Flow flow flow, we all say flow!

16 Nov 2005
UK, Cambridge
I upgraded from AIO to a hardline build back in August 2021 flow rate was 150 L/H then slowly got worse and worse, thanks Aquasuite flow rate graphing!!

Now it won't go over 94 l/h at 100% pump speed, support article says min of 100 L/H is safe.

I recently had my D5 pump replaced as it appeared to be dying from the evidence I had to hand. Replaced it and also added an active backplate into the mix whilst I had it drained but flow rate still the same.

I had previously drained the whole loop, taken it apart, flushed the rads, dismantled the blocks and cleaned them and still the same result.

Also spent a long time rocking/turning/upending to ensure I got all the air out and bled air from the distro plate.

This link indicates that having a number of fittings in your build shouldn't impact flow massively either https://www.ekwb.com/blog/do-angled-adapter-fittings-really-reduce-flow/

Coolant under load hits around 36c.

So I guess what am I asking is..

1) People with similar hardware as below, what is your flow rate at 100% pump speed?
2) Is it possible to pressure blast the radiators incase there is a blockage my basic flush didn't pick up?
3) Sweet baby cheeses make a suggestion I haven't thought of.

Thanks for reading!

Lian Li Dynamic XL
13 x Angled fittings
4 x offset fittings
2 x tube bends
EK-Quantum Reflection PC-O11D XL D5 PWM D-RGB - Plexi
EK Water Blocks EK-Quantum Convection D5 Pump Cover - Nickel
EK Water Blocks EK-Quantum Vector MSI Trio RTX 3080/3090 D-RGB Water Block - Nickel + Plexi
EK-Quantum Vector TRIO RTX 3080/3090 Active Backplate D-RGB - Plexi
EK-Momentum MSI Z390 MEG Godlike D-RGB - Plexi
EK-CoolStream PE 360 (Top)
EK-CoolStream SE 360 (Side)
EK-CoolStream XE 360 (Bottom)
EK Fittings and 16mm Acrylic Tubing
Aquacomputer flow sensor high flow NEXT
Mayhems XTR Nano Tech Premixed Coolant
Man of Honour
16 Mar 2005
Clevedon , Bristol
Are your component temps OK ?

If so, then no need to panic.

In my many years of water-cooling i have never heard of an 'unsafe' flow rate ( alright, zero would be bad ).

Standard for D5 is set it to max at the start to push the airblocks out, then gradually reduce till you find a decent noise level/component temps level.

A lot of people who have been water cooling for years, wouldn't of had access to a digital flow meter, and they didn't have any issues.

Take EKs 'unsafe' with a large pinch of salt.

My current rig ( in Sig link ) runs at 78 l/min and the sky has fallen in on me :D
Last edited:
21 Dec 2019
Planet Thanet
As brizzles said
Are your temperatures OK?
Been watercooling a very long time
Never had flow sensor or coolant temperature measurement

Have seen people say they blasted their radiators through
Using a garden hose
Though never tried it myself

The other thing to consider would be is whatever hardware and software
That's reporting the flow rate may be faulty
Or giving an inaccurate reading
5 Mar 2017
As suggested, run at 100% for some time to make sure there’s no air affecting your flow. While doing that, safely move the case, to get rid of any potential air trapped.
One thing I noticed was that a 90 degree fitting, straight out of the pump output would dramatically affect flow rate.
The active backplate doesn’t affect that much, if it was fine before. Angled fittings, not a worry. You’re more likely to have issues depending on your rads and blocks. EK rads aren’t restrictive, neither are the blocks. Just one thing to note, there’s a recommended input and output. When I was using the 3090 was quite shocking the difference when using the wrong ports.
A single D5 was able to cope with 2 PE 360 + 1 XE 360, flow meter, active backplate and monoblock. Many angled fittings. The difference, I was using a pump/reservoir combo and you’re using a distro plate.
16 Nov 2005
UK, Cambridge
Hey everyone

Thanks for the replies.
  • The component temps seem fine.
  • The below graph starts around August 2021 until around now. The highest point being 160 L/H and then over the months slowly dropping down to 96 L/H which is why I was concerned something had become blocked/stuck as it was the only thing I could think of that would cause a trend downwards when I hadnt changed anything.
  • Pump has been running at 100% since August and I'm running the replacement at 100% too.
  • The outlets are fitting free, just straight pipe runs, although they do have an angled adapter at the end for the rads
  • With the active back plate attached to the GPU block it says you can use any but I went with their recommended defaults.
  • I guess there could be a fault with the "Aquacomputer flow sensor NEXT" I could pickup a second hand cheaper flow sensor and see if they are close in number.

