When you are constructing your solar thermal installation, one of the things you will need to do is select an acceptable pipe size. This is a pipe size that is suitable not only for the requirements of the system but also for the area and complexity of the installation. The size of the pipe has a direct impact on the total volume of fluid that will be contained inside the system, which is in turn influenced by the needed flow. If you choose pipes that are too thin, you will experience an excessive amount of friction, which will result in a slower flow and the need for a larger pump to push the fluid around; if you choose pipes that are too large, you will experience an excessive amount of solar fluid running around the system, as well as higher installation costs. For a solar thermal installation with up to four collectors, the recommended pipe size is three-quarters of an inch (1.905 centimeters); for up to eight collectors, the recommended pipe size is one inch (2.54 centimeters); and for anything larger than that, there is no rule of thumb! 
In any event, the table located on this page can be utilized by anyone interested in determining the optimal pipe size for a certain flow rate (in gallons per minute) Sizes of pipes that should not be exceeded as recommended for solar thermal water heating Do not go the route of employing smaller pipes to save money; doing so will need you to run a pump with a larger capacity to force fluid through the congested area, which will increase overall expenses. It is preferable to have pipes that are somewhat larger than necessary rather than ones that are inadequately sized. The ability to expand the system at a later time is another benefit of using larger pipes.
- Fluid Volume Calculations
You will be able to calculate the amount of fluid that will be present in each component of the system at any given moment once you have determined the size of the pipes that will be used. 
You will have to determine how much liquid you require based on whether you opt for a drainback installation or a closed-loop pressurized system to be installed in your system. In either scenario, you are going to need the table that is linked to this post since it provides accurate estimates of the amount of liquid that can be carried by a single foot of a variety of pipe lengths and diameters.
- Calculating solar fluid volume for typical pipe sizes
Drainback systems require a volume of fluid that is equivalent to the size of the drainback tank as well as the size of all the pipes that run vertically below it: The volume of the drainback tank plus (Length of pipes below the tank in feet x Volume contained per foot) Because the solar loop has to be full at all times, pressurized systems are more difficult to understand. You are going to want to begin with the collectors and determine how much liquid each one can contain (the manufacturer or the SRCC will know this). It would be a mistake to disregard any collectors that have a capacity of up to 2 gallons on their own. 
Next, you will need to measure all of the pipes and determine the total amount of fluid that they will contain for the whole solar loop; this should not be limited to the distance above or below a certain location. In the end, you are going to have to inspect each component that makes up the system. However, some of them, namely wraparound jacket-style heat exchangers, may store several gallons, so you shouldn't make any assumptions about anything! The vast majority of them won't retain very much at all and can be safely disregarded. (The length of the solar loop pipes in feet multiplied by the volume contained per foot) + (The volume contained by the solar collector model multiplied by the number of collectors) Plus (Volume contained by components)
- Expansion Tanks
One further critical component of the system, the expansion tank, should be taken into consideration in this context. One of these is required in every pressurized system to withstand the pressure fluctuations that are brought on by the seasonal shifts in temperature that occur throughout the year. 
Solar fluid isn't like water that's just been sitting there; under typical conditions, it goes through a massive temperature swing. Because of this, it's vital to have an expansion tank that's appropriately sized for your needs. If the pressure drops too low, you run the chance of air getting sucked into the system, which will prevent it from operating as smoothly as it should. If the pressure rises too high, you risk blowing the relief valve or possibly a joint. The makers of expansion tanks each have their recommendations for the optimal size of the tank, which are based on the volume of fluid already present in the system as well as the temperature range that is typical for your region. Make sure you review their advice and then make sure you properly follow their recommendations.