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Old 10-16-12, 12:55 PM   #18
Daox
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I re-ran my calculations for the same floor, but using 6 parallel lines (similar to the 5 parallel lines shown in the image below) since the TEE fitting pressure head was so high in the previous example.

Looking back, the tubing pressure head added up to a measly .62 inches of water, and the pressure head from the TEEs was 2.3 inches of water (but, I believe it was calculated incorrectly). Anyway, it makes sense to reduce the number of tees. This will reduce the pressure head the TEEs create, but it will also increase the pressure head from the tubing. So, this is an exercise in finding the sweet spot between too many tees, and not enough parallel plumbing runs.

I'll run through the calculation step by step.

This is the planned plumbing layout (roughly). Each parallel run is the same length and therefore will have equal flow.




Alright, lets assume we have the 1gpm flow rate coming into the flooring. In reality, on the inlet side, as the water travels down the header pipe more and more of the flow is being diverted down the parallel lines, so this isn't 100% true. On the outlet side, it is the exact opposite, you won't get an actual full 1 gpm until the last parallel line has teed into the header. So, we're going to use a little more than half the tubing length as a guess. Each header pipe is roughly 9 feet long, so we'll just say 10 feet for our calculations to make it easier.

Using this spec sheet: http://huduser.org/portal/publicatio...sign_guide.pdf

We can see that at a flow rate of 1gpm through 10 feet of 1/2" PEX tubing is going to create a pressure head of .17 psi. To convert psi to inches of water you multiply by 2.3, so we have .39 inches of water for the header tubing.

At this point, our flow diverges into 6 parallel lines that are about 28.5 ft long. Each line will see a flow rate of .167 gpm. Sadly, our spec sheet doesn't go quite this low, so we'll have to use the .2 gpm pressure head. The pressure head for 28.5 feet of 1/2" pex at .2 gpm is .065 inches of water.

Now, we still have to add the pressure loss from the TEEs that need to be put into the system. Last time this is where a sizable amount of the pressure head came from. This time we have reduced the number of TEEs from 36 down to 12. The water flowing through the system will flow straight through a TEE 4 time, and flow through the TEE as an elbow 2 times. To get the pressure head these TEEs create, we must use the chart below:




So, we have 4 'run tees' for a total of 4 X 2.2ft = 8.8 ft of additional tubing pressure head. The flow rate these fittings will be seeing is exactly the same as the header pipes will see because as the water flows down the header pipes more and more is being routed down each parallel branch. So, we can calculate it at a little more than half the 1gpm flow rate. We'll use .6 gpm for these. According to our spec sheet above, that gives us .14 inches of water pressure head.

We also have 2 'branch tees' for 2 X 10.4 = 10.8 ft of additional tubing. Now, these branch tees are only going to see the amount of flow going down each parallel line, so the flow rate is .167 gpm. So, we have 21.6 ft at .167 gpm which gives us a pressure head of .049 inches of water.

So, now we know all our pressure heads and can add them together.
header tubing: .39
parallel tubing: .065
run tees: .14
branch tees: .049
Total: .644 inches of water

In the previous calculations, I didn't break apart the tee fitting calculations, and that made for some pretty huge differences in calculation. I believe I have it correct now. This is 1/10th the power it takes to pump the same flow rate of water through a single loop of 170 feet of 1/2" pex tubing. You should get the same exact heat transfer.

The only disadvantages I see are:
1) Tee fittings are a possible point of leakage. You could possibly put them below the floor so you could fix them if need be.
2) Tee fittings add additional cost to the project.
3) You can't configure the parallel tubing runs as well as you can with a single line. Normally, the inlet and therefore hottest tubing is put along the outside wall which helps keep the room slightly more comfortable.
4) Purging air from the system may prove more difficult.

The advantages are fairly obvious:
1) Drastically reduce pumping power required which means lower energy consumption and a much smaller pump required, both of which save money.


When I get some time I'll probably run a cost analysis on this setup and compare the two.
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Last edited by Daox; 10-16-12 at 01:33 PM..
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