solar heater plan what is wrong with it

WillyP

I am posting a diagram of a system I am going to build soon. It uses heat exchangers in large underground tanks. The heat exchangers will be copper coils, the heat will be produced by evacuated tub heaters. The entire system will be inside of a greenhouse with (insulated) underground walls, below the frost line.
It is basically two systems working together. One system will be the vacuum tubes supplying heat, to a pair of tanks. These tanks will supply heated water/glycol to the heat exchangers inside the under ground tanks. The under ground tanks will supply hot water for a radiant floor heating system.
I am doing it that was so I can better regulate the temperature of the water going to the floor. It will be a closed loop system. There is also a discharge tank. This will prevent cold water from the slab, from mixing with the hot supply water. The discharge tank will be heated then returned to the underground tanks.
One last note. I originally was going to use oil inside the vacuum tubes. But I changed that to water/glycol. It is still labeled as oil in the diagram.
I ask what is wrong because I am hoping to find any flaws before I build it.

hot_rod

how large are the A & B tanks? filled with glycol? How many evac tubes, how will you handle overheating of the array, in summer for example.
Are you pumping between all these tanks? Seems like a lot of tanks and insulation?

WillyP

The under ground tanks are 850 gallons. I plan on using metal tanks for the water heated by the vacuum tubes(tanks A&B). A thermal switch will cut on the flow (to the heat exchanger) when the under ground tanks start to cool below 95 degrees F. As for over heating in the metal tanks it shouldn't be a problem. I've never heard of those heating above boiling. But it will get hot in that room in the summer time. But that should work to my advantage in winter, if the metal tanks are sitting at over 140 degrees F, it will keep everything nice and warm. At least that is the plan...

WillyP

as for how many tubes. I am still hoping for some feed back on that. I am kind of leaning towards two ten tube arrays. Maybe two fifteen tube arrays. My thinking is if I have two heaters working at once, it will raise the temperature faster as it will move move fluid through the system. I could be totally wrong about that though. If anybody has experience with that I would love to hear about it.

hot_rod

If you go to the SRCC website see if that brand of evac tube is listed. There you will find actual test data for output under various conditions. The better you match load and array the more efficient and problem free the system. Download Caleffi Idronics # 3 and 6. Great info on how to decipher and use that test data. The best method is to define the load , then determine what % of that load you want to cover with solar. SF or solar fraction is the number to play around.  Usually it comes down to how much $$ you want to spend chasing “free” energy. KISS, I’ve seen systems where it costs more to run the pumps than the system is capturing

WillyP

I can really relate to spending thousands of dollars, chasing free energy. I sort of plan on doing just that. But there is a method to my madness. I am building a retirement home. My plan is to spend the money up front, now while I still have an income. Then once I retire I won't have a heating bill, for as long as I live.
Of course I will still need to have a heater, no matter what type it is. A heater will probably cost at least ten grand. So with that in mind, spending twenty thousand on a solar heater isn't as bad.

hot_rod

I’d spend money on solar PV and water to water or air to air heat pumps. You use electricity everyday somewhere on the property. But solar thermal really only covers DHW in summer months, maybe 400 bucks worth of hot water?
Properly designed the heat pumps get you 2-3 COP, to leverage that PV power. A heat pump water heater also.

WillyP

Oh I plan on using PV as well. I will be installing a massive PV array on the roof. I am looking at twenty LG Neon2 panels. There will be a grid tied system.

Larry Weingarten

Hello @WillyP , Have you done what can be done to make things efficient? My experience is that it's not too hard to cut the energy use by half, but is possible with more effort to cut it by 80%. Insulation, air sealing, day-lighting and efficient fixtures seldom need their glycol checked, pumps unfrozen, or sensors replaced.

I'm a fan of solar, having first installed in in 1978. It's just that too many times I've seen people build big complex systems and not be happy with the results. Efficiency shrinks the load and gives you more choices.

Yours, Larry

WillyP

Larry. I agree completely. I actually worked my way through college as an insulator. I learned as a very young man the importance of a well sealed wall system. I also plan on having well placed windows. But what you said is great advice.

PerryHolzman

Your biggest flaw - and the one that has doomed every solar heat storage system I have ever seen in person (that did not work) is that those underground tanks have to be significantly insulated - including under them with adequate insulation that will support their weight. Otherwise, you loose most of the stored heat into the ground.

