Water-cooling Reservoir Theory and Construction Guide

Excellent How-To – RoboTech (Lee Garbutt)


I have been experimenting with water-cooled PCs for several years now and incorporate water reservoirs into most of the systems I build. Reservoirs are optional – you don’t have to use one (and many people don’t). But for me, the advantages of having a reservoir usually outweigh the disadvantages of not having one. The purpose of this guide is to discuss a little reservoir theory and illustrate a simple technique for constructing your own inline reservoir.


The reservoirs used in PC water-cooling systems come in many different sizes, shapes and styles. Some reservoirs contain a submerged pump while others do not. I personally prefer the in-line style of reservoir, which is the type we will primarily be discussing in this guide. Here is a brief overview of some of the advantages and disadvantages of using a reservoir.


  • Provides a reserve volume of water to make up for losses that can occur over time
  • Allow for expansion and contraction of the water volume (minimal)
  • Can serve as an air-trap to help remove air bubbles from the system
  • Provides an easily accessed fill point
  • Can help reduce NPSH (Net Positive Suction Head) of the pump
  • Submerged pump may run quieter and cooler


  • Bulky – reservoirs may be hard to locate in small systems
  • Potential source of flow restriction (if not designed and/or installed properly)
  • Potential source of leaks (if not manufactured properly)
  • Cost – they can be relatively expensive
  • Submerged pump transfers motor heat to water

Many of the online water-cooling specialty sites sell commercial reservoirs. They range from small, flimsy plastic containers to large, machined aluminum units. Some are designed to mount inside a spare 5.25″ drive bay while others are intended to mount outside the case. I frequently find these commercial offerings fall short of my expectations for several reasons.

  • Too small
  • Sometimes poorly constructed (prone to leaks)
  • Restrictive to flow
  • Too expensive
  • Or, some combination of the above

There are many ways to build your own reservoir. Some people use Tupperware containers, watertight electrical junction boxes or assorted PVC pipefittings. Others like to build their reservoirs from scratch using Plexiglas (acrylic) or Lexan (polycarbonate). One of my favorite techniques is to use a plastic bottle with clear, flat sides and install brass pipe barb fittings as connection points.



RoboTech (Lee Garbutt)

Reservoir Theory

Let’s take a closer look at some of the reasons you might want to include a reservoir into you water-cooling system. The two most obvious advantages are providing an easy access point to fill or add water to the system and helping get rid of trapped air bubbles (especially at startup).

Most reservoirs come with a screw cap or plug. It is much easier to remove this cap and add a little water than it is to disconnect a line from a fitting somewhere. Some people like to use a combination of inline valves for filling and draining their system, but multiple valves can decrease flow which will ultimately reduce performance.

During the initial fill and startup of a system, there will always be some air bubbles circulating around in the water. These air bubbles must be removed for best performance (water transports heat more efficiently than air).

After the main components have been rocked and tilted to dislodge pockets of trapped air, adding a little surfactant to the water (Water Wetter, Purple Ice, etc.) can help remove microscopic air bubbles still trapped on the internal walls. Caution: When using surfactants, more is not always better. In fact, using too much surfactant can cause foaming and trap air bubbles back into the system.

As the water enters the relatively large volume of the reservoir, the water velocity will decrease dramatically (Don’t confuse velocity with flow – the overall flow rate will remain constant in a closed system.) Air bubbles can become trapped in high velocity fluid streams and may only rise to the surface when the water velocity slows down enough to release them (This is one reason why fill-tubes are not very efficient at removing air bubbles.)

In general, the higher the overall system flow rate, the larger the reservoir volume needs to be to effectively remove air bubbles. Based on prior experience, I use the following rules-of-thumb to determine the minimum size for my inline reservoirs. In this case, larger is better but it still has to physically fit somewhere:

  • 250 ml (Flow rate ~ 50 GPH)
  • 500 ml (Flow rate ~ 100 GPH)
  • 1,000 ml (Flow rate ~ 150 GPH)

Remember, just because your pump is rated at 300 GPH free flow, you will not see anywhere near that kind of flow once the pump is placed into a real system (an example HERE). In addition to the total volume, the shape of the container and use of baffles and screens can have a large affect on a reservoir’s ability to remove trapped air bubbles.

PII Case Diagram

Two less obvious advantages of using a reservoir are that it will act as an expansion volume and provide a source of makeup water for the system.

At the temperatures most PC water-cooling systems operate, pressure changes caused by the expansion and contraction of the water volume will be negligible. A less well-known fact however, is that the internal water volume will decrease over time, even in systems with no obvious leaks. Water vapor can pass thru the walls of most plastic tubing. Silicone tubing is much more permeable to water vapor than vinyl.

In the winter time (low ambient humidity), I frequently have to add 20 ml to 30 ml of water to a rig that is using ½” ID Silicone tubing. Vinyl tubing (including Tygon and ClearFlex) is less permeable to water vapor so the water loss is much slower.



RoboTech (Lee Garbutt)

Reservoir Location and NPSH

One of the nice features of an inline reservoir is that you can mount it most anywhere you want in your system. Now some people think that having the reservoir at the highest point is best – after all, air bubbles will rise to highest point, right?

