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Choosing a radiator for pc watercooling

Updated on May 13, 2011


What radiator to use with x components is probably the most common question seen when getting into water cooling. It seems to be a very mystifying process that many people just can't get a grasp around. Choosing the right parts the first time means less headache, and much less money spent.

Comments on message boards such as read the stickies, and go to this or that website, while good advice, can be very over whelming, and still not make the answer perfectly clear. I will be the first to admit that going to watercooling websites blind is one of the most overwhelming experiences in all of learning the art of water cooling.

There are numbers, and graphs, and phrases that make absolutely no sense and very little is done to prepare the new person for this amount of information overflow.

Understanding your wants.

The first step in the process involves deciding what you want out of your cooling system.

Do you want a pure performance oriented loop that gives you the lowest possible temps, or perhaps you want a quiet set up that still maintains decent temps? Perhaps you would like a balance of the two?

This is a very relevent question. By answering this question first, it allows you to narrow down the amount of radiators in the market place.

Where will you mount your radiator?

If you want to mount this radiator inside your case, will it fit? Will you need to mod your case? This is very important to keep in mind when choosing your components.

Maybe you can only fit a 120.2 radiator up top and you have decided you need a 120.3. You can always use a 120.2 and 120.1 to make up the full radiator.

Keep this in mind when you start looking for parts.

*NOTE* If you choose to break up your radiators, you will have to pay attention to pump selections. I will go over this in another hub. 120.1, 120.2, and 120.3 are descriptions of the radiator sizes. 120 stands for the size of the fan that will be mounted, and the second number is how many fans on that rad. So a 120.3 uses 3 120mm fans.

Understanding your heatload.

Your heatload is the next step in choosing the proper radiator, but what is this and how do you find this out?

Heatload is the amount of thermal energy you need to dissipate from your loop. Too little rad and you have a hot cpu or very high speed fans; too much and you wasted money.

The answer to finding your heatload depends on one simple question. Is your cpu overclocked?

If the answer is no, simply search cpu name + tdp in google. I.E. If you have an I7 920 you would search for "I7 920 tdp." The Intel site will be the first link and will tell you max tdp for that chip. In this chip's case it's 130 watts.

If the answer is yes it gets a little more complicated. By using the following formula, you can find the heatload of your overclocked chip.

((TDP*OCF)/SF)*(VC^2/VID^2) = Overclocked Heatload

TDP = Thermal Design Power (Point). Get this with google by using "<chip name> tdp."

OCF = Overclocked Frequency in MHZ. This is how far you have overclocked to. If you have a 4ghz overclock, this entry will be 4000

SF = Stock Frequency in MHZ. This is pretty self explanatory. 2.66GHZ will be represented as 2660.

VC^2 = V Core squared. This is the V Core you have set for your overclock.

VID^2 = The stock V core setting. You can find this with google by using "<chip name> stock vcore."

How to work this out.

Using an I7 920 overclocked to 4GHZ with a V Core setting of 1.23v we figure it as follows.


Start with the parenthesis of tdp*ocf.

130*4000 = 520,000

Then divide that by the sf

520,000/2660 = 195.4887218

Next square your vc/vid.

1.23*1.23 = 1.5129

1.27*1.27 = 1.6129

Then divide vc/vid

1.5129/1.6129 = 0.937999876

Now multiply the answer to the first parenthesis with the answer to the second parenthesis.

195.4887218*0.93799876 = 183.3683968 watts. This your heat load.

*NOTE* Since not all of the energy contained within a chip has to be removed you can multiply either of these with .8 to get a more realistic idea of your actual heatload. For the stock tdp it would equal 106 watts and the overclocked would be 147 watts.

Understanding Delta T

Delta T is a fairly simple concept. Delta simply means a change, and T stands for Temperature.

In water cooling, Delta T (Known as DT) is the difference in the temperature of a stabilized water loop, and the ambient air temperature.

For instance, if you have an ambient temperature of 23 degrees Celsius and a water loop temperature of 29 degrees Celsius, then you have a DT of 6 degrees.

When water cooling a CPU, you want to shoot for a DT of 5 - 10 degrees Celsius.

*NOTE* I will cover how to find a theoretical DT later in the article. This is extremely important when you start selecting parts.

Understanding FPI

For our purposes here we will say that you are looking for a quiet loop with good performance. You are cooling your overclocked i7 920 with 147 watts of power.

You look for your radiator, but with all the choices how do you choose one from another? Here we go over Fins Per Inch (FPI)

This is exactly what it sounds like. How many fins are in each inch of radiator.

Using the above image you count each fin both up and down.This particular radiator has 8 fins per inch. 8 fpi.