I once designed a "thermal battery" that would use phase change materials for my house with the assumption that I could store all the heat I needed for a cold winter in one big "thermal battery."

My plan was 1st to do that and if it was anyway reasonable to then figure out an optimum system that assumed that I got some solar heat during the winter.

I analyzed the heat losses from my storage battery and the amount and type of insulation I would need and pipe runs (pipes loop down to near bottom of the tank level to stop convection heat loss when the system is shut down runs to minimize heat loss) so that the thermal battery would actually work.

Ground buried failed due to heat loss to the ground (it's almost impossible to keep buried insulation dry). Having the thermal battery storage in an vault worked much better. Also, to insulate the bottom and support the tank I ended up using the kind of insulating ceramic tiles developed for the Space Shuttle heat shield and multiple support feet to distribute the weight. I also ended up with about a foot of ceramic fiber blankets wrapping the storage unit (with joints overlapped by the next layer).

My thermal battery estimated cost was about $100,000 (it was designed with materials and construction techniques to last at least 100 years - and likely several times that); and I dropped the phase 2 analysis for an ideal system.

A storage tank without phase change materials is much cheaper to build. However, as it would likely operate at temperatures above 150 F; it would need more insulation to prevent heat loss.

I believe you still need to put them in a vault; and insulate the living heck out of all sides, pay attention to losses caused by pipe runs that just go up, and also allow for a leak containment system for when the tanks leak (the legal and financial liability could significantly wipe out most - if not all - the value of your house when those tanks leak - if not contained).

Given the weight of the tank - you might get by with a couple feet of insulating plaster to support it (which might handle the tank weight); or its likely you will need those ceramic space shuttle tiles (they are available - but not cheap). 10-15 ft of "insulating" concrete would likely work as well (insulating plaster and concrete has expanded "popcorn" minerals dispersed in it which makes it less dense and a better thermal insulator than normal plaster or concrete). Rebar subtracts though.

Perry

WillyP

WOW! I wonder if I can find a scrapped space shuttle, to get some insulation from. But seriously. For a battery I intend to build an underground wall eight feet deep. The exterior will be insulated with closed cell poly. That will be wrapped with pond liner so the critters can't get it. Above will be a greenhouse. SO the whole system will be contained.
Inside the green house, I will have a small insulated room. The room will contain the discharge tanks and the two storage tanks for the water from the tube heaters. My hope is storing the extremely hot water with the discharge tank will help heat the discharge tank.

Jamie Hall

@PerryHolzman 's tale is a good -- and delightful -- look at some of the pitfalls! Thank you!

Steps 1, 2, and 3 of any solar thermal installation are to get the thermodynamics right. Don't even start to think about hardware until you figure out just how much heat you need and for how long you need to store it. Then what temperature swings in your storage are reasonable (phase change materials have some real advantages).

Then you can start tackling the hardware aspects -- how big a tank? How much insulation? (lots), How big a collector? How does the hot heat transfer fluid get to the tank? How does it get to where it is needed?

And keep it simple!

WillyP

I think when you start getting into phase change materials you really move into the mindset of spending thousands, to save a few dollars. I thought about a huge paraffin bubble around the underground tanks. but even that would cost a lot.
My belief is clay serves as a good storage material, if it is insulated (it will be). The biggest mistake most people make is thinking they can heat the earth. My design has the large storage tanks inside an insulated cement bunker, under ground. On top of it is a glass greenhouse.

WillyP

Oh and the water from the tube heaters will be stored in tanks. Those tanks will feed heat exchangers inside the underground tanks. The reason I am doing it that way is; so I can better regulate the temperature of the large tanks, which will feed the radiant floor heat.

PerryHolzman

How much heat are you looking to store. In my case - it was easy to estimate based on the natural gas bills and the efficiency of my Viessmann Vitodens 200 over many winters (at least for my phase 1 concept study).

No way could I store the amount of desired heat long term in a non-phase change tank of fluid.

I think you better evaluate how much heat you need to make your system work - and how much must be stored for say a week of operation during cloudy/rainy days. Then calcualte the temperature rise and drop of your fluids heat capacity per gallon to figure out how large of storage tanks you need. I suspect that you will be surprised (unless you live in a mild weather environment).

Making phase change materials work is also a trick. You cannot have just a huge tank of it. I used a variety of different temperature range phase change materials inside tubes (my battery looked a lot like an industrial tubed heat exchanger - and would have been constructed in a HX shop: note that part of my background is a plant heat exchanger engineer - and I know how to build them to last 100+ years too). This allowed the circulating water to get to the phase change materials and melt them when storing energy, and solidify them when extracting energy.

