Low head loss hydronic floor heating

[SolarUtah]

Great thread. The system I installed under the basement floor uses 3 parallel circuit with a single pump, I think it is a Taco 9, uses 135 watts.

See graphic of the system below. Works well, I used about 450' of Pex tubing and about 70 ' of copper. Only used 45 degree fittings to limit pressure drop.

[Daox]

Great to hear I'm not the only one thinking along these lines. How is the floor working out for you so far?

[Blue Bomber Man]

I used to in stall hydronic floor systems with my father years ago.... this was the preferred layout due to the way it averages out heat distribution.

Additionally we would decrease the spacing between parallel runs near exterior walls or other heat sinks to improve comfort

[SolarUtah]

My system works well, I was concerned with the air entrapment issue using a parallel arrangment. By installing a common manifold, I was able valve off one or more circuits to purge the air, as well as balance flows for even temperatures. See pics below of the Manifold I used.

There are actually 3 circuits, not 2 as shown in the graphic.

[Daox]

I'm bringing this thread back. I'm pretty close to starting to work on the office hydronic floor. I am going to go with a few parallel lines in it. I don't know quite how many yet. Somewhere between 3 and 6 is most likely. I'm also still trying to decide between 1/2" and 3/8" pex for it.

[Mikesolar]

Remind us how many ft2 the floor will be? When we design these for boilers we are using trying to get a 20F dT and this usually meant we needed about 250' of 1/2' tube per loop. Head loss is quite reasonable and I recently did an 8 loop floor with a Alpha pump and the display goes back and forth from about 3gpm to a power consumption, when it settled somewhat down, of 12-15w. It is hard to get much better as this was the whole house.

[Daox]

This specific room is about 14x12, so 168 sqft. The whole house is about 1600. I'm also planning on using 3/8" tubing for the floor versus 1/2" due to space constraints.

[stevehull]

The flow through each of the parallel circuits (each of the legs) will only be the same, if the diameter of the supply conduit is slightly larger than any of the parallel legs.

Here is why.

Imagine a flow circuit with the parallel "legs" (all the same diameter) being vertical and a supply manifold "header" is at the top of each and a return manifold is on the bottom. The parallel circuit is fed water from the upper left.

If the manifold and each of the legs are the same diameter, then the parallel legs closest to the inlet get the most water flow (heat). The further you are from that inlet, then the lesser the flow. This is because there is a pressure drop along that manifold.

The reason for this is that there is a finite resistance in the manifold "header" tubing. But if you put it in slightly larger in diameter, then the pressure drop along the length is negligible compared to any one parallel resistance.

In physiology, this is why supply arteries (aorta for example) are always larger than the downstream arteries (femoral, iliac, renal, etc). The downstream arteries are all in parallel. Such a system keep the pressure at each downstream artery virtually the same and thereby flow through any one organ system is simply regulated by the resistance of the arterioles (smallest arteries) in that organ. This is an example of a constant pressure system with flow being regulated simply by resistance and being directly proportional to changes in pipe diameter.

If the supply manifold (aka "header") is the same size as the parallel circuits, increasing pressure in the system somewhat negates the pressure drop along the manifold pipe. But in general, radiant floors are low pressure systems.

Flow always goes into the parallel circuit with the lowest resistance (electricity or water). In the above parallel system, there will be flow in each of the parallel legs, but the flow will be highest in the legs that are closest to the inlet supply.

By increasing the manifold diameter slightly, you decrease the resistance in the manifold markedly. Only a 11% increase in diameter will decrease resistance in half (double the flow). This is because flow is proportional to radius to the forth power.

Having a larger manifold diameter also allows you to have parallel circuits of differing lengths have similar (not identical) flows.

You can model this as either a constant flow inlet - or constant pressure inlet system, but the results will be the same.


Steve

[Zwerius]

Just read this discussion.
What I didn't read in this discussion, are these two things:
1. The heat transfer from water to the pipe is depending on water velocity.
As soon as waterflow is so low, that the flow isn't turbulent anymore but it becomes a laminar flow, the heat transfer from water to the inner pipe wall is reduced considerably.
So by dividing the flow over 18 parallel pipes (as in the first example), the water velocity is only 1/18 th of the original value, so there's a good chance velocity is too low to keep a turbulent flow.
So heat transfer might be much lower than expected.
2. It seems nobody was taking Bernouilli's law into account.
Let's assume the first lay out with the 18 parallel pipes.
Assume the input is at the bottom right of the drawing and the output is top left.
Since a radiant floor heating is in one horizontal surface, there's no difference in height between the pipes.
Then according to Bernouilli's law: 1/2*rho*(velocity)^2 + Pstatic = constant.
This leads to the fact that (contrary to what many people think), the flow is not evenly distributed over the 18 pipes. The highest flow will be in the most left pipe en the lowest flow in the most right pipe.

[Mikesolar]

My manifolds are always piped supply opposite to the return or "reverse return". Flow is partially defined by pump speed which means that the dT can be designed, to some extent. While turbulence does help in heat transfer, 50% or more will still occur with laminar flow. When the heat loss of a particular building gets close to passivHaus standards, it is very difficult to pump fast enough to have a high reynolds number and meet the design criteria for the system.

If we are using the heat pump as a heat source and you want to have a 10C dT over the loop (typical with a boiler), some the capacity of the buffer tank will be quickly overcome. The HP may have some trouble keeping up. The dT over the loop will probably have to be lowered which means shorter loop lengths.