Propylene Glycol + DI Water in Thermal Systems: The Practical Guide
Table of Contents
What you will learn
Learn how to mix propylene glycol and DI water for heat transfer systems. This guide covers concentration ratios, freeze vs. burst protection, inhibitor selection, material compatibility, and maintenance best practices for HVAC, chillers, and solar thermal applications.
💡 Frequently Asked Questions
Find quick answers to common questions about propylene glycol + di water in thermal systems: the practical guide.
Mix ratios, freeze protection, inhibitor selection, and maintenance best practices for hydronic loops, chillers, process cooling, and solar thermal applications.
Who This Guide Is For
If you work with closed-loop thermal systems—hydronic HVAC, chillers, process cooling, or solar thermal collectors—you've probably dealt with glycol at some point. This guide is for the people who need practical answers: facility engineers, HVAC technicians, mechanical contractors, and maintenance teams.
We'll cover the essentials: why inhibited propylene glycol matters, how DI water affects system longevity, what concentration to use for your climate, and how to maintain your loop for years of reliable service.
Before starting any glycol work, review the relevant Safety Data Sheets and ensure your approach aligns with equipment manufacturer guidelines.
Why "PG + Water" Isn't Enough
You can technically mix any propylene glycol with any water and call it a heat transfer fluid. But if you want that fluid to protect your system for years instead of months, the details matter.
Inhibited vs. Uninhibited Glycol
Propylene glycol on its own provides freeze protection—but it doesn't protect your system from corrosion. Over time, glycol solutions can become acidic, especially when exposed to oxygen or high temperatures. Without corrosion inhibitors, this creates problems:
- Metal degradation: Copper, aluminum, steel, and brass can all corrode in uninhibited glycol solutions
- Sludge formation: Corrosion byproducts create deposits that reduce flow and heat transfer
- Seal and gasket damage: Acidic conditions can accelerate degradation of elastomers
- Premature component failure: Pumps, heat exchangers, and valves don't last as long
For closed-loop thermal systems, always use inhibited propylene glycol formulated for HVAC/industrial heat transfer. Uninhibited or food-grade PG isn't designed to protect your equipment long-term.
Closed-Loop Best Practices
Even with inhibited glycol, system design matters. Air separators, proper venting, and expansion tanks help minimize oxygen ingress—the primary driver of glycol degradation and corrosion. A well-designed, well-maintained closed loop can run for 15–20+ years on the same glycol charge.
Why DI Water Matters (And When It's Worth It)
The water you use to dilute glycol concentrate has a real impact on system performance and longevity. Here's why deionized (DI) or distilled water is the professional choice:
Scale and Mineral Deposits
Tap water contains dissolved minerals—calcium, magnesium, silica—that can precipitate out as scale inside heat exchangers, pipes, and pump housings. Scale acts as an insulator, reducing heat transfer efficiency and increasing energy costs. DI water eliminates this risk.
Chlorides and Corrosion
Many municipal water supplies contain chlorides. Even at low concentrations, chlorides are aggressive toward stainless steel, copper, and other metals commonly found in thermal systems. Using DI water removes this variable from the equation.
Predictable Chemistry
When you start with DI water, you know exactly what's in your loop: glycol, inhibitors, and nothing else. This makes testing more reliable and troubleshooting easier. If something goes wrong, you've eliminated a major variable.
In some situations—emergency top-offs, budget constraints—tap water gets used. If so, at minimum use water with low hardness and low chloride content. But recognize you're accepting some risk. For critical systems or long service life expectations, DI is worth the investment.
PG Mix Ratios: A Practical Cheat Sheet
Propylene glycol concentration is always expressed as a percentage by volume. Higher concentrations provide more freeze protection—but also increase viscosity and reduce heat transfer efficiency. The goal is to use enough glycol for your climate without overdoing it.
Mild Climates & Indoor Systems
Suitable where minimum temperatures rarely drop below 15–20°F (-9 to -7°C). Common for indoor process loops, data centers, and southern U.S. applications.
Lower viscosityModerate Climates
The "middle ground" for most commercial HVAC in temperate zones. Provides reasonable freeze protection down to roughly 0 to -10°F (-18 to -23°C) depending on exact concentration.
Most commonCold Climates & Solar Thermal
For systems exposed to extreme cold or outdoor collectors where stagnation could occur. Provides protection well below -20°F (-29°C). Note: higher viscosity may require pump sizing adjustments.
Maximum protectionGoing above 50–60% PG concentration is counterproductive. Heat transfer efficiency drops significantly, viscosity increases (harder on pumps), and you get diminishing returns on freeze protection. More is not better. Match concentration to your actual minimum expected temperature with a reasonable safety margin.
Freeze Point vs. Burst Protection
This is one of the most misunderstood aspects of glycol specification. Let's clear it up.
Freeze Point
The freeze point is the temperature at which ice crystals begin to form in the solution. At this temperature, the fluid becomes slushy—it doesn't instantly turn into a solid block. Flow is impaired, but the system isn't necessarily damaged yet.
Burst Protection
Burst protection (sometimes called "freeze protection") is the temperature at which the slush becomes solid enough to potentially damage pipes, heat exchangers, or other components through expansion. This temperature is significantly lower than the freeze point—typically 10–20°F (5–10°C) below it.
| Spec Type | What It Means | Practical Implication |
|---|---|---|
| Freeze Point | Temperature where ice crystals begin forming | Flow degrades; system may struggle but probably won't break |
| Burst Protection | Temperature where physical damage risk begins | Actual equipment protection threshold |
When sizing glycol concentration, base your decision on the lowest temperature your system could realistically experience—including power outages, equipment failures, or overnight exposure. Add a safety margin, but don't go overboard.