Of course, phase change materials are not cheap. Nor are tubes to put them in (the support structure for those spaced tubes is cheap - standard HX baffle plates). I would have had a double o-ring seal, crimped ends, and a seal welded pug on each end (with the phase change temperature range indicated on each plug). But the cost of the phase change materials and very long life tubing made the concept cost prohibited. Note that I did assume a few tubes would eventually leak phase change material (its not cost practical to find all flaws - and flaws tend to get worse with time); and my battery was designed to allow that without affecting operation or the heating circuit system and components.

I wish you the best with this...

Perry

PerryHolzman

I also believe that you are underestimating what you need to build, and how to insulate it.

You talk of insulated concrete for your vault.

I don't think you understand the difference between residential and commercial structures.

I'm not going to look for it for this discussion; but a while ago I watched a video on how to build an insulated basement floor.

You put down a layer of a specific type of foamboard, put poly on top of that, and pour your basement floor on top of that.

That's for a residential basement rated for I believe about 100 Lb/Sq ft (average). My memory was that insulating foamboard was rated for an 125 lb/sq ft. Above that it crushes down to nothing - with no insulating value.

Your storage tank based on sensible heat of the contents will weigh a lot more than that. You might get away with a 250 Lb/sq ft floor if you can distribute the weight; but I'd guess that you probably will need a 500 Lb/Sq ft floor or higher (and I'm used to working in plants that the minimum floor design is 500 Lb/Sq ft and certain areas are 1000, and 2000 Lb/Sq ft.

You cannot insulate that with various poly foams. They cannot withstand the weight; and will crush (you will end up with a broken sunk floor and heating the earth though the bottom of the tank.

You're planning to build an industrial system. It needs a proper industrial structure and evaluation of all factors.

I was really big into Solar and Alternate energies in the 1970's and 1980's. The only solar storage system I have ever seen that worked as designed was on a NASA demonstration house at Langley (Hampton Virginia) in the 1970's (I still have all the NASA materials on that house in a box somewhere - including the FORTRAN computer programs they used). The typical homeowner misunderstands the need to really properly insulate the storage, miscalculates the size of storage needed, and builds it like standard home construction - which fails as they are dealing with industrial weights and components.

Even the use of phase change materials is almost always done wrong. You mentioned paraffin. That is actually a reasonably priced effective and safe phase change material. It's also a reasonably good insulator and you have to break it into strands or plates to be able to transfer heat into and out of it in a timely fashion. The phase change materials I looked at that were better at transferring heat were more toxic or corrosive if they leaked (and I designed my "Solar Battery" based on paraffin as it was safe, would float to the top of the storage tank, and leakage would not chemically damage other heating system components).

I'm not saying that your idea will not work. Lets start with the math on how big of storage tank you need and the fluid.

Sensible heat calculations are easy. How much heat energy do you need to store and for how many hours or days?

That will drive the size of the tank. From there you can calculate the weight of standard sized tanks, and the floor loading (add 50% for the floor design).

How long you want to store the heat, with the maximum and minimum fluid temperature vs the ground temperature (48F?) will drive how well you have to insulate the tank. I recommend allowing no more than 10% heat loss in the storage period you are looking for, and 5% would be better.

Then how do you insulate the bottom of the tank (including support feet) with materials that will more than adequately support the tank (assume the 50% higher floor loading number from above as what the insulation must handle) - while insulating.

Get back with those numbers - and send me a message and I'll run a confirmation calculation and tell you if it looks reasonable (PM me if you wish).

Perry

WillyP

You mention some good questions. The main one being how much water do I need to store for a week of operation. Months ago, I set out to find that particular answer. SO far nobody has been able to give it to me. But dozens of people have told me the system can't be built unless I know it. It has gotten to the point where I believe people only ask it to prove, well I am not sure what they hope to prove....
I am going with 850 gallons of hot water to run the slab heater. This will be stored at 95°F. My estimate is that will heat the slab for 24 hours. But I would really love to hear from anybody who has successfully built a closed loop system. Then measured how much water it uses/pumps in a day.
I couldn't agree more about using closed cell poly under concrete. I have read all the data and heard how it is supposed to hold up a slab. But I don't buy it. I am building a T shaped foundation which will be insulated on the outside. Rather than pouring a slab over Styrofoam. The walls will be six inches of poured concrete (8 feet deep) with radiant tubes inside. The outside of the walls will be insulated with 3 1/2 inches of sprayed cellulose, sandwiched between 1/2 inch plywood. Then there will be two inches of ridged foam boards.

