"Unbalanced loopfield flows can rob your loopfield capacity and produce unexpected system performance degradation." I recently saw this statement and I can't help but think that this is what encourages the use of balance valves on each supply and return in multiple group loop fields. Frankly, I don't believe that the penalty of imbalance is worse then the penalty of imposing a permanent extra pressure drop on the loop field and the resulting extra pump energy.... If the length of supply & return pipes is causing a balance concern then reduce the diameter on the closer groups and create better balance while also saving money and pump energy. I want to challenge the geothermal community to quantitatively support or dispute the use of balance valves. I look forward to a interesting discussion.
In support of your position, also consider that the pressure drop across each branch is (approximately) proportional to gpm^2. So, small mismatches in lengths of parallel branches will tend to be self-equalizing, with gpm-mismatches being consierably smaller than length-mismatches. ...Mother Nature is occasionally on your side, Looby
We've had lively debates about uneven loop lengths and throttling for balance. While many of the pros seem to agree creating resistance will not improve efficiency, most still feel mis matched loop sizes can have huge consequence. That, reverse return and turbulent flow mantras often fail to consider that any loop system will work as long as adequately sized. Some will be more efficient, but the energy saved may be quite miniscule. IMHO Joe
Been designing Working on a design for a geothermal vault (commercial) based on just that - self balancing. Challenge accepted
All of our commercial vault work is designed with a bucket load of valves for isolation and balancing. Having been back on numerous jobs at this point after completion the valves are not used, all wide open. So self balancing? On residential we have been requested to run tails into mechanical space to be hooked up to balancing manifold by others, same result. I admit that I am anal about plus or minus 5% for the loop lengths may be an indicator of results. I feel this may be a bigger issue with hydronics like in floor radiant where the circuits are harder to balance by friction loss being the same? Eric
Any professional designing geothermal loops should read the ASHRAE commercial geothermal book. It states that variations in flow between loops does not cause lack of performance for the loop field. I also talked to Dr. Bose and he stated that experiments show that an imbalance of flow as great as 25% does not change the total heat exchange of the loop field. The balance valves do consume alot of energy and take away total flow rates and thus actually reduce the effectiveness of the loop field and add to the pumping power. This creates lower efficiency rather than enhancing it. The use of balance valves comes from engineers who are so anal that they believe the flow must be the same as they calculate, ignoring the fact that the real world is different from the computed. Most engineers create systems that need way too much pumping power rather than designing systems that need minimal pumping power to work. They ignore efficient operation as a design factor. I often use larger pipe to reduce pressure drop. I also refrain from using 30% propylene glycol as that antifreeze at that concentration kills loop performance.
I was told by one engineer that balance valves were needed to make sure the flow in one loop was exactly the same as every other one, otherwise the performance of the entire field would be compromised and total heat exchange would be reduced. The same engineer told me that flow rates had to be kept at the design rate all the time, even with multiple heat pumps and only one heat pump operating, otherwise turbulence is lost. I have found, and again experiments show, that a low flow rate when the load is low does not hurt heat exchange in the field despite the loss of turbulence because the size of the field is large enough to make up for the loss of turbulence.
When we design commercial loopfields, we don't usually use balancing valves. Why pay for pumping energy just to dissipate it with a circuit setter? Turbulence in the loopfield is not necessary during off-peak conditions (i.e. - when only a few heat pumps are running on a large multi-heat pump system). If only one heat pump is running on a loopfield designed to accommodate 10, that single heat pump is going to be very happy, with or without turbulent flow..
Good point. So, 'self-balancing' actually works at two levels: - differences in BTU/hr are smaller than differences in GPM - differences in GPM are smaller than differences in length .
You have got to know Flow I am not on either side of the turbulance issue as I design and build hydronic heating and cooling systems. I have no means to test Mr. Reynolds theory. ....but if I am buying pipe in the ground and paying for it. I want every BTUH I can get my hands, or fluid on. I know what waterpirate is saying is true, all the valves are open. I think that is because no one knows why the valves are there, so no one ever balances the loops. I do lots of radiant work which uses the same numbers a loop field uses but backwards. Think Star Trek, matter and anti-matter. It is just give and take.again I want every foot of plastic to be ALL that it can be. I never saw or heard of ballancing manifolds for loop fields until I started typing here. I also know there is no really new knowledge, so I may have missed them being used. I think it is impossible for a system to self balance as moving water is lazy. I takes the easiest path on it's journey. So.... ...if a "selfbalanced loop field" makes the heat pumps and the people happy, why would anyone consider that all of the buried pipe is not being used. I would but I am anal in that dirrection. In a radiant floor it shows up as a cold area. In the dirt or the pond who is uncomfortable. Thanks for allowing the rant, but I spend a good deal of my efforts in this field fixing screw ups and BAD engineering. Remember only you can CYA your own A. (I'll fix the grammer and some of the spelling after I cool down).
Mark, I agree with what you are saying, if some of the radiant loops have fast flow and low delta T and some move slow and have high delta T, the difference will show up in floor temperatures. If the radiant was for a big open space, heating the building would be the same for the balanced as the unbalanced flow paths, within reason (say 25% difference) The hotter areas would give of more heat and the cooler areas give off less. But if you measure the delta T in the main pipe you cant tell if the loops are balanced or not. The ground loop is the big room and the total exchanged by the slow flow and the fast flow will balance as long as the difference is moderate. I use close manifolds and the lengths of all the loops are within 10% of each other so in essence I am within the tolerance for reverse return anyway, without actually doing it. The close manifold assures that the pressure on each loop is the same, and the balance is close just from the physics of it.
What if the owner's work space is over the cold spot. I look bad and the loop field is not doing the job it is paid for to do. Collect all the heat possible. Reality and physics are not the same to the guy writing the check.
