Flushing the solar thermal systems:  Using glycol based solutions in solar thermal systems it is beneficiary for heat transfer. HTF Glycol based is able to carry more temperature load than pure water. Usually at temperature range of 90F to 212F, glycol solutions behave optimal offering moderate low vapour pressure, high thermal stability, low viscosity and in this way a laminar flow which in turn require pumps with a lower power and lower velocity making the system energetic viable. In some of this cases DC pumps are very suitable being able to carry the heat transfer load, in the mean time they are connected to a solar photovoltaic panel mounted close to solar thermal panels. In this way the thermal system works using a natural thermostat(solar photovoltaic panel). In such a situation we have to make sure that the PV module and the dc pump, which is a low head pump, does not air lock the system. When the HTF temperature rise over 212F (90C) the glycol based fluid start to deteriorate and there for it is more as necessary to use propylene glycol rated for 325F thermal capacity for one year duration. A very important problem with HTF Glycol based solutions is that they are toxic and the system has to be leak proof tested.

Another situation refers to the cold period time of the year when ground temperatures can go as low as 4oC and even lower and some of the glycol traces in the solar thermal panels are frozen and are becoming slushy and there for by system start even if the collector temperature is in the range 75oF to 90oF we will have a deterioration of the HTF carrier inside system. Depending of the microclimate where solar thermo panels are installed it may occur an excessive soiling for those panels and that may lower the HTF quality and  in turn we will have a full stop of the fluid inside system's transfer loop causing the temperature to raise temporary over 212F. Through repetition HTF carrier will deteriorate.

In all this situations a complete system draining and system flushing before refill is highly recommended.

To flush the pipe work and solar thermal collectors we have to use a passive solution which will not react with copper and has a ph between 7.2 and 10. Most of the time this type of solutions contains corrosion inhibitors to treat the copper pipe work and that it means glycol in excess of 50% concentration to water. We can not use any other mix or pure water for the flushing fluid and that based on the fact that it is not possible to perform a full drain of the solar collector and to totally remove flushing fluid. When we drain the system through opening the lower drain valve we will have a real scalding danger. This situation has to be very closely monitored.

Charging a solar thermal system.

Before charging occurs: we may consider to pressurize the system close to the system working conditions if the system it is not a close loop system the pressure level is in the range 50 to 60 psi. Using leak detection fluids we can spot problems in the system and in this way we aim to keep the fluid lost in range of operational performance year

Charging the system Time of charging is a real issue. We have to charge the system during non functional hours and in this way to prevent scalding or in mode accidental to induce pump cavitations. Therefore a full automated set with a transfer capacity of 5gpm will fill the system with 50 gallons of HTF in an optimal time, Some installer are recommending to cover the solar thermal panels when we pressurize the system before filling them and to keep the cover on top even after during the time the circulation pump is discharging the air bubbles.

Pressurizing the system has to be done in 5psi to the system pressure.  Because of the system venting during the filling it is a good measure to double the pressure value, 30 PSI plus 1psi for each 2.3 feet pump head is an optimal value.  All together the system pressure has not to exceed in this situation 65 PSI the filling has to be performed slowly and we have to make sure that air is vented. Actually we will go through a repetitive cycle where the system is refilled vented and again refilled and pressurized.

Post charging system checks: When we monitor the system pressure we have to consider the outside temperature. A higher external temperature will induce a higher HTF transfer loop pressure. It is recommended not to have more as 10psi dropped of pressure from the initial value in the following 10 days. If the circulation pumps are DC pumps  for the first days we have to monitor the pumps close. A higher amount of air bubble generated by heat and a poor system venting may collapse the pumps; they tend to work with a lower head. Therefore by dimensioning the system or by retrofitting the system with a dc pump we have to use 45o fittings to smoothes the pump head and eliminate vapours and air locks.

At the very end the installation has to be labelled to prevent accidental opening of the drain valves and the service dates has to be visible marked.

The figure in our representation shows the main flushing and filling valves uncapped.