WillyP

The floor of the battery will be twelve inches of poured concrete. It will be insulated on top with foam board. This should give me both the insulation I need, with the strength required to support 14,000 pounds of water.

hot_rod

It’s fairly easy to calculate energy stored in water The biggest variable is tank loss. Take a look at the Cocoon foam tanks. I know of some that have been buried as solar storage tanks. One in frigid Minnesota😎

WillyP

Let me throw some random thoughts out there. I worked in horticulture for about twenty five years. I spent a lot of time working with (and in) green houses. I have seen many green houses maintain warmth all winter, simply by heating the soil in the bottom of the green house. Therefore it is very hard to convince me that a green house wont keep itself warm in winter. Or to quote one wise old man: There are a lot of people who can show you the math, that proves; what we have done for years, is impossible.
Soil, especially clay, works great as a solar battery. But it does have to be insulated well below the frost line. This is a fundamental design feature in many green houses. Cooling happens, but it also reheats during the day. If built properly a green house will hold more heat then it loses.
I hate foam insulation. It is a great insulator, but if it is not done correctly it can be worse than useless, because it traps in moisture and can do all sorts of damage to any wood products attached to it.
In Maine, we consider Michigan a southern state where it barely gets cold enough to run a snowmobile ;-)

Jamie Hall

Greenhouses are interesting beasts from the heating standpoint. There are two points, one of which you mention, which often get missed with them. The one you mention is that if the insulation is carried well down, the mass of soil -- which is more or less directly heated -- will store a remarkable amount of heat. Enough, in fact, to limit the temperature gain during the day to something reasonable, even on a sunny day -- and, correspondingly, limit the temperature drop at night to something at least tolerable, if not entirely reasonable for a living space. The other, though, is the solar gain: it is transferred directly to the storage medium -- the soil -- over the entire area of the greenhouse at any time the sun is visible. The efficiency is remarkably high, both in terms of capture -- and, perhaps more importantly, in that there is never any high temperature fluid to lose heat.

Thus, in a greenhouse, you are capturing most of the solar power as very low grade heat in a very large mass, over the entire area of the greenhouse.

This is not the case for any active design, such as you are proposing. You are capturing high grade heat -- and storing it at high temperature -- which gives you much larger losses.

Now how much heat do you need to store? Not hard to figure out. You have, presumably, a Manual J calculation of the heat loss for the structure, in BTU per hour. Times 24, that gives you the total BTU you need to store. The heat capacity of water is -- conveniently -- 1 bTU for each degree Fahrenheit change for each pound of water. For the sake of mental convenience (where's my calculator?) suppose that your Manual J figure is 100,000 BTUh. For 24 hours, that 2,400,000 BTU. Now suppose that your storage tanks have a total capacity of 2,000 gallons, or on the order of 16,000 pounds of water. To store 2,400,000 BTU, you will need a temperature drop in that water, over 24 hours, of 150 degrees Fahrenheit. Since you minimum usable temperature is around 90, your peak storage temperature will be 240 Fahrenheit.

Umm... I think I see a problem with that. In fact, no can do with that volume.

Let's try again. Suppose we decide that our peak storage temperature will be 140 F, with a minimum usable of 90. Now we have a delta T of 50 Fahrenheit, so we need about 6,000 gallons of water.

And that's for one 24 hour day. I seem to remember that in New England we can go several days without sun...

Which brings us to how much collector do you need. Start off with our New England average of 3 hours of usable sun per day. The solar constant is 1 kilowatt per square meter, or -- close enough for government work -- 100 wats per square foot. We decided that we needed to store around 2,400,000 BTU. 700 kilowatt hours. We need to collect that in 3 hours, so around 250 kilowatts capacity. Making an assumption of 50 percent collector efficiency (which is very optimistic) our collectors need an area of 5,000 square feet.

And again -- remember that we are only collecting enough energy for one day.

