Combined domestic hot water and house heating system

[mike90045]

The voids between the walls, and the tank grid is filled to depth with peat moss. The peat is an outstanding insulator, as well as generating some of it own heat. 

if your peat moss is generating heat, it's gotten wet, and will continue to compost itself to something other than peat moss.  Filling the voids with styrofoam packing peanuts is a good way to dispose of the peanuts for 50 years, and add more insulation.

You may also need some "legs" in the slab, to transfer weight past the blue foam.  At some point, the foam is going to crush, and your tanks will tip/leak/break connections.

[BruceM]

Regarding the ply box design:
Jens-
Hardly rocket science, but lots of guys don't have an intuitive feel for statics, strength of materials and structural design, and a failure of an above ground 500 gallon tank could be a serious matter as well as expensive.  

A proven design defining all the structural elements and attachment methods would save a lot of time and get rid of a lot of risk, especially for a TLAR design (that looks about right) with zero structural analysis.

Regarding the use of poly drums:
Some of the standard 55 gallon drums are HDPE, and can't be used for hot water storage. Polypropylene can, and I did find some sources that sell them.  Some used, cheap PP marked drums might make this approach appealing. At $63 bucks each plus shipping for new ones, perhaps not. The multi drum situation makes heat exchangers more difficult, too.  Polypropylene (PP) tanks are also available and might be a good in ground solution, if insulated (spray foam) on the outside.

Blue board shouldn't be used for the foam touching a hot water tank/barrel , at least that one layer needs to be isocyanurate foam board, at least that's what I've read.

[Lloyd]

It's unlikely that the foam below the slab would cause any settling issues, a five inch reinforced slab will be one big diaphragm, with the drums evenly distributed the weight will flow to the insulation evenly.

Here is one source for drums http://www.usplastic.com/catalog/default.aspx?catid=459&parentcatid=458, I'm sure with a little local investigation used drums could be had for much cheaper.

At no point does the insulation come in direct contact with the heated barrels. The reason for the drains is for the moisture from the peat, replenishing the peat would be in relation to the anaerobic process controlled by moisture and oxygen allowed to the compost function of the peat. Less conductive lose will result.

Lloyd

[mbryner]

Welcome Lloyd.   Your idea is interesting, but I agree about the rotting peat moss.   Lining some sort of cube shaped tank (whether plastic, steel, wood, or concrete) with polyisocyanurate foam would be far easier and cheaper by far, IMHO.   Making concrete forms for decent sized tank >300 gal is not as easy as building the whole thing out of wood, and not cheaper.   That was how my Dad and I got to talking about septic tanks, because we were talking about forming up concrete walls.

If I went with the buried septic tank idea, I don't think a drain on the bottom would be necessary.   If the tank needed to be emptied for any reason, it could be pumped or siphoned out, eliminating one more potential leak.  Anyone see any problems with this reasoning?   Think the building dept will let me put it under the slab of my basement?  That way it's a straight shot upstairs to the kitchen and baths, and the propane on-demand heater can heat it easier. 

I'm going down to the concrete place to get some info on tank interior dimensions, price, etc.   I've been on-call in the hospital for 3 days, so it's good to get outside.

Marcus

[Lloyd]

hi Marcus,

Thanks for the welcome.

My plan was also to use either a septic tank or utility vault.  But after considering the conductive losses to the ground. I had the peat moss epiphany.

It would take many inches of foam to get to the r value of peat that is active.

Lloyd

[BruceM]

Marcus, (sorry Brett, I got your name here by mistake)-
I've seen plenty of solar home designs where the storage tank was under the floor.  With HDPE liner inside a concrete septic tank, that's pretty secure!  

I hope you can find a good deal on a single compartment concrete tank!

Llyod- I don't think you really want composting peat- once it breaks down, how will you replace it?

[Fat Charlie]

P.S.  And what about building copper heat exchangers?  Have any of you ever made them?   I've read about using smaller diameter copper tubing, filling first with sand or salt to prevent buckling, then winding around a pipe.   Does this work well?   Multiple coils sets of a smallish diameter copper connected to form a larger heat exchanger.   Anyone with experience here, maybe when making when making Listeroid exhaust gas heat exchangers?

A homebrewed heat exchanger?  Too easy:
http://www.olderascal.com/brewing/wortchiller/index.html

You get some piping hot water out of those things.  Ought to work just as well in the opposite direction.  This is the tube bender he's talking about:
http://www.homedepot.com/h_d1/N-5yc1vZ1xn8/R-100647978/h_d2/ProductDisplay?langId=-1&storeId=10051&catalogId=10053
I'd rather do that than fill with sand and keep my fingers crossed.

[bschwartz]

Bruce,
Although I've been following this thread, I'm not involved......

-Brett

[BruceM]

Sorry Brett, I put your name in my post where I meant Marcus.  My bad.