Rich (BB code):
18 Oct 2002
The fact that you say two different pumps give you the same flow, and there is no signs of blockage anywhere, I would suspect the flow sensor itself. It is possible the bearings for the little rotary impellor in the sensor are wearing, causing the impellor to not rotate as freely as when it was brand new, giving a lower flow. If this is the issue then I imagine it would look like a downwards trend as it would probably be a gradual effect. Either that or the fluid viscosity is changing over time, but I guess you've replaced with new fluid a few times so probably not that.
10 Dec 2020
@kosch - I believe most people don't have flow meters so are mostly just concerned with temperature, as others have already advised. My system, in flow order and linked with EK ZMT 10/16 (total around 1.5m), is as follows:

Dual pumps: 90° fitting -> EK-XTOP Revo dual D5 PWM -> straight fitting
X570 block: 90° fitting -> EK Quantum momentum -> 90° fitting
GPU block: straight fitting -> Aorus RTX 3090 waterforce WB -> straight fitting
Rad 1: straight fitting -> Alphacool XT45 140 X-Flow -> 90° fitting
Rad 2: 90° fitting -> Alphacool XT45 420 X-Flow -> 90° fitting
CPU block: 90° fitting -> EK Quantum momentum -> straight fitting
Meter: straight fitting -> Aquacomputer High Flow Next -> 90° fitting x 2
Rad 3: 90° fitting -> HWL SR2 420MP -> 90° fitting
Reservoir: 90° fitting -> EK-Quantum volume FLT 360 -> straight fitting

With those pumps both at max (about 4500 rpm) I measured around 290 l/h. I would need to check, but I believe that it can still achieve this. Over the year or so I have had the system I have had reason to drive them at max (which I only do whilst accessing BIOS) and I don't remember being concerned that the flow rate had dropped.

That's probably all the information I can provide to meet your specific request, but it might be beneficial to tell you more.

Amongst the usual objectives in building a new gaming PC, mine was for it to be as quiet as possible both during office work and gaming. My last PC was air-cooled and it was "obvious" level loud when I turned it on, and if I played a game that pushed it's performance then it would get "jet engine" loud. The main problem was that the fan control was basic. So my design objectives for the new PC was to have full control over pumps and fans. Hence once I had built the PC I spent a lot of time setting it up for a good balance between performance and noise, and importantly this meant that water temperature itself was not particularly important. Probably like you I searched the web for advice and read lots of "rule of thumb" advice which I started with, but to be honest in the end I discounted a lot of it.

Possibly the best advice I read was about flow rate having little impact on water temperature. I confirmed this with my own testing. Yes I can reduce the water temperature by 1-2C with a higher flow rate but does that really matter? I decided that it does not, so I tested my pump speed for flow rate and noise and ended up deciding that I wanted to have a rate of about 65 l/h during idle/office work. Then I checked the high flow rates and found that although I could get quite high before the pumps themselves were annoying, I found that at some speeds I would get resonant vibration from parts of the case. The pump assembly is damped and the mounting is damped so there was not much I could do other than add some anti-vibration tape to the case glass panel which helped a bit. But the conclusion was that I did not want the flow rate to be above 135 l/h. So my BIOS is set to control pump speed based on a temperature sensor in the loop to automatically increase flow and during office work I see 65 l/h and during gaming it gradually rises until a maximum of 135 l/h. I haven't tested this extensively but I guess that this helps my maximum water temperature to be lower by 1C or so during gaming.

