In a typical geothermal water-source heat pump (WSHP) system, the water piping is set up in what we typically call a two-pipe reverse-return configuration. One pipe ‘begins’ at the point where the well-field enters the building, and acts as ‘supply’ from the ground to the heat pumps in the building. The other pipe ‘begins’ at the point where the supply ends, and brings return water back to the well-field. This configuration includes redundant pumps on the return side that circulate the water throughout the system piping. These pumps are oftentimes controlled by a variable frequency drive (VFD) and pump speed is reduced as the system calls for less water, based on feedback from motorized shutoff valves located at each heat pump. These valves are interlocked to only open when the heat pumps are operating (i.e. when the unit compressor is running).

A one-pipe design is almost a retro technology in terms of application. The idea was common for fan coil heating systems as many as 60 years ago, but has recently fallen out of fashion (if such a word can be used in the HVAC design world). Implementation of this design strategy in a WSHP system theoretically offers energy through operational costs as well as upfront installation costs (which can average $1.00/sf) . The latter is achieved through reduced equipment, controls and installation costs while energy savings come into play via decreased fluid circulation. And though historically these system types have been assessed as less efficient overall than their two-pipe counterparts, studies with modern high efficiency heat pumps have shown to result in much less of an efficiency gap. The one-pipe system also has the added benefit of increased heat transfer to the ground during off-peak load condition from an overall turbulent flow condition.

As mentioned previously, one-pipe system operation can result in decreased fluid circulation energy since flow to each individual heat pump is delivered through a dedicated pump. These savings are realized through the near constant pressure of the main piping loop, compared to a two-pipe system with a VFD that could result in pressure variations and require the system to make up for it with flow regulating valves (which add pressure losses). The one-pipe system theoretically eliminates this over-pressure potential since the main loop pumps are controlled via feedback from the main internal loop’s overall temperature.

Further savings in a one-pipe system can be realized through maintainability of a more simplistic system (without VFDs, pressure controllers, motorized isolation valves, and advanced programming).

So why doesn’t everyone utilize a one-pipe design strategy and reap the benefit of these savings? That’s not an easy question to answer, as with many mechanical designs there are many solutions for a similar problem. Each project may have different requirements and what might be applicable for one doesn’t necessarily translate well to another. It is the responsibility of the mechanical engineer to consult with the owner and determine what’s most appropriate for each job.

Related Advice:

• Geothermal Heat Pump Systems: Do New Tax Credits and Grants Make Them Affordable?



• Geothermal Heat Pumps: How Do They Work?



• Geothermal Research: What to Consider with Geothermal Heat Pump System?



• Online Geothermal Training: What are my Entry Level Training Options?



• Geothermal Tax Credit: Why Doesn't My System Qualify?



• Geothermal Tax Credits: Can I Replace my Heat Pump but Keep the Existing Loop?



• Fuel Cell vs. Solar and Geothermal: What is the Best for Residential Energy Savings?