Turning on the Heat

Solar Thermal System Commissioning

Low natural gas prices and lack of public awareness of solar thermal technologies are the two most oft-cited barriers to significant expansion of the industry within the US. Unlike photovoltaics, which is relatively well known and certainly well publicized, solar thermal continues to be an esoteric segment of the solar technology spectrum.

While the activities of the solar thermal industry cannot impact the price of natural gas, the technology’s public profile can be greatly improved with the deployment of high-performance solar heating systems that meet customers’ expectations. Professional commissioning ensures long-lived, reliable and efficient installations. These systems, along with public outreach and education, are the keys to widespread solar thermal application awareness and acceptance.


Picture the solar thermal installer in a mechanical room, deep in the bowels of a massive commercial building. After months of hard work, a new solar heating system is nearly complete. The copper pipes have long since been roughed in, insulated and jacketed. Long banks of flat-plate collectors fill the roof, mounted on a properly engineered, secured and flashed racking system. In the mechanical room, the solar storage tanks are strapped in and full of water. The pumps, flow meters, sensors, and control and monitoring systems are wired up and plugged in. Commissioning, the official start-up of the new system, is the last major procedure to complete. Considering the overall scope of a project, commissioning does not take a significant amount of time, but doing it right is vital to the performance, longevity and value of the system.

As with solar thermal system design, project commissioning requires various levels of sophistication. For a residential application, it may be a simple procedure, while an industrial project may take several days. Residential domestic hot water systems, generally comprising four or fewer collectors, are the least complicated. Commercial and industrial systems that provide heat for applications such as process heating can include hundreds of collectors. Commissioning these systems is complex and time-consuming. Combi systems that provide heat for multiple loads and processes are typically the most difficult to commission. The wide variety of system types and elements means that a separate commissioning procedure must be developed for each application.

Some elements are common to commissioning each of these system types. In this article, we focus on the systems that we most often commission: commercial-scale active closed-loop glycol systems that provide domestic hot water to the facility. In the end, the goal is the same: to ensure that the solar thermal system remains in use for decades to come and performs optimally. After commissioning, the system is no longer a work in progress, but a work completed.


Failure to commission solar thermal systems properly can have drastic consequences including property damage, injuries and delays in payment. Failed commissionings also damage the reputation of the installer, the technology and the industry as a whole. Over the course of many years in the solar thermal business, professionals have encountered all manner of commissionings gone wrong. Before we tackle specific procedures and approaches, here are some realworld examples where proper system commissioning would have saved installers time and money, and, in one case, would have prevented an injury.

Leaving a system manually on. One case involved a large commercial system with an unpressurized storage tank. At start-up, the system seemed to be functioning properly. The pumps were on and appeared to be operating normally during the sunny day that the commissioning took place. In reality, one of the sensors had failed and the system control was programmed incorrectly, bypassing the high temperature setpoint limit. The result was that the pumps were locked in the On position. After the installer left for the week, the unpressurized storage tank began to overheat. It got so hot that the mechanical room turned into a steam room. When a maintenance man grabbed the door handle to enter the room later that week, he suffered second-degree burns to his hand. The room’s walls were destroyed and the entry door was warped from the heat.


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