Glycol calculator

Find the glycol concentration needed for the lowest temperature in your system and how many litres of concentrate to add, for a new fill or for topping up an existing mixture.

How the calculator works

The calculator answers two questions that come up on every glycol system. First, what concentration protects the circuit at the lowest temperature it will see. Second, how much concentrate you need to reach that concentration, either in a new fill or in a system that already holds a weaker mixture.

The required freezing point is the lowest expected temperature minus a safety margin. For outdoor pipework and dry coolers the lowest temperature is the design outdoor minimum of the site, not the average winter. For a process circuit it is the lowest fluid temperature, which on the evaporator side can be several kelvin below the leaving water setpoint. A margin of 3 to 5 K covers measurement uncertainty and slow dilution by top-ups with water.

The formulas

For a new fill, the volume of concentrate is the system volume times the target concentration divided by the strength of the concentrate. For a system that already contains glycol, the tool assumes you drain part of the mixture and replace it with concentrate:

X = V x (c2 – c1) / (s – c1)

X is the volume to drain and replace, V the system volume, c1 the current and c2 the target concentration, and s the strength of the concentrate, all in per cent by volume. Example: 1,500 litres at 20 % ethylene glycol, target 35 %, pure concentrate: X = 1,500 x 15 / 80 = 281 litres.

Typical concentrations

Lowest temperature to protect	Ethylene glycol	Propylene glycol
-5 °C	about 22 %	about 25 %
-10 °C	about 29 %	about 33 %
-15 °C	about 35 %	about 39 %
-20 °C	about 41 %	about 44 %
-25 °C	about 45 %	about 48 %
-30 °C	about 49 %	about 51 %

The values include a margin of 5 K and are rounded up to the next full per cent. Exact values depend on the product: commercial heat transfer fluids contain inhibitors and sometimes water, so read the data sheet of the fluid you use.

Example

A dry cooler circuit in northern Germany has a design outdoor minimum of -16 °C. With a 5 K margin the fluid must not freeze above -21 °C. The calculator gives 36 % ethylene glycol or 40 % propylene glycol. If the system of 2,400 litres currently holds 30 % ethylene glycol, about 206 litres of the old mixture must be drained and replaced with pure concentrate. After circulating, a refractometer reading confirms the result; if it is still low, the system volume was larger than assumed.

Which glycol to choose

Ethylene glycol has better heat transfer and lower viscosity at the same protection, so it is the usual choice for industrial cooling and HVAC systems. Propylene glycol is less toxic and is used where a leak could reach food, drinking water or people, for example in food and beverage plants. Never mix the two types, and do not mix products with different inhibitor packages without checking compatibility with the supplier.

Practical advice

• Measure the concentration with a refractometer that has a scale for the glycol type you use. An ethylene glycol scale reads wrong on propylene glycol.
• Mix well: after adding concentrate, run the pumps for several hours before taking a sample, and take it from a point with good flow.
• Do not overdose. Every 10 % of extra glycol raises viscosity and pumping energy and lowers the capacity of the heat exchangers.
• Check pH and inhibitor reserve once a year. Old glycol becomes acidic and attacks steel and solder joints, even if the freezing point is still fine.
• Top up leaks with premixed fluid, not water, or the protection will drift away over the years.

The full freezing point data for every concentration are on the glycol dilution table page. If your glycol is old or contaminated, see our glycol testing and replacement service.

Frequently asked questions

How large should the safety margin be?

For most systems 3 to 5 K is enough. Use the upper end for outdoor pipework, dry coolers and systems that are often topped up with water, where the concentration drifts down over time.

Why does the tool raise low concentrations to 20 or 25 %?

Inhibitor packages are designed for a minimum concentration. Below it, corrosion protection becomes weak, and propylene glycol mixtures at low concentration can support bacterial growth. If you need less frost protection, it is still better to stay at the minimum or use inhibited water.

What if I do not know the system volume?

Add up the pipe volumes by diameter and length, the buffer tank, the chiller evaporator and the coils from the data sheets. As a check, measure the concentration before and after adding a known amount of concentrate and calculate the volume backwards.

Can I add concentrate without draining?

Only if there is room in the system, for example after a leak. In a full closed system the added volume must go somewhere, so drain first or use the expansion vessel capacity, which is usually small.