I was in the field recently witnessing flow and pressure drop testing of an installed ground-coupled heat exchanger (aka a geothermal ground loop or GHX). As the contractor was preparing for the work at hand, he commented to me that he had not done any of these tests before. Although this contractor has done a lot of GHX installs and does quality work, I was not surprised by this statement. Skipping this test is not uncommon but doing so is a mistake that can lead to a system that underperforms or even fails at some point in the future.

Common Practice?

Doing a flow and pressure drop test should be automatic on every GHX install and these tests should be done without fail. If they are not specified or planned for, installers themselves should insist on doing. The test is straightforward and it gives certainty that you have full, unobstructed flow through your entire loop or circuit. It is also a required install step spelled out in Standards from the International Ground Source Heat Pump Association (IGSHPA) and included in most project specifications.

Tests Process

Here is how a flow and pressure drop test works. After a multi-bore, vertical GHX circuit is completed, the circuit is filled with water and purged of all air. The water in the loop is then circulated at a specific flow rate and the difference between the entering and leaving pressures is measured. This is the pressure drop across the circuit. The measured pressure drop is compared to a value calculated based on the as-built circuit details. If the value is not within 10%, then there is something wrong that needs to be corrected.

Potential Issues

The most common issue that is identified in doing this test is that you do not have flow to one or more vertical bores. This is relatively easy to spot because the flow rate on the remaining loops goes up and results in a predictable increase in the differential pressure of the circuit. Frequently, the underlying cause is that air is still trapped in the GHX. A bit more purging will solve that problem. However, sometimes there is something else blocking flow such as an improperly installed fitting or debris that needs to be addressed. If not addressed, these issues can lead to long-term performance issues for your system.

Potential Impact

So, what if you skip this test? More than one installer has said to me, “If I get the required flow rate (gpm) through my GHX, then I assume it is good.” This assumption misses two important points. First, in most cases, you can get the specified flow rate if your pump has sufficient head even if your flow is restricted somewhere in the GHX. If you do not check the pressure drop, you will not know if the target flow is happening at the expected system pressure drop. If that pressure drop is higher than its design, this means your system requires increased pump energy to operate at the design flow rate. Over the lifetime of your system that can add up to a lot of extra pump energy.

Second, if flow is blocked to one or more of your boreholes, then you have reduced your system’s capacity. Even if there is extra capacity, your system’s loop temperatures will run warmer or cooler than necessary resulting in less efficient operations than would be realized if your entire GHX were operating as designed. Furthermore, this means the money spent drilling and installing the flow-blocked borehole was wasted.

A number of years ago, we got a call about a system where the heat pumps were shutting off due to high water temperatures coming from the GHX. We recommended that they reach out to a skilled installer to have them check the loop based on our test scripts. That installer ran flow and pressure drop tests and quickly determined that flow to several of the system’s boreholes were blocked. The issues were corrected by excavating the borefield and replacing several fittings that had been improperly installed. The system has been working fine ever since. However, this situation could have been avoided if the original install team had just done the flow and pressure drop testing when they installed the GHX.

Conclusion
So, whether you are an installer, an inspector, a general contractor, a commissioning provider, and especially if you are a project owner, you should be insisting that a flow and pressure drop test is done. The little bit of extra time it takes could save you from big issues down the road.