Well actually, no. Air bubbles will only freely rise to the highest point when there is no flow – system off. The normal flow in an operating system is usually enough to keep air bubbles trapped and circulating in the fluid stream. Only when the water velocity suddenly decreases in the reservoir do the tiny air bubbles have a chance to make it to the surface.

Now there may be nothing wrong with mounting the reservoir in the top of your case, but there are two other criteria I feel are more important in choosing the best location. The first is to pick a location that will use the shortest lengths of tubing and fewest numbers of fittings. This will help maximize flow and overall cooling performance.

My favorite place to locate an inline reservoir is right before the pump. As you can see in most of my examples, I mount one of the reservoir fittings at the same height as the pump suction fitting. This allows connecting the pump to the reservoir with a short piece of tubing. This also means that you can use an oversized piece of tubing for this one connection.

For example: Even though you use 1/2″ ID tubing throughout your system, the short piece that connects the pump suction to the reservoir can be 5/8″ ID or 3/4″ ID. The larger the tubing ID, the lower the water velocity and therefore the less flow resistance or pressure drop. This in turn minimizes the pumps NPSH requirements.


A pump operates by creating a high-pressure area at the discharge and a low-pressure area at the suction. This differential pressure causes fluid to flow through the external system. Centrifugal pumps require a relatively unrestricted flow of fluid into the suction to prevent the suction pressure (negative pressure – vacuum) from becoming too low, causing cavitation.

Based on each pumps design and the fluid operating temperature, a slight positive pressure must be maintained at the pumps suction to prevent cavitation. This is defined as the pumps NPSH (Net Positive Suction Head). Eliminating any flow restrictions near the pump suction helps insure more efficient pump operation.

If you are cooling multiple devices in parallel, you can eliminate Y connectors by routing all of the return lines directly to the reservoir.

For example: If you have TEC (Peltier) coolers on both your CPU and GPU and a waterblock on your Northbridge chip, you might want to run these three devices in parallel rather than in series. Normally, you would combine the three exit lines into one line before returning to the reservoir and pump. In this case you could install three return line fittings into the reservoir to help maximize flow.


Construction Details

OK, enough talk – let’s build a reservoir… 🙂 If you are a DIY type and have a few basic tools, you should have no trouble with the technique presented here. There are three main steps involved in making your own inline reservoir:

  • Choosing an appropriate bottle or container
  • Acquiring the fittings you will need
  • Assembly


RoboTech (Lee Garbutt)

Choosing an Appropriate Bottle or Container


The first thing to do is find a bottle or container that will suit your needs. There are a couple of requirements though, so not just any bottle will do. The container should have:

  • Flat sides – to allow for properly mounting the fittings
  • Wide-mouth cap – large enough to place the fitting nut inside
  • Plastic, but not too flimsy – semi-rigid walls are desirable
  • Large enough to be useful, small enough to fit
  • Clear sides – personal preference

The container should be plastic and not glass because we will need to drill holes in it. If you know someone who works in a hospital or science lab, ask him or her to save some old reagent bottles for you. They make great reservoirs!

Acquiring the Fittings (the hard way)


Believe it or not, this may be the hardest part of the whole operation. As you can see in the picture above, we need three pieces for each fitting to be installed:

  • Brass hose barb with male NPT fitting
  • Flat Stainless Steel (SS) washer
  • Brass nut (to fit NPT fitting)

You may get lucky at your local Home Depot finding the hose barbs but good luck finding the right size washer and nut!

Brass Hose Barb

I recommend using the largest fitting size you can on your reservoir. I always use at least one size larger than what I am using in the rest of my system.

For example: If I have a 3/8″ ID system, I will use 1/2″ OD fittings (which have a 3/8″ bore). If I am making a reservoir for a 1/2″ ID tubing system, I will use 5/8″ OD fittings (which have a 1/2″ bore). Most 1/2″ ID hoses can be forced over a 5/8″ OD fitting (especially if you soak the ends first in hot water).

I tend to be a little anal about flow restriction. Before using standard brass fittings I frequently bore them out to the largest ID I can and chamfer the ID at both ends… 🙂

SS flat washer

The purpose of the washer is to give the fitting stability and provide a large sealing surface. Finding a SS washer the right size can be tough. If you can’t find one, you can take a standard SS flat washer and open up the ID to the desired dimension by boring, drilling, filing or grinding.

Brass Nut (to fit NPT fitting)

The brass nut goes inside the bottle to hold the fitting tightly in place. What you want is a brass nut with a straight thread that matches the same pitch and diameter as the tapered thread on the NPT fitting.

I found some bulkhead fittings in my junk box that had straight thread nuts I could use with 1/4″ NPT and I have made my own nuts by cutting two 1/2″ slices off both ends of a 1/2″ NPT brass coupling. You will need to run a 1/2″ NPT tap into both ends first to open up the threads so the nuts will freely run up onto the barb fitting when you are done.