[Ronmar]

With the price of copper and the ammount of time you spend fabricating it, you can get a brazed flat plat heatex that will out perform it by a large margine on Ebay, for what you would spend in copper alone.  I had originallly planned on making my own heat exchanger for collecting the heat from my 6/1's cooling system.  After calculating the temperature differences I wanted/needed, I determined the ammount of surface area in copper pipe I would require.  At the going price in my area, it was well over $100 in copper pipe, plus fittings and my time to fab it.  I bought my 600 SQ/In flat plate heatex from a wood fired boiler vendor on Ebay for around $95 delivered...  Coiled copper pipe heat exchangers really are not all that efficient, especially considering the ammount of material used in their construction. Copper is a good thermal conductor, but low surface area and laminar flow or lack of turbulence are their largest drawbacks. 

My .02 after having done the math...

[mbryner]

When I build my large heat storage tank I will depend on the heat stratification of the stationary hot water for additional efficiency.

Ditto.

ronmar, how would a flatplate heat exchanger work in a large tank?   Don't they have to have a counter-current set-up of transfer fluid and fluid to be transferred to?

[mbryner]

So, just in case anyone is interested in what I came up with today (and documenting for my own purposes):

At the local concrete plant, at 1000 gallon septic tank is $970 + ~$30 delivery.   The tank is a rectangle, made on-site to order.  They can put in whatever I want for a manhole, but standard is a 24" plastic cylinder with cap.  They can insert any number/size of pipes desired in the lid during casting.   The tank is poured first and then the lid is poured on top after the bottom and sides are solid.   
The inner dimensions are 61" D x 86" W x 55" H.   
With 8" of interior insulation: the volume is reduced to 122,850 in3 or 531 gallons.   
With 6"       "           "                   "             "         155,918 in3 or 675 gallons.

My favorite local lumber yard (i.e. where I have an account) carries insulation in 1", 1.5", 2", and 3" thick 4'x8' sheets.   3" thick is $60/sheet.  Every 1.5" is about R value = 10.
8" of Rmax polyisocyanuate insulation: R = 53.
6" of Rmax           "                "         R = 40.
Think R40 is enough?
For 6" thick I'll need 12 sheets of R max = $720

Estimated cost of EPDM pond liner:  $150

Excavation costs are minimal because they are excavating the house foundation anyway next week.   I'll just have Derrick dig the hole at the same time.

Cost for 1000 gallon septic tank lined with 6" polyiso foam (R=40) and sealed with liner:   $1870

The local pipe and electrical store has flexible copper coils:  1/2" diameter = $3/foot,  3/8" diameter = $2/foot
Guessing I'll need to shell out about $500 for copper in the end.
Any of you engineers remember off hand the equations for heat exchange to help me figure out length of coils necessary?

For the domestic water exchanger I was figuring on using 1/2" coils at least.   Maybe one, or should I use 2 in parallel?    For the on-demand propane heater coil and solar/woodstove coil I was going to use 2 coils each of 3/8" in parallel.   They would be running of circulator pumps anyway at lower flow ~1-2 gpm (assuming here, need to get data on the Laing and El Sid pumps, etc.).

@BruceM:

Marcus, (sorry Brett, I got your name here by mistake)-

No problem.   Figured you'd catch it sooner or later...

@FatCharlie:

Thanks for the tip about the tube bender!

Marcus

Edit: spelling and clarity

[mbryner]

And, for those of you wondering about smashing the insulation: from the Rmax data sheets,  the compressive strength is 20 psi (= 2880 lbs/ft2).   Put in perspective, a 4 ft high column of water with a 1 ft square footprint is about 30 gallons or 240 lbs.

[vdubnut62]

I am just curious as to how are you going to fasten your pond liner. Are you planning to line and insulate the inside top also?
The tanks around here are cast in halves, a top and a bottom with a seam around the middle, think mickey D's hamburger box.
Sounds like a good plan but I'm just not bright enough to cipher this one out. Pictures help! I'm a visual learner.
(8 x 10 color glossy pictures with circles and arrows and a paragraph on the back of each one telling what each one was)
Ron

In the immortal words of Miss Emily Litella, "Never mind" on re-reading your post I saw "The tank is poured first and then the lid is poured on top after the bottom and sides are solid."

[Lloyd]

Llyod- I don't think you really want composting peat- once it breaks down, how will you replace it?

Hi Bruce,

I don't want to try and convince anyone, but the idea has many merits. First one has to understand peathttp://en.wikipedia.org/wiki/Peat.

Unless there is a reason to remove the peat it could last thousands of years with just a topping off. It can be controlled by anaerobic process, it can be easily removed by a large shop vac.

I have an old Swed friend who said that the only way he could keep his underground pipes from freezing was to fill the pipe trench with peat.

It could even be a Methane gas producer http://www.peatnet.siu.edu/Assets/G.pdf, which could be used as an additional heat source.

Everyone's gardens love the stuff, it can even be used as fuel to burn.

Ok that's it.

Lloyd