. ....pick to come

Daox 05-15-20 04:51 PM

2 Attachment(s)
I am working on this project again. I really want to get the solar panels installed this summer. So, I cleaned off the tank (materials) because things things had been on it for years.

Time to clean up the area, figure out where I want everything to run and find out where to put the pump and pressure tank, etc.

bennelson 05-15-20 05:08 PM

Glad to see you're back at it!

Daox 05-16-20 10:23 AM

I agree. Its great to be getting back to this project!

menaus2 05-18-20 08:29 AM

Never too late to start again! I like how this project is half Marie Kondo half Engineer. :thumbup:

Daox 05-25-20 08:28 AM

I've been working on this project a little bit here and there. I ordered the EPDM tank liner. 90 lbs of rubber! It'll be fun wrangling that thing around haha.

I'm also starting to think about how I want to do the heat exchangers on this thing. I'm going to need 3 different loops. The first loop will be to put heat into the tank from the solar panels. The second will be for domestic hot water. The third will be for heating purposes. I like the idea of a big PEX heat exchanger like I've seen on build it solar for DHW setups. However, I'm not sure that'll work for the other two loops. Do you guys have any suggestions?

menaus2 05-25-20 09:44 AM

I've been impressed with how copper heat exchangers have worked on my tank. You can always buy copper wort chillers on Amazon if you want something premade that will look nice. They can be run in parallel if necessary. Running your space heating on an open loop using tank water directly would be 100% efficient & save exchanger space, provided you aren't worried about it freezing.

jeff5may 05-25-20 06:38 PM

The material you use for heat transfer makes a massive difference. Copper pipe is the king when you consider raw heat transfer and time lag versus dollars . Pex is about 6 times worse per surface area than copper at heat transfer, and also introduces lag. If you have the volume, pex will work (best for slab heating and ground loops). For domestic hot water, and other high drain, not so steady-state processes (quick, high dT/dt pulses), copper pipe is the cool kid.

Rule of thumb for copper is 100 feet of 1 inch will reliably move 100k btu per hour with a dT above 25 degF, steadily, forever. Two parallel runs of 3/4 inch, four runs of half inch, move equivalent amounts of heat. With PEX or poly pipe, the bigger the diameter, the worse the heat transfer. So if you push your luck, half inch might have a factor of 4 or 5 times less effective.

The 1 inch and larger diameter poly pipe has a much, much thicker wall. This thermal resistance and slow flow isn't insignificant. A 1000 foot spool of 1 inch poly pipe will not let you take a 15 minute hot shower with a 150 degF thermal store. Once the stagnant water in the pipe runs out, the water comes out lukewarm. I'm not talking about an eco low flow, locker room showerhead, and a crispy cool shower here.

If you need fast, high density, compact heat transfer, it's hard to beat a brazed plate heat exchanger. If you're thinking about a beer chilling thingy, they work great for sea water. Both are pricey compared to a roll of copper tubing for the raw BTU capacity.

WillyP 07-20-20 03:13 PM

I've been reading this thread and want to toss in my two cents. I build a lot of wooden stuff. If you are looking for the best structural bang for your buck, invest in high quality wood glue. "Woodtight3 is awesome stuff that wont change your fitting dimensions (unlike liquid nails). The level 3 adhesive is water proof, but shouldn't be submerged in water (not that you would do that). But it is what you want to use anywhere near water.

The metal joiners are almost a waste of money. You are much better off just double lapping the wood and using good glue.

On a 4X8 plywood side, you should really put a support at least every two feet. Two by fours are really cheap. Adding a couple of extra 2X4s, can prevent some serious problems latter on. Glue them to the plywood, then add a couple of screws, and you will have a wall that will hold a lot of water.

It is a really good idea to rip about a quarter inch of wood off the 2X4 (if you have a table saw) This will give you a much tighter edge. Even though the wood is thinner, it will be stronger because the (glued) fitting will be tighter.

Always drill a pilot hole when using wood screws. The pilot hole should be the same diameter as the solid bolt under the threads. A good test is to hold the bit on top of the screw. You should only see the threads of the screw. The switch it so the bolt is over the screw, the bit should disappear (from view, not reality).

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