Heat carrier fluid
This panel selects the fluid that will circulate in the network. The density ρ and the dynamic viscosity µ enter every pressure loss the solver computes, so the fluid chosen here sets the friction of every pipe, the operating point of every pump and the pumping energy of the system. The current version assumes one and the same fluid for every network of the drawing, evaluated at a different temperature for each — see Where the temperature comes from below.
Heat carrier fluid - Five fluids are available: pure water, ethylene glycol, propylene glycol, methyl alcohol and ethyl alcohol. Their properties come from the Melinder correlations (Melinder, 2010), which are fitted from 0 to 60 % mass fraction and, in temperature, from the freezing point of the solution up to 100 °C for the two glycols and 40 °C for the two alcohols. The way those correlations were built is summarised in the author's thesis (Melinder, 2007), which shows the comparison against the source data for ethyl alcohol and refers to earlier work for the glycols. GLN evaluates them inside a narrower window that stays within that domain: from −15 °C, or the freezing point of the mixture if it is higher, up to 50 °C for water and the glycols and 40 °C for the alcohols, and from 0 to 60 % in concentration. A 20 % propylene glycol solution, for instance, freezes near −7 °C, and a network declared colder is evaluated at −7 °C. A network configured outside the window is still solved, but with the properties taken at its edge rather than with an extrapolation that could return a non-physical density or viscosity. When that happens, the fluid name reported in the console shows both the configured temperature and the temperature at which the correlation was actually evaluated. Changing the fluid or its concentration clears the results on screen and solves the network again with the new fluid.
Antifreeze mass concentration - The mass concentration of antifreeze, in kilograms of antifreeze per kilogram of solution, reported as a percentage, assuming pure water as the solvent.
Where the temperature comes from - The fluid type and its concentration belong to the network, and the temperature at which they are evaluated belongs to each independent network inside it, set in the Network section of the Hydraulic network panel. A drawing holding a primary loop at 30 °C and a ground loop at 0 °C is therefore solved with two different densities and viscosities.
Summary & charts: the fluid, network by network
The table beside the selector reports the freezing point of the mixture, and then one column per network: the temperature that network is solved at, and the viscosity, thermal conductivity, density and specific heat the solver reads at that temperature (read from the same Melinder evaluation the hydraulic solver itself calls). Each column heading carries the network's number, the same as in the Network section of the hydraulic panel and in the console.
The charts below plot each property against temperature over the range the correlations cover; they show how steeply the viscosity of a glycol solution climbs as the loop cools. A vertical line marks the temperature of each network.
GLN in practice: what viscosity costs a ground loop
Do not over-concentrate the fluid. The freezing point should sit a few degrees below the coldest temperature the loop will see, and no lower. More antifreeze adds no safety: it thickens the fluid, raises the pump head, lowers the Reynolds number and pushes a marginal leg toward laminar flow. Take the concentration from the minimum leaving fluid temperature GHE Analysis predicts, then solve at that temperature. The choice between propylene glycol, ethylene glycol and the alcohols is rarely hydraulic alone — toxicity, code, compatibility and disposal usually weigh as much as friction.
A common field mistake. When a heat pump trips on high pressure, the fix often proposed is more antifreeze. It raises the viscosity, reduces the flow the installed circulator can deliver, may drop a marginal leg into laminar flow and moves the return temperature further from the ground temperature, which worsens the fault. The antifreeze is also usually added without measuring what the loop already holds, which may be at or above the design concentration. A field hydrometer answers that in a minute: draw a sample, read the concentration and compare it with the value set in this panel.