But what does that mean? Well the lower the fin count, the lower the fan speeds you need, but also lower total performance. The higher the fpi, the higher the fan speed needs to be, and subsequently better total performance. It should be noted that lower performance is not a bad thing here. It is simply lower than the top performers, but gives quieter fan speeds.

So for our above requirements for a quiet system, we know we can look for lower fpi radiators.

For this article we will call low noise fan speeds 600-1200 rpm (8-12 fpi). Medium noise speed 1300-1800 (12-17 fpi). High noise speed 1900+ (18+fpi).

Going to

We now have a better idea of the radiator we want to chose for our loop. We decided on a quiet set up that can remove 147 watts of cpu heat load. Time for our first trip to skinneelabs! is the premier website for obtaining non biased, real world results on the most popular water cooling products. Using this website you can pick the best products for your loop, and have full confidence that they will perform the way you need them to.

Going to Skinnees the first time can be very intimidating. There is a lot of information being presented, and very little explanation on how to use it. The great news? You, as a first timer looking, can throw most of the information away. Skinnee posts quite a bit of information to show how he came up with his analysis of a radiator.

There are several radiator reviews here, and the best thing I can tell you is to read the articles for the specs of the radiators looking for ones that fall in the fpi range we are looking for (8-12 FPI).

For the sake of simplicity I will use the classic Thermochill PA120.3. It has an FPI of 10 so it falls right in where we are wanting.

The first page is a general overview of the radiator in question. It tells us the fpi, the materials it's made from, etc.

*NOTE* Always make sure your radiator uses brass and/or copper in its water tubes. The fins can be aluminium, but the tubes need to be the copper/brass. The reason for this is that mixing the copper in your water block with tubes made of aluminium will result in galvanic corrosion of your water block. That's bad.

The second page is flow rate and pressure drop. The first table, you can mostly ignore it as you have no frame of reference for the numbers, however the wording of the article is very important. Using the graphs at the bottom of this page we see that the radiator in question has very low restriction, and that is a good thing.

The third page is about how he tests and what equipment he uses. You can skip most of this as it's useless to us. There is one thing on this page, though, that you need to know about.

C/W. How skinnee gets his c/w is, in all honesty, irrelevent to us. However what you can use C/W for is very relevent to us. It will let us know what kind of Delta T,(You read that article above, right?), you can expect with what speed fans.

How you use C/W is you take C/W and multiply it by your heat load, and this tells you how much hotter your water loop will be over ambient air, i.e. your DT!

The fourth page has loads of information on it, and it's this page that we can find out if our rad of choice will be able to handle our heatload (147 watts) with low speed fans (600-1200 rpm).

You only need two pieces of info from this graph.

1. The fan speed on the far left.

2. The c/w that is outlined in yellow on the far right.

For the 600 rpm fan speed, we see it has an average C/W of 0.04180077. Now we multiply that by our heatload.

147*0.04180077 = 6.14471319 DT. 

THIS IS GREAT! We know we are shooting for a DT between 5 and 10 degress Celsius. Looks like this is our rad.

If you continue down to the graph, you see that each of the lines represents a fan speed identified by the key at the top of the graph. 

The purple line is the 600 rpm fan choice.

The numbers going up the left side represent your target DT.

The numbers going along the bottom are your heatload in 25 watt increments.

Following the heat load number over to 150 and heading up you will see where each fan speed crosses that line. The 600 fan speed is crossing right after the 6 degree Delta T mark. Perfect.


Now you have found a radiator that will cool your overclocked cpu, and do it silently.

You can use this to find any combination of radiators whether it's a high performance, high fan speed set up, or a silent set up as seen here. It can also be used for any combination of waterblocks added to the loop. As long as you have a goal, and your heatload, you can head to skinnees and find the perfect radiator for your application.

Happy Water Cooling!


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    • profile image

      Multiman 5 years ago

      Awesome article very detailed and useful knowledge on teh subject. Voted up.

    • profile image

      Matt 4 years ago

      Just read this and for the first time I actually now understand how to calculate proper heat exchange figures rather than rely on peoples guesswork. Thanks!

      PS I'm assuming I can use the same calcs for the GPU's if I have them on the same loop as the CPU?

    • Daemonkin profile image

      Daemonkin 4 years ago from Ringgold, Ga

      Figuring GPUs is still guess work, to be honest.

      If they aren't overclocked then you can simply use their TDP to figure in their heatload, however the above formula does not calculate their heatload properly when overclocked.

      I do know that a 120.2 radiator will sufficiently cool a GPU, and a 120.3 will handle a crossfire or sli set up. I have never heard of someone needing more than that unless they have more cards.

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