Of course the other part of the noise is the fans and I too have the great Aquasuite software to control them. I won't bore you with the details but obviously this was easier to judge because 140mm fans can be almost silent at slow speeds, so it became a pure matter of trading water temperature for noise during gaming. This is where I realised that I really did need to ignore all that advice about water temperature. For what its worth mine is 33-34C at idle, and at full load it can reach 40C or maybe slightly higher depending on room temperature (our home office can get hot and so I have an air conditioner, but I use it sparingly). Originally my target was 38C max until I realised it was irrelevant and so I changed the fan profiles to be a bit quieter. I'd be happy to change them again and allow it to go above 40C but at full load my PC is quieter than my old air-cooled one was at idle so it really doesn't bother me. As you know the only thing that matters is that the water temperature keeps the CPU and GPU below your preferred limit. In my case the GPU never gets that hot so I just checked that my undervolted 5900X does not reach AMD's thermal throttling threshold.

My final point is that I can confirm that these set flow rates at idle and full load have not reduced over time (nearly a year). I hope that this is of help to you.
5 Mar 2017
@therealdippy , very valid points.
Personally, I tried flow meters few times, more often than not, they presented some clicking noise.
My way to check if flow was decent, after checking temperatures, was checking the reservoir and notice how fast the coolant at the inlet was. Not measurable, by any means, but you should be able to notice if the flow is poor.
Normally you should expect to see the coolant entering the reservoir with some pressure, nothing crazy, but like when you open a tap. If flow is poor, you'll notice no pressure, as if the coolant is "being poured". But temperature is king.
When I tried the O11 classic distro and the rubbish DDC pump included, using 3 Corsair 360mm 30mm thick, the pump couldn't even flush the air from the radiators.
As I mentioned before, I can't see anything potentially affecting his flow.
EK rads aren't restrictive at all.
Same for EK blocks.
If GPU/CPU block fins are clean, I don't expect them to be the issue.

@kosch I just mentioned the intake from the active backplate because I was trying some funny tubing run here. If I remember well, I messed using lets say the port closer to the power connectors of the card from the GPU block as intake, and the port closer to the I/O port from the active backplate as outlet. Don't ask me why... :D:D:D

My first approach to watercooling was silence. At some point I became crazy about coolant Delta, then decided to drop the whole thing as it was consuming a lot of time and I was obsessed.
Rad area helps a lot, until you reach a point.
The best way to achieve best temperatures at the lowest noise possible is playing around the case you're using.
For the O11 XL, I always achieved best results using the top radiator as exhaust, right after the CPU/GPU. Don't need a radiator between CPU and GPU, but the heat from them should be directed ASAP as exhaust. Bottom radiator intake and side you can use either intake or exhaust, depending on your preference regarding positive/negative pressure.
You're limited by the distro, but if was a combo, I would suggest pump/reservoir - bottom XE as intake - GPU - CPU - top PE as exhaust - side SE as intake -pump/reservoir, or pump/reservoir - bottom XE as intake - GPU - top PE as exhaust - CPU - side SE as intake - reservoir/pump.
When you exhaust most of the heat, even if the other two radiators will get to work with coolant warmer than room temperature, the air from them won't choke the rest of the system, but have a feel on the air from the top exhaust and note how hot it can get.
Once you manage to get rid of the heat as efficient as possible, you can run your fans at lower rpm and get good results.
The most important thing I would suggest is a temperature sensor for the coolant and one for ambient (or find a way to measure it). That would allow you to measure how efficient the loop is dealing with the heat.

Just to add, cooling the backplate of a 3080/3090 + VRM from motherboard will add a lot of extra heat for the loop to cope.
Angled fittings won't affect flow as much, only noticeable if straight on the outlet of the pump.
8 Nov 2006
OP I run 4 radiators in a XL and 4 blocks, flow rate is fine.


On a single D5 I get this at 100%


Though these flow meters usually over estimate. Still, my temps are fine


Those are my temps at around 85% on the pump under a 500w load. I allow the pump to drop down to around 2000RPM on idle/desktop for complete silence. Flow rate measures about 0.42l/min, but this is absolutely fine for no real load.

I've played around with 60~85% on the pump under load, basically no temp difference, maybe the graphics card going up 1 degree between those two speeds. So I just settled on what has the most tolerable noise on the pump and that is around 85%. It's more high-pitched whirr than the deeper sound around 60~70%.
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