Multi Pump Setups – Parallel vs. Series

Posted: April 26, 2011 in Pumps

Testing Pumps In Series

Series pump configurations requires no special fittings, you simply connect the outlet of one pump with a section of tubing to the inlet of the second.  You can also place the two pumps pretty much anywhere within the loop as long as you’re careful about not running either pump dry when filling.  Folks that want to place the second pump further away from the first that has a reservoir or T-line, should probably leave that second pump disconnected while filling (avoid running dry).  In this test, I simply connected the two pumps with about 4″ of tubing as shown below. A little bit of tubing in between does help isolate vibrations between the two pumps.  Hard connecting two pumps via solid fittings is not recommended due to the impact/vibration stress you’ll be inducing on that threaded connection.

Series In Testing

Series Detail

Series vs Single Pumps, Add pressure (RECOMMENDED!)

The test in series performs very well.  It is as simple as adding the two pump curves in the Y or pressure direction.  The little bit of tubing between the two pumps is not a measurable restriction and the curve results are very much as expected morphing the two pump curves into one stacked curve.  While this does double the pressure of two like pumps, it should be noted that it does not double your flow rate.  Because the restriction pressure drop curves are not linear, you generally net around 30% more flow rate from running two pumps in series.  This would be the recommended method of running multiple pumps in one loop and it does NOT have to be the same pump.  A modified DDC-1 and PMP-450(D5 Vario) work together very happily together.

Series Pumps Redundancy Check

Ok, so how does series handle the loss of one pump you might ask?  Obviously you lose the power of the additional pump, but I wanted to see what happens to the remaining pump. Many forum members have suggested that a pump is highly restrictive when stopped and imply that performance in the remaining pump is degraded heavily…is it true?

Series PMP-450 Off Detail

Series (PMP450 off) vs Single DDC Pump


While you do lose the power of the second pump that was turned off, the restriction of the still pump is is very small.  On average you will only see about a 2% loss in pumping power vs. a single pump setup.  Series setups retain nearly full pumping power of single pump setup and therefore pass the redundancy check with flying colors.  Not only do you get adequate redundancy, you pretty much get full single pump setup power.  No problem…

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  1. Rori Woodman says:

    Hi Martin,

    Love reading your reviews, just wondering if you could answer a quick question for me?

    According to your 360 rad shoot out, the Black Ice GTX 360 has some pretty high restriction for a rad, I had read other reviews first and bought the Black Ice GTX 360 before coming across your review. I also bought the same rad, but a 480 to go in series with it, along with 2 EK HD 7970 water blocks, the Koolance 380i CPU block, a flow meter and a few elbows, not many as I have used the rigid acrylic tube and made most of the bends.

    My question is will the XSPC Twin D5 Dual Bay Reservoir with obviously the two D5’s in series be enough to achieve a minimum of 1gpm through the loop? Whats your thoughts? I think there would roughly be 2.5m of tubing all up. Not sure if its important, but the tube size is 3/8 ID and 1/2 OD.

    Thanks and hope to hear from you soon!

    • Martinm210 says:

      Yeah, no problem. One D5 is really plenty for most loops. Even if you drop below 1GPM, it’s generally more than enough pumping power. Some of the AIO kits like the H220 max out around .7GPM, I consider .5gpm the minimum for decent self bleeding performance, 1GPM if you want to be sure it bleeds itself easily, but thermal performance really doesn’t start dropping off fast until you get down in the .3GPM range.

  2. Mafieuso says:

    Hi Martin!

    You have a really nice blog and all the test and experiments are very useful.

    I would like to add that when you put 2 pumps in parallel you always have to put a check valve at least on the output of the weakest pump! It is the reason why the results are “strange”, when you have a pump with a higher head than the other you might have negative flowrate in th weakest pump!

    The theory is: 2 pumps in parallel are use to increase the total flowrate but not the head (you add the flowrate for a constant head).

    Finally, in parallel it’s always better to have 2 indentical pumps.

  3. Kevin Jin says:


    Love your pose, read it twice.
    Based on your articular 2 mixed pump in a serial loop works great?
    It would not create convection?
    How do you calculate the flow rate with two mixed pump in a single loop?