Whether you’re building your first custom water cooling loop or you’re a seasoned liquid cooling enthusiast, choosing the right type of water to pour into your reservoir is a critical step. Two of the most commonly recommended options are distilled water and deionized (DI) water. Both appear very similar — clear, colorless, and seemingly “pure” — but they have noteworthy differences that can impact the longevity and safety of your expensive PC components.
TL;DR (Too Long; Didn’t Read)
Distilled and deionized water are both commonly used for PC liquid cooling due to their high purity levels. Distilled water is generally safer and more reliable for long-term use in closed-loop cooling systems because it’s less reactive than deionized water. Deionized water, while extremely pure, can potentially be corrosive to certain metals due to its aggressive nature. For most users, distilled water is the more accessible and lower-risk option.
Understanding Water Purity
The main reason distilled and deionized water are favored for custom PC cooling loops is their ultra-low mineral content. Tap water contains minerals, ions, and impurities that can lead to scaling, corrosion, biological growth, and electrical conductivity — all of which can damage your hardware and degrade cooling efficiency.
Here’s a quick breakdown of how these two water types differ in terms of purification method:
- Distilled Water: Obtained by boiling water and condensing the steam back into liquid, leaving most contaminants behind.
- Deionized Water: Created by passing water through ion-exchange resins that remove ionized salts and minerals.
Distilled Water: The Standard Choice
Why is distilled water the go-to for PC liquid cooling? Simply put, it hits the sweet spot between availability, low conductivity, and non-corrosiveness. During distillation, biological materials, dissolved solids, and contaminants are left behind, resulting in highly purified water that’s inert to most metal surfaces.
Here are some advantages of using distilled water:
- Widely Available: You can buy it at almost any pharmacy, grocery store, or online — no special suppliers needed.
- Cost-Effective: It’s one of the most budget-friendly solutions for enthusiasts.
- Non-Aggressive: Compared to DI water, it is significantly less reactive, meaning it’s unlikely to strip ions or metals from your loop components.
However, distilled water isn’t 100% free of impurities — some volatile organics and trace minerals can make it through, but they generally don’t pose a threat if you regularly maintain your loop or use additives.
Deionized Water: Ultra-Pure, with Caveats
Deionized water takes purification a step further by specifically stripping out ionic substances — making the water ultra-pure and virtually non-conductive, at least initially. This makes it incredibly attractive for certain industrial and laboratory applications.
Benefits of using deionized water in PC cooling include:
- Exceptional Purity: Performs even better than distilled water in terms of initial conductivity and contaminant-free operation.
- Low Residue: Leaves behind almost no deposit if leaks or evaporation occur.
However, this ultra-purity can be a double-edged sword. Deionized water has a strong electrical potential to re-acquire ions. When introduced to metal parts in your cooling loop, it may actively pull ions from them, leading to corrosion or pitting, especially if you are using mixed metals like aluminum and copper. Over time, DI water can reduce the lifespan of pumps, water blocks, and reservoirs if not handled carefully.
Conductivity Concerns in a Loop
Both distilled and deionized water have very low electrical conductivity when first poured into the system — a trait that is highly desirable in case of a leak. However, once the loop has been running for a few weeks or months, both water types will accumulate ions from metals, tubing, and fittings. This natural ion accumulation increases the conductivity of the coolant over time, rendering both less “safe” from an electrical standpoint.
To combat this issue, many enthusiasts recommend using anti-corrosive and biocidal additives designed for PC loops. These help prolong the water’s effectiveness and keep conductivity from rising too quickly.
Should You Mix the Two?
Some builders have experimented with combining distilled and deionized water in an attempt to get the best of both worlds. However, this usually doesn’t offer any real advantages. Once deionized water is contaminated — even slightly — it behaves no differently from distilled water. Since DI water begins soaking up ions immediately upon exposure to air or metal, mixing it with distilled water only results in a temporary improvement in purity, at best.
Best Practices for Using Water in Your PC Loop
Regardless of whether you choose distilled or deionized water, following basic water cooling best practices is essential for performance and hardware longevity.
1. Stick to one metal type in your loop: Avoid mixing copper and aluminum, as this can accelerate corrosion when water impurities inevitably accumulate.
2. Add corrosion inhibitors and biocides: Commercial coolant additives or silver kills coils are recommended for keeping microbial growth and metal corrosion at bay.
3. Regular maintenance: Drain, flush, and refill your loop at least once every 6–12 months. This ensures minimal buildup and prevents long-term degradation of components.
4. Avoid pre-mixed coolants when purity is a priority: Many colored or cosmetic coolants use dyes and other reactive substances that may cause residue or clogs over time.
When to Use Distilled vs Deionized
Let’s summarize when to choose one over the other:
- Choose Distilled Water If: You’re looking for a safe, non-reactive, and widely available coolant base that will work reliably with typical loop care and maintenance.
- Choose Deionized Water If: You are building a short-term showcase system where maximum purity is critical and you’re using uncontaminated components with strict maintenance intervals.
Note: If you’re setting up a loop for the long haul, using DI water exposes you to higher long-term corrosion risks unless you have lab-grade materials or are prepared to maintain strict conditions.
Environmental and Storage Considerations
Both distilled and deionized water should be stored in clean, sealed containers away from sunlight. Once exposed to air, they begin to absorb CO2 and may start to re-ionize, meaning they will eventually lose their high purity. Avoid storing the water in containers that can leach plastics or metals into the water over time.
If you’re purchasing a large quantity for future use, make sure it’s properly sealed and labeled to avoid cross-contamination. Also, never use softened tap water or bottled drinking water in your loop — these often contain salts, minerals, or even sugar-based flavor additives that will gunk up your system fast.
Final Thoughts
Both distilled and deionized water have roles to play in PC liquid cooling. That said, for the vast majority of enthusiasts and even professionals, distilled water is the safer, more forgiving choice. It offers nearly the same level of purity as DI water without the added risks that come with its aggressive ion-hunting behavior. Paired with the right additives and loop maintenance, distilled water provides an optimal base for cool, clean, and consistent performance.
So the next time you’re filling up your custom loop, remember: the water may be clear, but the choice isn’t — and now, you’re equipped to make the right one!
