Too much of a good thing!

GCHP system designers and contractors working in colder climates need to protect the heat transfer fluid from freezing if the temperature of the GHX is expected to drop below about 40°F (5°C). Adding antifreeze to water increases the density and viscosity of the fluid and changes the flow characteristics of the fluid. As the fluid is cooled by the heat pumps the density and viscosity increase and can result in temporary failure of the system. 

Three fluids are commonly added to water: methanol, ethanol or propylene glycol. Other fluids have been used but are banned in many jurisdictions. The graphs show the impact of temperature, flow rate on the Reynolds numbers and the impact of concentration of methanol, ethanol and propylene glycol on the Reynolds numbers flowing through typical GHX circuit piping.

The two photos graphically illustrate the difference between laminar flow and transitional to turbulent flow. Heat transfer is greatly reduced when the flow is laminar: i.e. Reynolds number < 2,500). 

An example of what can happen on a real project. Heat transfer fluid comprised of approximately 50% ethanol and water was installed in a system. The company that installed the system was called several times after a lengthy cold period because the heat pump would not operate. Every time the technician arrived at the home to fix the problem, he found nothing wrong. The heat pump was operating perfectly and the temperature of the fluid circulating through the ground was well above the minimum required for the heat pump. Yet a few hours later the heat pump quit operating again. 

The heat pump would operate again for a few hours after the technician left. As the fluid temperature dropped, the fluid became more dense and viscous...and the flow became laminar again heat transfer from the ground was reduced, the temperature of the fluid dropped and the heat pump quit operating again. Once the problem was identified, the concentration of the antifreeze was lowered, the flow did not become laminar again and heat transfer improved.