viscosity for any oil gets higher when the temp gets colder but there are systems running in the Canadian arctic with 50% glycol. I have done a number of systems in Fort Francis, Ontario with 40% and AFAIK there has not been any issues.
How deep can you bury the piping and can you place a 2" sheet of SM over the tubing to allow the ground heat to come up closer to the tubing? It's just one way to help it out.
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