Materials, Inhibitors, and Compatibility
Inhibited propylene glycol formulations are designed to protect the metals commonly found in thermal systems. But not all inhibitor packages are identical, and material compatibility still matters.
Common System Metals
- Copper and brass: Common in heat exchangers, fittings, and some piping. Quality inhibitors protect these well.
- Steel and iron: Found in boilers, some piping, and structural components. Ferrous metals are susceptible to rust without proper inhibition.
- Aluminum: Used in some heat exchangers and automotive applications. Requires specific inhibitor chemistry—verify compatibility.
- Stainless steel: Generally corrosion-resistant, but can be attacked by chlorides. Another reason to use DI water.
Why Inhibitors Matter
Corrosion inhibitors work by forming a protective film on metal surfaces or by neutralizing corrosive agents in the fluid. They're consumed over time—which is why testing inhibitor reserve is part of regular maintenance. When inhibitor levels drop too low, corrosion accelerates.
pH and Oxygen
Fresh inhibited PG solutions typically have a pH between 8.0 and 10.5 (slightly alkaline). Over time, oxidation and thermal stress can cause pH to drop. Acidic conditions (pH below 7) accelerate corrosion. Minimizing air contact through proper system design extends fluid life significantly.
If your system contains aluminum or you're unsure about material compatibility, check the glycol manufacturer's technical data sheet or contact the supplier. Not all inhibitor packages are formulated for aluminum.
Maintenance and Testing
Glycol isn't "fill and forget." Regular testing helps you catch problems before they become expensive failures.
What to Test
- Freeze point / concentration: Refractometer testing confirms your glycol percentage hasn't drifted due to leaks, evaporation, or top-offs
- pH: Should remain in the 8.0–10.5 range for most inhibited PG formulations. Dropping pH indicates degradation.
- Inhibitor reserve: Some test kits measure remaining corrosion inhibitor. Low reserve means it's time to boost or replace.
- Visual inspection: Fluid should be clear or slightly colored (depending on dye). Cloudiness, particulates, or dark discoloration indicate problems.
Testing Frequency
Annual testing is the minimum for most commercial systems. Critical systems, high-temperature applications, or systems with known issues may warrant more frequent checks—quarterly or even monthly.
Top-Off Best Practices
When adding fluid to compensate for losses, always match the existing concentration. Adding straight water dilutes your freeze protection and inhibitor levels. Adding straight concentrate increases viscosity and can throw off the balance. Pre-mix your top-off fluid to match what's already in the system.
When to Flush and Replace
If testing reveals significant pH drop, depleted inhibitors, or visible contamination, it's time for a flush and fresh charge. Typical service life for well-maintained inhibited PG is 3–5 years in standard applications, potentially longer in ideal conditions. Solar thermal and high-temperature systems may require more frequent replacement.
Document your test results, top-offs, and fluid changes. This history helps identify trends and makes troubleshooting easier. It's also useful documentation if warranty questions arise.
How to Choose the Right Glycol Blend
Follow these steps to select the right propylene glycol product and concentration for your thermal system:
Identify Your Minimum Design Temperature
Determine the coldest temperature your system could experience, including power outages, overnight exposure, and equipment failures. Add a reasonable safety margin (typically 10–15°F / 5–8°C below your expected minimum).
Check System Material Compatibility
Identify all metals in your loop—copper, steel, aluminum, brass, stainless. Verify the inhibitor package is compatible with your specific materials. Pay special attention if aluminum is present.
Decide: Concentrate or Premix
Concentrates offer flexibility and lower shipping costs but require careful mixing with DI water on-site. Premixed solutions save time and ensure correct ratios—ideal if you don't have access to DI water or mixing equipment.
Calculate Required Volume
Determine your system's total fluid volume. Account for piping, heat exchangers, expansion tanks, and any other components. Order enough for the initial fill plus some extra for top-offs.
Source Quality DI Water (If Using Concentrate)
If mixing from concentrate, secure a supply of deionized or distilled water. Don't substitute tap water for critical or long-life applications.
Review Documentation
Before filling, review the SDS and TDS for your selected glycol product. Confirm handling requirements, storage conditions, and any specific application guidance.
Plan for Ongoing Maintenance
Establish a testing schedule and keep matching fluid on hand for top-offs. Document your initial fill concentration so future maintenance stays consistent.
Choosing the Right Alliance Chemical Products
Alliance Chemical offers several options depending on whether you prefer to mix your own or use ready-to-go solutions:
Inhibited Propylene Glycol Concentrate
For buyers who want to custom-mix to a specific concentration. Dilute with DI water to achieve your target ratio. This option provides maximum flexibility and often the best economics for large-volume applications.
Premixed Inhibited Propylene Glycol
Ready-to-use solutions (such as 50/50 premix) that eliminate the mixing step. Ideal when DI water isn't readily available on-site or when you want to ensure consistent, factory-blended ratios.
Deionized Water
For diluting concentrates or topping off existing systems. Using DI water protects your investment by eliminating scale, chlorides, and mineral contamination.
Not sure which option fits your application? We're happy to help you choose.
Frequently Asked Questions
Ready to Source Your Glycol?
Browse our selection of inhibited propylene glycol, premixes, and deionized water—or reach out for help choosing the right product for your system.
Need documentation? [LINK: /pages/sds | View Safety Data Sheets]
