What is a reverse return pipe?
The concept of reverse return piping is simple. Just reverse the chilled water return pipe such that the first air handling unit (AHU) that receives the chilled water supply is the last AHU that provides the chilled water return. Hence, reverse return piping is sometimes known as first in last out.
What is a reverse return system?
A reverse return system is a type of closed loop system where the return header is connected to the most hydraulically remote load, as shown in Image 1.
Can I use reverse return piping on a short manifold circuit?
Again, because the pressure drop along the short manifold is extremely low in comparison to the pressure drop along a typical radiant panel circuit, the use of reverse return piping will have virtually no effect on individual circuit flows.
How does bore size affect reverse return piping?
The larger the bore of the manifold relative to the number of circuits it serves, the less effect reverse return piping has. Keep in mind that the floor circuits may not all be the same length and don’t necessarily need equal flow rates.
What is a Reverse return piping system?
A reverse return system is a type of closed loop system where the return header is connected to the most hydraulically remote load, as shown in Image 1.
How does a reverse return system work?
1:553:34hvac-Direct Return vs Reverse Return Pipe - YouTubeYouTubeStart of suggested clipEnd of suggested clipIn Reverse return piping the losses from the supply through the unit and out the return are balancedMoreIn Reverse return piping the losses from the supply through the unit and out the return are balanced through the three units.
Why is Reverse return piping system more desirable than direct return?
Why is reverse return piping system more desirable than direct return? All paths have the same pressure drop with a reverse return system, making the system easier to balance.
What is a two pipe direct return system?
Two-pipe direct return hydronic systems have a pipe circuit for the supply and the return. The advantage it has over the single-pipe system is that the hot water directly routes to each radiator terminal at the same time.
What is hydronic piping system?
A hydronic system is a system designed to circulate chilled or hot water with the connections between piping and terminal units (heating and/or cooling devices). Most hydronic systems are closed and are usually made in a series loop.
How is hot water controlled to a zone?
Each zone valve is controlled by its own zone thermostat. End switches in each zone valve control the central circulator and the burner. On a call for heat, the valve is opened.
Where is air vented from a hydronic heating system?
top of the air-separating chamber as the flow enters the air scoop. Air accumulates at the top of the chamber and then upward into an automatic float-type air vent that ejects it from the system.
Where should the water pressure reducing valve located in a hot water hydronic system?
Where are pressure reducing valves located? They are located on the main water supply line.
How does a centrifugal circulating pump create flow in the hydronic system?
Force the hot water from the heat source through the piping to the heat transfer units and back to the boiler. These pumps use centrifugal force to circulate the water through the system. The rotating part of a pump that causes the centrifugal force to develop fluid flow and pressure difference.
Which is better one pipe or two pipe system?
The two pipe system is much more efficient than the single pipe system this system but still needs to be balanced properly at the end of the installation.
What is the advantage of a two pipe system?
The advantage of the two pipe system is that the water in the primary 'flow' pipe is only used in one radiator before being returned to the primary return pipe for reheating. This means it is now possible to achieve equal temperature in all radiators unlike the older single pipe system.
How does a 2 pipe heating system work?
A 2-pipe HVAC system is one that uses the same piping alternately for hot water heating and chilled water cooling, as opposed to a 4-pipe system that uses separate lines for hot and chilled water. Two-pipe originated 50 or 60 years ago as a cost-effective way to add air conditioning.
How Reverse Return Piping Works?
The concept of reverse return piping is simple. Just reverse the chilled water return pipe such that the first air handling unit (AHU) that receives the chilled water supply is the last AHU that provides the chilled water return.
Why Use Reverse Return Piping?
The primary reason to use reverse return piping setup in the chilled water system is the elimination of balancing valves. However, most people would not agree with that.
Disadvantages of Reverse Return Piping
Although reverse return piping may be attractive, there are a few critical disadvantages associated with it. Perhaps, they are the reason why many design engineers stop using the reverse return piping setup in the chilled water system.
Pressure Independent Balancing Control Valve (PIBCV)
Nowadays, new buildings fitted with the chilled water system are mostly using pressure-independent balancing control valves (PIBCVs) instead of the traditional manual balancing valves.
Reverse Return Piping in Hot Water System
New buildings are starting to use the heat pump hot water system rather than the traditional gas boiler system. In this case, the reverse return piping setup can significantly improve the performance of the hot water system.
Conclusion
If you ask me, I don’t think reverse return piping setup in the chilled water system will be considered in the future given that most buildings nowadays are proceeding to use variable flow and pressure independent balancing control valves (PIBCVs).
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Introduction
Example
- Figure 1 shows a simple representation of reverse return piping through two identical components, each in its own branch piping. It also shows a direct return piping layout for comparison. The reverse return arrangement has the potential to create approximately equal flow resistance through each branch and produce flows that are closer to equal. Th...
Operation
- However, just because two or more branches are piped in reverse return does not imply that each branch will operate at the same flow rate. Anything that creates a difference in the flow resistance of the supply or return piping between points where the branches connect will affect branch flow proportions, as would a different type or size of heat emitters in any of the crossovers. A differe…
Applications
- Reverse return piping subassemblies make sense when two or more identical devices require equal flow proportions and have a common supply. An example would be multiple solar thermal collectors, as shown in Figure 2.
Usage
- Notice that the supply pipe size decreases as the circuit moves away from the heat source while the return pipe size increases. Pipe size changes are made to keep flow velocity or head loss along the main approximately equal. For pipe sizes of 2 inches and smaller, flow velocity should generally not exceed 4 feet per second. For larger pipes, sizes should be selected to maintain th…
Advantages
- In systems with relatively short supply and return mains, the designer may elect to use the same pipe size for all mains. This decreases system head loss, which may, in turn, reduce circulator size. However, it also increases piping cost.
Cost
- Such an arrangement requires a third pipe sized to carry the full system flow from the dead end back to the mechanical room. While this arrangement still provides approximately equal supply water temperatures, and is closer to self-balancing relative to a direct return system, the third pipe running the entire length of the system adds substantial cost.
Effects
- Although reverse return systems are closer to self-balancing than direct return systems, they still cause a fixed-speed circulator to experience changes in differential pressure due to changes in flow as the valves controlling flow through the branches open, close, or modulate. If uncorrected, these changes in differential pressure can cause flow velocities in active branches to increase a…
Books
- This article was originally titled Reversing course in the November 2016 print edition of Plumbing & Mechanical.