When your system is installed and operating, you can monitor its performance through the storage tank temperature and the heat exchanger’s inlet and outlet temperature difference. Tiny, slow leaks are tell-tale signs of poor performance and can be monitored on the pressure gauge. When the temperature of the circulation fluid changes, the system pressure rises and falls but the average pressure at rest should be the same throughout.
In hot weather, there will be more hot water produced than required; the electronic control module turns off the circulation pump to prevent any further temperature rise that would exceed the preset limit. Circulation fluid already in the solar collectors will keep absorbing energy to increase their own operating temperature. Note that drain-back systems do not experience this condition.
When the circulation pump stops operating, "collector stagnation" begins. If this condition persists, the propylene glycol solution breaks up, depositing a sticky sludge in the plumbing system. Manufacturers have come up with a few preventive measures for this problem: for example, EnerWorks installs a smart metal thermal actuator underneath the solar collector’s flat plates. When the system functions normally, heat will exit from the solar collector through the circulating fluid with the actuator closed. During stagnation, the excess heat inside the solar collector system opens up the top and bottom actuators to allow fresh air in to ventilate and cool the whole system.
To avoid "wasted" heat when the storage tank is full, a bypass valve can be installed to direct the energy to other uses in the home. For example, the heated water can flow into a hot water radiator baseboard as auxiliary space heating or another heat exchanger can be used in a hot tub or swimming pool. Whatever the application, the heating contractor needs to install the appropriate valve and control components to ensure that domestic hot water is heated before any auxiliary loads.
The antifreeze circulation fluid should be checked bi-annually. You can test this fluid in two ways: firstly, you can send samples to Dow Chemical Company for free annual analysis. They return a thorough summary report inclusive of a results table with a cover letter that explains the sample’s condition and outlines any maintenance needed. (You can contact Dow Chemicals at www.dow.com/heattrans/index.htm.)
Secondly, you can use ACUSTRIP® Company's ( www.acustrip.com) simplified fluid test kit on the propylene glycol antifreeze solution. Just place some solution in a clean glass container and immerse a test strip then compare its color to the color chart. The percentage of propylene glycol concentration can be determined by the pH (acidity) level. The ideal mixture concentration depends on your location, and is often around 50% propylene glycol and 50% distilled or de-mineralized water, at which point the solution’s pH is ideal in the 8-10 range. An overly acidic or overly alkaline level requires partial draining of the system to add water or glycol as appropriate.