Acquiring the fittings (the easy way) McMaster.com

PII Case Diagram

If you have a hard time coming up with the fittings you need or don’t feel like going thru all the work to modify them, you can just jump online and order the parts from the Granddaddy of all hardware stores – McMaster-Carr. The disadvantage is cost – they only sell most items like this in packs. Here are the part numbers for my favorite combination (5/8″ OD barb x 1/2″ NPTM).

  • Brass hose nipple 5/8 x 1/2 NPTM – #5346K66: $12.27 pkg of 10
  • Shim washer, 18-8 SS 7/8 x 1-3/8 – #98126A703: $7.38 pkg of 10
  • Brass lock nut 1/2 NPT – #50785K144: $1.73 each


RoboTech (Lee Garbutt)


Once you have a container (it doesn’t hurt to have a spare) and the proper fittings, the rest is relatively easy. We will be drilling holes in the container and then mounting the barb fittings into place. You will need the following tools and supplies:

  • Electric drill and step-drill bit (Unibit)
  • Silicone RTV sealant (clear)
  • Marking pen
  • Small wooden applicator

1. Layout the Holes

Layout and mark the centerline position on the container for each of the fitting locations.

If you will be mounting the reservoir next to the pump, then set the reservoir outlet fitting at the same height as the pump’s inlet fitting. The return line fittings should be located well above the outlet fitting but below the water surface level. You can place fittings on any side of the container you want. Look at the layout of tubing in your computer and see what orientation will work best.


2. Drill the Holes

Drill a hole in the container for each fitting. A step-drill bit works great for drilling holes in thin wall plastics. Make the hole approximately 1/16″ larger than the NPT fitting OD. I like to mark the right diameter step with a felt tip pen before drilling.


After all the holes are drilled, de-burr any rough edges and clean out all the plastic chips from inside the bottle.

3. Install the Nuts

I have found from experience that it can be very difficult (if not impossible) to hold the nuts in position inside the container while screwing-in the barb fittings. The best way I have found to accomplish this is to first glue the nuts into position:

  • A Drop a nut into the bottle and position it so the large end of the tapered thread is facing up, right below the hole. Using a wooden applicator, dab a little Silicone RTV around the inside surface of the hole, forming a small circular bead:

  • B I then insert a brass tube or wood dowel thru the hole and into the center of the nut. This will act as a guide to position the nut over the hole when the bottle is turned over:

  • C Turn the bottle over so the nut falls down on top of the hole and bead of sealant. Carefully center the nut over the hole. Set aside and let the Silicone sealant cure.


  • D Repeat steps A through C for each fitting.


RoboTech (Lee Garbutt)

4. Install the Fittings

Before installing each fitting, carefully run a tap thru each nut to remove any sealant that may have gotten into the threads. This will help insure that when you install the fitting it will screw-in without jamming. Next, run a narrow bead of Silicone RTV sealant around the threads and base of the flat washer:


Now carefully screw a fitting into each hole of the reservoir:


The fittings only need to be snug, not overly tight. Wipe off any excess sealant from around each fitting. Set aside and let cure for 24 hours.

Adding Accessories

You should now have your very own custom-made in-line reservoir! But why stop now? Let’s finish it off by adding a thermometer and some LED lights.


Any water-cooling afficionado is interested in knowing what the system water temperature. Some people like to place a small thermistor or thermocouple in their reservoir to monitor water temperatures. I frequently mount a lab thermometer in mine.

  • Select a thermometer of your choice
  • Drill a suitable hole in the reservoir cap
  • Insert thermometer (the hole can be sealed with a dab of Silicone RTV)


If you are using a DigitalDoc5 or CompuNurse type digital thermometer, you can easily apply one of the flat sensors to the outside of the bottle with some sticky-foam tape – the probe doesn’t have to be in the water.

The temperature will equilibrate and read just fine thru the plastic wall. The foam tape serves to both hold the sensor in place against the bottle wall and insulates it from the ambient air. Clean the side of the reservoir, position the sensor where you want it, and then cover with a piece of sticky-foam tape:


LED Lighting

Adding LED lights is a very popular mod and works great with clear reservoirs. The color, quantity and placement of the LEDs is up to you. I mount LEDs in a little stand or base I make out of a 3/4″ thick piece of aluminum, Delrin or wood. Make the base the same size as your reservoir and then mill (or Dremel carve) a cavity into the under side.

Drill holes into the base to mount the LEDs so they will shine up into the reservoir. Secure them with a drop of super-glue or a dab of Silicone RTV sealant. In this example I used four LEDs, one in each corner:


Once the LEDs are mounted, they can be wired together in series. You can use either a 12 VDC 4-pin Molex style connector or a 3-pin fan connector to power the LEDs. You will also need a current limiting resistor in series with the LEDs. The value of this resistor can be found by adding all the LED forward voltages together and subtracting this number from 12. Next divide that value by 0.02 (typical LED current). For example:

  • Blue LED = 3.4 VDC
  • Red LED = 2.0 VDC
  • 12 V – (3.4 + 3.4 + 2.0 + 2.0) = 12 – 10.8 = 1.2 V
  • 1.2v / 0.02a = 60 ohms (1/4 watt)


OK, that’s it! I hope you have found this guide both interesting and useful. Be cool… 🙂

RoboTech (Lee Garbutt)

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