Low head loss hydronic floor heating

[ecomodded]

I could believe the freshly heated hot water would head down the first sections available,as the flow is so slow it could or would find the path of least resistance.
I think the coiled line is superior now, after some thought.
I want too make this system as well, as i have leaned,here, it is the best method for heating.

[Daox]

Yes, it will flow down the path of least resistance. The path of least resistance is a balanced flow between the parallel lines. Each line has the same pressure loss across it if analzed individually, so you should have equal flow amongst them all.

[hamsterpower]

I have a friend with a system like this in his warehouse space. 14 parallel loops starting and returning to manifolds on a common utillity wall. All loops start and end with valves. I can only guess that these valves allow some flow ballancing/adjustment. The manifolds are 2" copper feeding 3/4 pex. I have no idea if it adds up to a low head overall.

[Minimac]

The fluid will always choose the path of least resistance, thus short circuiting the sections that are further away. The only way to "balance" this would be the addition of throttling valves in each "loop". In practice and real world, more than just a few loops are practically impossible to balance.

[Daox]

Yes, it will choose the path of least resistance. Once it starts going down one tube lengh it will start seeing increased resistance as is normal of any liquid flowing in any tube, thus it'll start creating a pressure head and that pressure head will push liquid down another leg, and so on and so forth.

What this does is mimic exactly what a solar panel does. They use the exact same pricipal except in this case I'm shedding heat instead of collecting heat. They do not have issues with flow balancing, they don't have issues with high pressure head or they would be made differently.

[JRMichler]

The sketches show a parallel system with supply on one side and return from the opposite side. The flow path from supply through any one branch to return is exactly equal to the flow path through any other branch. The system will self balance, even if the flow velocity through the branches (I'm too lazy to do the calculation) is laminar.

I expect possible trouble from two sources.

The water velocity may be too low to push trapped air out of the branches. Then you will have unbalanced flow.

The pressure drop will be extremely low in the branches. If there is any elevation differences, temperature / gravity effects need to be considered.

[Daox]

Thanks JRMichler. I hadn't thought about air bubbles. That is a good point. You would just have to make sure you can achieve the necessary velocity to get rid of air in the lines. This could hopefully be done by shutting down other loops in a home run type setup, but it would have to be calculated.

[Daox]

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.

[Minimac]

It sure looks like false economy to me. By having to move everything to below the floor(to be able to fix leaks), it won't be as efficient, heat wise. Drastically reducing pumping power will make it harder to purge. The flow loss through a tubing bend is less than using tees. Don't forget too that if your using water, you'll always have air being introduced into the system either through make up water or just from the water it self. Calcs are fine but sometimes real world works differently for some reason.

[Daox]

Thankfully its not false economy. I've been talking an engineer I work with (who has done hydronic heating installs) and pressure head increases with the square of flow velocity. So, reducing flow velocity reduces pressure head very quickly.

The reason I am posting this is because I don't have real world experience with hydronic floor heating. On paper this sounds great. Talking to the few people I know who have done hydronic floor heating they say its totally possible but don't like it for the reasons I've listed (mainly the connections in the floor). I'm asking if anyone has any experience or ideas that says this is a bad idea because so and so. I have yet to really see any reason saying that this is a horrible idea because of whatever reason.

I don't think having connections in the floor is a huge issue especially if you pressure test the system before putting the flooring in. I'm on a well system so I could just hook it up to my well and crank the pressure up. Or, maybe you just pay a bit more attention to the connections you make and make sure your crimp tool is properly calibrated. I'll have to do some reading on reasons pex connections fail to help reduce the chance of failure.