I hope that helps with your calculations. You can, incidentally, do exactly similar calculations for a greenhouse -- which will show you that they do work, You have indeed done it for years, and the math shows you how it works -- if it's done correctly of course.

hot_rod

Download one of the free solar aps, walk outside place your phone where the array may locate for a quick read of what is possible. I use the SMA freebie.
In Portland January is 3.7 hours, July the high month around 6, 4 hours average across the year

PerryHolzman

@Jamie Hall & @hot_rod have provided the basics of calculating how much heat you need to store, and determining how much solar you can realistically collect in your area.

Lets get back to basic construction though. I don't care much that your floor is 12" of concrete. How strong is the concrete? 2500 psi, 3000, 4000, 5000, 6000 (or more). Also, how much rebar and wire mesh? It matters.

So lets approach it my way. You talk of a 850 gallon tank. That's 7094 lb of water. A lot of plastic tanks weigh about 200 Lb in that size range (7294 total weight when full - not counting piping and accessories), and are 4 ft in diameter.

Area of the tank bottom is (pi) r^ (squared). For a 4 ft diameter tank that is 3.14159 X 4 = 12.57 Sq ft.

7294/12,57=580.3 Lb/Sq ft. Note: Steel tanks will be heavier.

You need a floor designed to handle 750 Lb/Square foot. That cannot be build like you pour a common residential foundation or floor.

There are common engineering tables on how to build floors of different strengths (250, 500, 750, 1000, 1500, 2000 Lb/Sq ft).

Also, I'm not aware of any foam board that will withstand 580+ Lb/Square ft and maintain its shape or insulation value (insulating plaster might handle that. Insulating concrete would - but note that both insulating plaster and concrete would have to be above the 750 Lb/Sq ft floor).

Now you could get a more expensive tank that has say 4 individual feet. Which allows you to insulate under the tank, with only losses through the feet. But that then requires a floor that can handle about 2000 Lb point loads (and you should design for at least 2500 Lb or 3000 Lb point loads). Again, there are standard engineering answers on how to build such a floor. Its more robust than a 750 Lb/Sq ft floor.

You need a someone with a civil engineering background (or experience) to design that floor for you with whatever you end up with. Also to design the foundations to support that floor.

The fact that you are only looking to store heat for a day or two indicates that you can probably get by with a tank with feet as the looses through the feet will not be that great. In my themal battery study I was looking to store usable heat for theoretically 6 months and had to put the feet on space shuttle tiles to insulate the feet to greatly minimize the heat loss though the feet.

Of course, the weights and floor strength gets higher the larger the tank gets - unless you go with a custom large diameter tank that is relatively short.

Let us know when you calculate how much heat you need to store for 1 day, 3 days, or potentially 1 week, and the size of water tank you would need for your purposes.

Perry

PerryHolzman

A bit more information:

If you don't know how to calculate heat loss during the day or night. Slantfin has a calculator program for that (its for a normal house with walls and windows - but I think it would work if you designate the walls are all windows such as a greenhouse has).

You need to look up cloud cover and storm history to determine how long you need to store sufficient heat. My experience here in Wisconsin says plan for at least 4 days of heat storage with no additional heat input by the winter collection system due to the length of long winter storms if you are not going to have supplemental heat. My phase 1 conceptual study planned for 6 months of storage - and assumed that the system would be drained at least 5 months when it could freeze in winter. For a lot more $$$ with a solar system it could operate through the winter and collect partial heat (and reduce the size of the "solar battery": That was to be Phase 2 of the study if my Phase 1 solar batter appeared economical

Concerning insulating the floor or the bottom of the tank:

Ordinary concrete has an R value of 0.8.

Expanded perlite insulating concrete (1/8 mixture of concrete to perlite - which is the common roof deck mixture) has an R value of about 0.5; and a density of a bit under 50 Lb/Cubic Feet. So you could put down a foot thick pad for R6 under the tank and only add 50 Lb/Sq ft to the floor loading.

If I was building your scenario I would also put down at least a 1 ft thick pad (if not 2 ft) under the planned high strength engineered floor for at least R6 underfloor (or 2 ft for R12). Then at least a 1 ft thick pad under the tank for another R6 (+) between it and the floor (which will suck heat away in all directions).

Note that Extruded polystyrene (XPS) starts out at R5 for 1" which degrades in time to about R4 if you are in a low moisture area with good drainage (or R10 to R8 for 2" XPS). It can degrade much worse than that if in a bad drainage area - and all of that assumes you never load it above 125 Lb/sq Ft where it will crush. Once any foam board is crushed by too much weight - it looses almost all effective R value.

