When designing geothermal ground loops, this is an issue that a lot of people get hung up on. Because of the advantages of, we use parallel circuits in ground loops almost exclusively in our industry. Read more to hear why.
When loops are tied in series with one another, they will all see the full system flow rate (because there is only one flow path) and the pressure drop through the loops add together. Because there is only one flow path, the pump must overcome the pressure drop through each consecutive loop as the fluid travels through the system from the supply to the return line. The pump will be required to produce the combined pressure drop from the series loops at a shared flow rate.
In a parallel system, the flow through each loop will be the same. We add individual loop flows together to get the combined total system flow rate on the supply-return line back to the heat pump. The amount of pressure required to overcome friction losses through each loop (because they equally share the total system flow) will all be the same. The pump will be required to produce the combined flow rate from the parallel loops at a shared pressure.
In a series system, the total length of the well pipes would have to be figured in calculating head loss while in a parallel system only one loop needs to be calculated.
Parallel flow: Individual loop flow rate adds at a common pressure drop
Series flow: Individual loop pressure drop adds at a common flow rate
Advantages include: Single flow path and pipe size, higher thermal performance per foot of pipe, since a larger diameter pipe is required.
Disadvantages include: Larger water or antifreeze volume of larger pipe, higher price per foot of piping material, increased installed labor cost, limited length due to fluid pressure drop and pumping costs.
Fabricated from smaller diameter pipe, which is generally less costly, special care must be taken in flushing to get all the air out of the piping loop.
Advantages include: Lower cost pipe, less antifreeze required
Disadvantages include: Special attention to assure air removal, attention to balanced flow, within plus or minus 10%, in each parallel path is required
See the below illustration. It may help.