Introduction to Water Source Heat Pump Systems Part 4: Piping Design
/By Chad Edmondson
One of the most appealing aspects of WSHPs in commercial applications is the overall simplification of piping. After all, you can eliminate the need for chilled, hot and condenser water piping. So instead of choosing between a 2- or 4-pipe system, you have the option of a 1- or 2-pipe system. But what does that mean when it comes to the actual piping layout? What are the differences and when might you apply one over the other?
Here are two side-by-side images of a 1-pipe and a 2-pipe WSHP system:
Right away you should notice a couple obvious differences between the two systems.
First, each WSHP in the 1-pipe system has its own dedicated circulator pump to pump water from the loop and through the unit. These circulator pumps operate independently of the central loop pump, providing the exact flow and head needed to satisfy the requirements of each respective zone, thus eliminating the need for individual control and balancing valves. This arrangement can save pump energy because (1) the circulator pump can be variable speed and turned off during periods of no demand and (2) the system loop pump does not have to overcome the pressure drop of multiple control and balancing valves.
In a 1-pipe system, each pump is piped in a primary/secondary configuration but without the need for any special valves or fittings. When the circulator is turned off there is no flow thru the unit. The circulator is controlled from the individual WSHP unit and is internally mounted at the factory, saving a lot of time and material at installation.
In addition to the individual circulators, notice also that the return water from each unit in the 1-pipe system goes directly back into the supply loop. In other words, there is no separate return loop. Each heat pump sees a different supply side temperature, as the water will become increasingly warmer (or colder) as it reaches the last heat pump. This must be considered when sizing individual WSHPs in a 1-pipe system. You may need multiple single pipe loops to control the WSHPs supply temperature depending on the building layout.
Unlike the 1-pipe system, 2-pipe systems have separate return and supply piping. This way all the WSHPs see the same supply water temperature since return water doesn’t mix with supply. No individual circulator pumps are needed because the entire system is pumped by the central loop pump. Each WSHP will need to be balanced to make sure it receives the correct amount of flow.
To meet ASHRAE 90.1-2013 standard in a 2-pipe system, a two-position automatic valve must be installed on each unit and interlocked to shut off flow when the compressor is off. We recommend these valves be mounted, wired and set by the WSHP manufacturer to protect the compressors.
2-pipe systems can be either direct or reverse return.
1-Pipe Vs. 2-Pipe
While 1-pipe WSHP systems are not well-suited for high rise buildings, they can be advantageous in many school, office and governmental building layouts and should considered as a way to reduce installation cost. However, keep in mind that that a 1-pipe system will not reap the full benefit of system diversity. Plus, all of the units in a 1-pipe system must be sized based on the assumption that all are either in heating or cooling mode at the same time. In these cases, the performance of the units near the end of the loop can be impacted because of higher/lower water temperatures. Also, morning start-up in a system with nighttime setback can be tricky with a 1-pipe system.
2-pipe systems work well any type of building layout. They are simple to design and provide uniformly consistent entering water temperature to all units at all times. A 2-pipe system also provides efficient energy recovery, which can be especially advantageous in buildings with an interior core that requires year-round cooling and a perimeter zone where heating is required in the winter.
Next up we will discuss geothermal heat pump systems which is one of the most energy efficient HVAC systems on the planet!