I believe you should now have the information necessary to calculate required heat load, size of tank, weights for floor loading, what strength floor you need (and have someone design it and the foundations to support it: strength of concrete and rebar sizes and patterns matter), and how to insulate the floor and under the tank, etc.

Please let us know what your results are:

Perry

WillyP

Perry my tanks have a foot print of 30 square feet. Which works out to about 250 pounds per square foot. I like the idea of rubber matts as insulation. I actually considered dropping a couple yards of crushed asphalt on the floor to help dissipate the weight load and help with insulation. But ultimately I abandoned that idea.

hot_rod

Foam board is rated psi not psf.
150 or 250 is most common so 15 or 20 psi. I had a load of 60 psi hi-load Dow board, used under heated runways. It was more like wood, very dense and heavy.
what city are you near? I have a solar Simulation program for thermal to give you some ideas of what to expect 

even in the sunny SW mountains, of the US, rarely did they design for over a 30% SF, solar fraction. The arrays just become too large and affordable.

Larry Weingarten

Hello, With all the discussion of heat storage, I haven't seen much on how to make the building really efficient. It's beginning to feel to me that is where dollars might be better spent first. Just FYI, I built my house using SIPS. In my mild central California environment (temps measured from 18 to 105) I still used 8" walls, 10" floors and 12" roof. I'm off grid, so the energy mattered. Just for chuckles, if you built your home like an ice chest using 12" SIPs on all six sides, what would your heat loss really be? The main concern about this system is to really seal it so moist indoor air cannot get into the panels or framing, which will rot otherwise.
With an efficient house, your energy supply problem should shrink so that you aren't needing space shuttle technology to make it work.

Yours, Larry

Jamie Hall

Well, yes indeed @Larry Weingarten ! The very first step in designing something for solar is to figure out how to drop the heating load to a minimum.

The only two comments to add -- having done a number of passive solar houses in New England in years gone by -- are first, use heat recovery ventilators (sensible heat only -- not latent) rated at at least 2 air changes per hour. Otherwise, the indoor air quality will be really awful. And second, be sure that there is a way to dump excess heat.

WillyP

I'm near Dover-Foxcroft.
I'll be using 2x10 douglas fir for studs. With sprayed cellulose in the wall cavities. The outside will have rigid foam board, covered with 45 mil EPDM. Then concrete siding. All in all it should be an extremely efficient wall system. My mind is not made up on windows, mainly because I am in a forest fire zone and am more concerned with fire resistance than thermal. Pretty much any modern window is going to be efficient, but not all do as well in the middle of hell fire.

PerryHolzman

@WillyP: OK, about 250 Lb/Sq ft for your planned 850 gal tank.

I can believe that some rubber or plastic materials can handle that (make sure and also that they are long lived and don't degrade into mush that leaks goo with the years). Not sure on the cost though (it might be worth looking at the cost of different insulation options to get the needed R value you are looking for). Also I believe that a 12" poured concrete floor using 3000 psi concrete will support that assuming the ground and fill is compacted correctly - without rebar (still likely useful to add wire mesh).

You need to do a calculation on how much heat you are going to loose through the bottom of the tank (the calculation is straight forward based of of max tank temperature, ground temperature, area, and R value). You can reverse that once you know how much you need to store - and how much you can afford to loose from the tank; and back calculate the bottom, side & top R values.

Have you figured out if that is the right size yet for the needed thermal storage (how much heat energy you need to store and if an 850 gal tank will do it)?

Perry

PerryHolzman

hot_rod said:

Foam board is rated psi not psf.
150 or 250....

I had a load of 60 psi hi-load Dow board, used under heated runways. It was more like wood, very dense and heavy..

Thanks for updating me: I was working from a video on building housing basement floors (which cited 125 Lb/Sq ft). That made sense to me as I have some old 1" extruded polystyrene here cut into pieces that I used to prop up & insulate under a ice chest in my car and other minor tasks (I cut up a 4x8 sheet likely 20 years ago into small pieces). If I kneel on it my knees sink right into it, which matches an 100+ Lb crush load.

I was also unaware of the current modern high density extruded polystyrene that is rated between 40 and 100 psi (which I am sure is not what is being sold at Home Depot, Lowes, Menards, etc).

The current ASTM C578 standard starts at 5 psi (or 720 lb/Sq ft) - and goes to 100 psi.

