Boiler room floor slab questions

[mark cline]

I will be pouring a floor for my boiler shed in the future and was wondering how thick a slab it should be. My shed is 12x16 which will have about 2 to 4 inches of rigid board insulation under the slab. My boiler is a 300k Econoburn indoor unit , about 2700 lbs full. Also 3 - 500 gal storage tanks , when full will weigh about 4500 lbs each , so about 16,200 lbs total for the shed floor . The tanks will be along one wall and the boiler along the front wall about 6 ft away .I'm thinking about at least 6 " thick, with alot of rebar and steel mesh . The slab will need to support the weight by the slab only , thinking that the rigid board insulation will not provide any strength as opposed to a gravel bed . Just thought about possibly putting a concrete pier under each tank location then pour the slab on top of the pier . Any thoughts?

 

[taylorfarms]

As long as you get the more dense foam I think I used 2" foamular 250, and plenty of rebar you should be fine with 6", in our shed I ran 2 1/2" rebars around the perimeter and a couple extras where our storage tanks are to be installed, in addition to the 6x6 9ga mesh throughout the shed. I put down and packed sand, then vapor barrier, then foam then rebar, be sure to pull the rebar up into the concrete a little bit as you go if you are not using chairs. Also we put I tiles dug to the outside and capped at concrete level to run our power and hot water lines through, rather than having to cut the concrete at a later date. I have had the skidsteer with the boiler, and the skidsteer with a pallet full of seed corn on this floor and see no signs of damage yet, even split a little bit inside that will put more pressure on the concrete than anything

we poked the 1/2" rebar in as we were pouring the concrete so I would not lose them in the concrete, as I did not feel like wiring them in. I the picture you can see the coffee can that covers the tile (poor mans sonotube) just had to pull the lid off once we were done striking off so we could finish over it.

 

[warno]

My garage slab has a 20" thick footer poured around the perimeter and the slab itself is 4-5" thick. My 750 gallon storage consists of 3) 250 gallon tanks stacked horizontally and it's right against the wall about a foot from a corner. The tanks sit on 4 large feet to spread the loud. So far 2 years and no issues.

 

[nhtreehouse]

For what its worth, I poured a slab over the summer. I went with the design which is well documented on the web, where you have a thickened edge, and a regular thickness slab. Search for "Shallow Frost Protected Foundation", also called an Alaskan Slab. This monolithic slab doesn't frost heave because you add wing insulation to trap the earth's heat as it attempts to reach the surface. I don't know that I would do that again, as the amount of foam required is substantial. I used reclaimed roof foam, and even at a deep discount, it was pricey.

In my case, I went with a 12" deep thickened edge and a regular 4" slab, all poured from a single truck in one long morning.

The foam can carry the weight if you choose to go that route. The number 250 in "foamular 250" is the PSI rating of the foam. You can work the math on this and would find that the area of a 36" circle (vertical tank) is a little over 1000 square inches. 4500 lbs distributed evenly over the circle is only 4.5 psi. Unless my math is wrong... You can also find documented cases where entire houses are built over foam. I seem to recall reading on the web that airport runways might also be poured over foam.

The big problem with this assumption is point loading from the legs on the tanks. This concentrates the load on the floor which is not the best thing to have. For that reason, I used a 1/4" plate under my tank and welded legs which form a cross under the tank. More work, more metal, but I was going with the famous adage "Nothing too strong ever broke." I put L bolts into the slab, so the plate is bolted to the floor, and I'll weld the tank legs to the plate. Lock everything down, just in case.

In the concrete side, I think you want to be considering the amount of metal more than the amount of crete. It's the metal reinforcing which matters. I have the usual 6" sq mesh in my 4" thick slab. My tank is 1000 gallons, vertically oriented, so when forming the foundation, I extended the 12" deep "haunch" perimeter under the tank. This area is reinforced with a rebar grid - the usual 1/2" stuff on 12" centers. That is, a 12"x12" grid. All the rebar in my slab was set of chairs to keep it off the stone and vapor barrier.

My two cents... As always, your mileage will vary. Here is a visual - the foam under the slab is 3.5" in two layers (1",2.5"), and the vertical insulation is 3" thick. You can clearly see the chairs and double rebar in the thickened edge.

 

[mark cline]

I'll have to research the Alaskan slab. It sounds like what I'll need . All the weight on one side of the slab had me thinking it would break off.
What underground piping product did you all use?

 

[nhtreehouse]

What underground piping product did you all use?

Before you spend any money on underground pipe, read the sticky on underground piping, twice. I have a short run - 50 feet of conduit just made it between the house and shed, and I just couldn't see the economy of doing the foam-in-trench. For one, I would have had to buy 300 feet of big PEX and that stuff isn't cheap. Secondly, I dug the trench down between 3 and 4 feet. I went with Logstor, it's dual 32mm PEX, which equates pretty closely to 1.25". The inside diameter is just larger than an inch. I see you are planning a bigger system, so think about head loss moving BTUs to the house. I believe Tennman used bigger PEX. See attached...

I did line the trench with 2" XPS foam on all sides of the logstor, and added more EPS I had kicking around on top. I figure it can't hurt. I should post my method on the underground lines sticky - just haven't had time yet.

 

[taylorfarms]

I used 1 1/4" thermopex from a local central boiler dealer $15 a foot including the end fittings, but no real measureable heat loss(.5 deg), in the 75' run to the house. I had crappy underground lines before, even if it was $30 a foot I would get the good stuff.

 

[vtwoodheater]

Even sakrete is 3K PSI. If the base material is good, the concrete will never crack. My boiler room floor is 4" thick, no rebar in it at all, over 2" foam. My storage is on the garage floor, 5" thick 5K psi mix. Hard to scratch that floor even with the studs on the snowmobiles.

Rebar will hold the slab together after the subgrade settles and the concrete cracks.

FWIW a 9K# Rotary 2 post lift has a footprint of 17 PSI

 

[nhtreehouse]

FWIW a 9K# Rotary 2 post lift has a footprint of 17 PSI

Interesting that the footprint on a rotary auto lift is so low. A woman's high heel might have an area of only 1/10 of a square inch. That means 100lbs = 1000 PSI, and broken toes. The analogy here is that tanks on legs without a way to spread the load to the floor are sort of like those high heels!

 

[vtwoodheater]

1,000 gallons of water 8,340#
2 tanks 1400#
hardware, fittings 300#
4 legs, 2"×2"/pc.=627.5psi

Way below 3K psi.

 

[jebatty]

Do your best to keep the pex at a depth of 2" from the top of the slab. Heat travels slowly through concrete, and you want the heat to quickly reach and stay at the surface of the concrete, not the bottom.

 

[mark cline]

I have read the sticky on underground pipe and totally agree 100%. I will get the good stuff but would prefer 1 1/4" -1 1/2" pipe if I can find it .