How many feet of Pex

[Alan Waters]

Hello. First post on the forum. Building a 40x80 with radiant. Had three courses of eight inch block laid. Layered about 12 inches of wet compacted sand. After the plumbing is done, two inches of polystyrene insulation will be laid on the plastic vapor barrier. There is a granite quarry nearby and the gravel crushing operation produces a granite powder residue. Seven inches of this granite powder will be laid and wet compacted. 3/4 Pex will be buried about two inches deep in the granite. Followed by four inches of concrete. This will give eleven inches of mass which will be heated by a larger, modified version of the water heater in the video. Don't know how many feet of Pex to use. Suggestions are welcome. Regards, Alan Waters If someone will tell me how to post photos and videos I'll post a few pictures of the progress.

 

[B.C.Biker]

Been lurking for awhile and this is my first post here also. So, Hello from B.C.!
I'm also putting up a shop but will pour a floor next year.
My question for you Alan is if you have compacted on pex before? Can you use the same force/pressure you would on a regular base without damage to the pex?

 

[Alan Waters]

Hello B.C. This is my first radiant heat project. I spent months thinking of a way to get more mass without using more expensive concrete. On my driveway I have what is called "crusher run" gravel. It is small gravel which has not been separated from the granite powder. A check with the quarry confirmed they did sell just the residue. Which brings us to your excellent question about compaction and Pex damage. It never entered my mind. So, I will run some test and let you know. I expected to hear some opinions about my question but as you can see……. I have decided to run 2400 feet. Pex universe.com sells four 300 ft rolls on a skid for 515.00 dollars.

 

[MagKarl]

Assuming your tubing runs the full length of your slab, then the length of tubing is a function of the tube run pitch or spacing. Seems most folks run on approximately 1 foot pitch, or 1 foot length per square foot area out in the field.

 

[Alan Waters]

Is that for all diameters of Pex? Is there a formula for pump flow? Since the material comes in three hundred ft. rolls I plan to use a complete roll to make a loop.

 

[Highbeam]

300 feet is the maximum recommended length per loop when using 1/2" pex for heating. You can use larger pipe but the 1/2" pex is standard for a reason.

Also when running 1/2" pex the pipe spacing is 12" which as you might imagine works out to 1 LF of pex per SF of building. With 3/4" you can space farther apart but you'll have more noticeable hot/cold spots.

The GPM per loop and temperature of water will be part of the boiler/pump design and at that point you'll need more design information like heat loss.

 

[engineer2]

If you run a single zig-zag line, the water might be cool as you get to the end of the run. Four ways to get more even heat:
Double serpentine: one line run doubled
Parallel sepentine: two parallel runs flowing in opposite directions.
Counterflow: Single run laid out in a parallel spiral
Multiple zones with short runs.

 

[Randy in Maine]

I did not think it ever got that cold in North Carolina.

 

[Alan Waters]

We rarely see single digits but 20's and 30's are common.

 

[Alan Waters]

If you run a single zig-zag line, the water might be cool as you get to the end of the run. Four ways to get more even heat:
Double serpentine: one line run doubled
Parallel sepentine: two parallel runs flowing in opposite directions.
Counterflow: Single run laid out in a parallel spiral
Multiple zones with short runs.

I like the sound of the parallel serpentine. Both on 30 inch centers which would give me a tube every 15 inches. Is that to close using 3/4.

 

[mygarageone]

Stick with the industry design criteria . 300 ft max , 12" centers or less . Even in the coldest areas of the country no one does what was suggested . A 10-20 degree temp drop in normal and expected .

 

[Alan Waters]

To run eight loops of three hundred feet, what size feed line on the manifold wil be required?

 

[mygarageone]

To run eight loops of three hundred feet, what size feed line on the manifold wil be required?

All the premade manifolds are 1"

 

[volleyball]

The pex should be in the concrete, not the granite. You will lose more heat into the ground if you do it in the dust.
You did not mention the vertical foam insulation. You want to go as deep as you can. At least to your frost depth and deeper would be better.
I would get a radiant designer to give you a layout plan. If the space isn't one big room, I would also zone it instead of manifolding it. Why heat under a dead storage area?

 

[Highbeam]

You can't get them too close together. You can get them too far apart.

 

[Alan Waters]

The pex should be in the concrete, not the granite. You will lose more heat into the ground if you do it in the dust.
You did not mention the vertical foam insulation. You want to go as deep as you can. At least to your frost depth and deeper would be better.
I would get a radiant designer to give you a layout plan. If the space isn't one big room, I would also zone it instead of manifolding it. Why heat under a dead storage area?

Why?

 

[Alan Waters]

How do you zone without a manifold? The granite is part of the total heat holding mass. It is insulated from the the ground with two inches of polystyrene and 12 inches of sand.

 

[volleyball]

How do you zone without a manifold? The granite is part of the total heat holding mass. It is insulated from the the ground with two inches of polystyrene and 12 inches of sand.

You can manifold after the zone is established. You just don't want a single zone system.
You want your heat source close to where you want the heat. If you put it as far down as you say, you will have nice warm ground and a colder shop. It will also take a long long time if the heat is off for it to build up to where you can feel it. To radiate from the concrete, it is best the pex is in the concrete. Below it and you will lose a lot larger portion to the ground downward and laterally.
I might go less dust and up the concrete if you are planning heavy equipment in shop.

 

[mygarageone]

How do you zone without a manifold? The granite is part of the total heat holding mass. It is insulated from the the ground with two inches of polystyrene and 12 inches of sand.

You won't have seperate zone control with out manifolds and pumps , you'll be fine with one manifold if your intention is to heat the whole building at the same time ? Other wise you'll need to think about seperate manifolds and pumps.
It is recommended that the pex be in the concrete and the granite will be a storage heat sink after the system balances out.

I did a job one time , where the engineer wanted no insulation below the concrete , we installed the pex as spec'ed tied to the concrete mesh and the building heated fine , the sand below the concrete was heat storage . It took a long time for the system to balance out but after it did it worked fine.

I don't think it was a good idea , because the amount of fuel it took to keep that mass upto temp.

I watched that Video , what he's done is nothing new . They have equipment made and designed doing the same thing but with all the modern controls and safety devices , self contained and approved for what your doing.
Looking at that combustion chamber , I Wonder if it would work for the volume of water you
will need to heat .
Personally I would avoid that home made water htr .

 

[BadgerBoilerMN]

What purpose does the "mass" serve?

This is a common practice where off-peak electricity is used as primary fuel, but serves no other useful purpose unless to collect solar energy, in which case a storage tank would be a more common and useful solution.

Try not to make stuff up since the science has been well documented since I started in the late 80's.

 

[Crazy68Dart]

There was an interesting document I ran across on the mass issue.

In general, it seems like it would take a year and a day to heat all that up. On the flip side, it should retain the heat longer... however, thermal conductance is going to play a big role. This granite slurry/powder, how well does it (or not) transfer heat? What if it is a very good insulator?

BTW, not saying you are wrong, and I am not expert, just some ideas.

 

[Alan Waters]

http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html

Granite does better than most as a thermal conductor. A lot better than concrete as you can see in the chart.