My pieces do not have any ASTM standard stenciled on them - and they may predate the standard. It may be that before the standard that 125/150 and 250 Lb/Sq Ft was being produced (and perhaps still is today as the lowest cost option).

Perry

PerryHolzman

@WillyP: I apologize; my mistake. Yes, you can get foamboard that will support and insulate your tank. I was operating on old information (see above post).

You are looking for Expanded Polystyrene certified to ASTM C578 which specifies products ranging from 5 psi (720 Lb/Sq ft) to 100 psi (14,400 Lb/Sq ft). Look for the psi rating. If in another country find the equivalent certification for your country.

At the same time I have taken the time to check the two home improvement chain stores in my area: Lowes and Menards (a midwest USA regional chain based in Wisconsin).

Both chains carry ASTM C578 foamboard. But, they also carry cheaper foam boards that are not ASTM C578 certified - which likely the most commonly sold sheets due to the lower cost. It's likely that these non ASTM foamboards have less crush resistance, as demonstrated by the old pieces I use at home that clearly crush if I kneel on them.

Perry

hot_rod

PerryHolzman said:

hot_rod said:

Foam board is rated psi not psf.
150 or 250....

I had a load of 60 psi hi-load Dow board, used under heated runways. It was more like wood, very dense and heavy..

Thanks for updating me: I was working from a video on building housing basement floors (which cited 125 Lb/Sq ft). That made sense to me as I have some old 1" extruded polystyrene here cut into pieces that I used to prop up & insulate under a ice chest in my car and other minor tasks (I cut up a 4x8 sheet likely 20 years ago into small pieces). If I kneel on it my knees sink right into it, which matches an 100+ Lb crush load.

I was also unaware of the current modern high density extruded polystyrene that is rated between 40 and 100 psi (which I am sure is not what is being sold at Home Depot, Lowes, Menards, etc).

The current ASTM C578 standard starts at 5 psi (or 720 lb/Sq ft) - and goes to 100 psi.

My pieces do not have any ASTM standard stenciled on them - and they may predate the standard. It may be that before the standard that 125/150 and 250 Lb/Sq Ft was being produced (and perhaps still is today as the lowest cost option).

Perry

Most often the box stores have the 15 psi, concrete suppliers have the 25, at least in my area. If you want 60, 80 or 100 psi, you need to order a truck load A semi truck!

Jamie Hall

I'm enjoying these comments on insulation... but... don't we have the cart slightly before the horse here? We still don't know what size tank we really need (850 gallons just isn't going to be big enough, unless this is a tiny house) nor what temperatures... shouldn't the basic design be done and validated first?

WillyP

Jamie. Thanks for pulling things back on track. One of the main reasons I started posting on this (and a couple of other forums) is I need to figure out just how much water I need. It does indeed seem as if 850 gallons will not be enough. Actually it is 1700 gallons because I planed on using two 850 gallon tanks. When I started planning this, my guesstimate was 850 gallons would be plenty. Then once again physics crushed my hopes and dreams.

Jamie Hall

Physics has a way of doing that, @WillyP . However -- any help we can give you, we will!

WillyP

Perry and Hot ROD I am far from an expert on concrete. But I have poured enough to understand the basics. There will be more than enough rebar. But the bottom of the vault has been a matter of great deliberation. A part of me says just insulate under it and run some extra pex in it. If I treat it like a basement floor it should solve a lot of problems. However I also have very little faith in the insulation lasting long enough. I fear it will crush and cause shift under that slab. One thing I do not want to deal with is a cracked foundation. I fear no matter what I do, I will lose sleep over it.

neilc

just throwing it out there, , ,
what does a septic tank type vault cost?
insulate around that ?

WillyP

Septic tanks have a lot fo extra baffles and stuff inside them. They cost more than a plain tank designed for underground usage. Yes I looked into it.

hot_rod

A 2000 gallon septic tank might be ideal. They can pour them without any baffles. I had a 500 gallon concrete septic bottom in my shop. I had several coils of 1-1/4 copper coils inside. It ended up with a smelly bacteria, I switched to a pressurized 500 gallon, former LP tank. The concrete septic tank would be easy to insulate. I used an EPDM liner in mine, basic rubber roofing material. Set it on at least 4” of foam, around the sides also.
Foundation waterproofing companies could waterproof the entire assembly.
Around here the put a fiber mat over the foam and spray tar that.

Did you look at the cocoon tanks, they are 6” foam and the are modular, capacity to you needs. Seal it drop into the ground and you are done.