Is there a minimum spacing size about underground piping system?

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I'm running capillary mat as ground heat exhcnager.

its pipes have 0.02m spacing.

then i run ep with this inputs

but it has severe error

but in another simulation, spacing is 0.1m, didn't have error.

so i want to know is there limitation at spacing between undergorund pipes.

if it have, how to modify limitation?

or, is there other method to simulate capillary mat?

looking forward to your answer.

thanks for reading.