# Reducing tubing size



## Jamie McGrath (7 Jun 2016)

Is there any problems reducing filter hoses from 25/34mm down to 16/22mm? Has anybody done this?


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## foxfish (7 Jun 2016)

I am sure lots people have done that but, it is obviously not recommended as the flow rate will be considerably reduced.
I assume you are talking about a high flow filter, if you want to keep the flow then you need the inside diameter to be 25mm.
The suction or feed side will need to be 25mm but you could split the return into 2 x 16mm.


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## Jamie McGrath (7 Jun 2016)

foxfish said:


> I am sure lots people have done that but, it is obviously not recommended as the flow rate will be considerably reduced.
> I assume you are talking about a high flow filter, if you want to keep the flow then you need the inside diameter to be 25mm.
> The suction or feed side will need to be 25mm but you could split the return into 2 x 16mm.


The reason I  want to do this is ive just bought a filter and it has the 25/34mm tubing. All my existing spray bar and intake pipe are for 16/22mm. I dont want to buy a new spary bar and intake pipe for the large tubing, im also struggling to find somwhere that sells it all.

Thanks for your reply.


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## ian_m (7 Jun 2016)

Not really a done thing, will more than likely damage the filter impellor running at reduce flow.

16mm pipe is about 200mm2 area. 25mm pipe is 500mm2. So greater than 1/2 flow reduction, seriously not recommended.


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## Easternlethal (8 Jun 2016)

I've done it for my sunsun and it doesn't seem to have any negative effect. You get more flow through the narrower tube to compensate for the reduced diameter but that's to be expected. I don't think it's even as bad as tightening a valve to reduce flow, which a lot of people do. Impellers are driven magnetically and would not really strain anyway as I understand.


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## alto (8 Jun 2016)

Easternlethal said:


> You get more flow through the narrower tube to compensate for the reduced diameter but that's to be expected


Did you measure actual flow rates?
I'm very sceptical


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## Easternlethal (8 Jun 2016)

Yup. Try it yourself. If it really did make such a difference we wouldn't have so many different shapes, brands, and designs of canisters,  impellers and so on all utilising the same 16/22 hose fitting...
Many filter manufacturers even offer hose adaptors.


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## foxfish (8 Jun 2016)

I guesse one would have to experiment to really know but, generally speaking a single elbow or tight bend will dramatically decrease flow so 2mts of smaller diameter tube will also have a similar effect.
At the end of the day if you want to try it out then do so, the filter might be fine for what you want it just wont be working at its full potential.
However it is the feed pipe that may cause the impeller to spin faster that it can pump because of the lack of water supply, this then causes air to be trapped within the impeller camber & slows everything down complete with added noise!
Of course that effect might be cancelled out due to the reduced flow caused by the smaller diameter return pipe


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## alto (8 Jun 2016)

Easternlethal said:


> Yup. Try it yourself


Now repeat the experiment with a pump that is already working at max capacity


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## zozo (8 Jun 2016)

Here too, did it in several occasions, reducing tube sizes to restrict flow or just to make it fit and take the reduzed flow for granted.. Till now i never had any issues with pumps failing because of it.. Not saying it can't happen because i personaly never experienced anything breaking because of it, but if it does and burns out a pump or breaks an impeller i would sincerely doubt the construction quality of the pump.

Many people use ball valves to adjust flow speed, especialy people running sumps need to adjust and regulate inflow and outflow capacities.. 90% of those people use ball valves in the tubing to reduce tubesize to reduze flow.

That said, regarding your pump brand and quality and or factory recomendations it's obviously at own risk if you do not know..

P.s now while writing i remember buying an Aquel powerhead i used it for years on a canister in the pond. I remeber is came with a stepdown reducer in the box for connecting 10 mm up to 25 mm hose..


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## alto (8 Jun 2016)

It's always recommended to reduce flow (increase pressure) on the outlet rather than inlet as these pumps are generally designed to push against pressure rather than pull against it  ... but some pumps are significantly over designed for the job & these will generally work for years without measureable impact


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## Easternlethal (8 Jun 2016)

It's because our canisters are magnetic drive pumps. These are very different to mechanical seal pumps where the impeller is directly connected to the motor. The same reason why magnetic drive pumps are more reliable, do not require motor alignment and not suitable for liquids with solids is the same reason why it does not hurt the motor if there is a blockage. The impeller just slows down without affecting the motor.

It's possible to configure your hoses in such a way that changing its size will affect flow (e.g by adding a bend) but the point is that it does not damage the motor.

In a fully efficient setup (where there are no kinks or devices in the way and the hoses run up and down directly to and from the tank), changing the tube size only affects the speed of the flow and not the overall volume of water displaced. This could become a problem if the water flow is huge and reaching limits where friction starts to play a role but that does not happen in our aquarium canister filters.


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## alto (8 Jun 2016)

Easternlethal said:


> changing the tube size only affects the speed of the flow and not the overall volume of water displaced.


If filter is running at 2 litres/min & tubing is changed out so that flow is now 1litre/min ie change in _speed of flow_, how can this not affect the volume of water displaced (ie "moved" )  in 1 minute


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## Easternlethal (8 Jun 2016)

Litre measures volume and should stay the same. But velocity will change.


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## ian_m (8 Jun 2016)

Righto....with standard magnetic drive pumps they will suffer if you reduce the flow, as if the impellor is no longer rotating at "full speed" due to reduced flow, it will slowly get demagnetised. Once the impellor starts getting demagnetised, the noise will increase and impellor will be prone to stalling.

Most decent filters, this is from JBL e1501, state that you should not reduce the flow too much.


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## alto (8 Jun 2016)

Easternlethal said:


> Litre measures volume and should stay the same. But velocity will change.


what are the units of velocity then?


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## zozo (8 Jun 2016)

ian_m said:


> Righto....with standard magnetic drive pumps they will suffer if you reduce the flow, as if the impellor is no longer rotating at "full speed" due to reduced flow, it will slowly get demagnetised. Once the impellor starts getting demagnetised, the noise will increase and impellor will be prone to stalling.
> 
> Most decent filters, this is from JBL e1501, state that you should not reduce the flow too much.
> View attachment 85748



The JBL statement tho obviously correct is also a bit vageuly put.. You would rather need to do some math and know some fluid dynamics to get to know when you reach the point of 50% capacity or lower, if you reduce the tube size by 50% by far does not mean you also reduce the pumps capacity by 50%.  You're quite up to a chalange to find that out without knowing the full specs of all components in the system involved.. For example not only diameter but also lenght of the tube, density of the filter media (which is dynamic rather than constant), size and number of holes in the spraybar, tube materials specification etc.


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## Easternlethal (8 Jun 2016)

Actually this statement in the JBL manual is referring to reducing the capacity via the stop valve (or 'stopcock' as they call it). The physics behind this is different from hose sizing. With a stop valve you are introducing a kink into the system which causes pressure to build where the kink is and cause turbulence around the area. This slows down both velocity and volume because of all the energy lost navigating the kink. You will not get this problem with a narrower hose.



alto said:


> pumps are generally designed to push against pressure rather than pull against it


This is another interesting point because if you look at an impeller the blades do not slant in a certain direction compared to, say a motorboat propeller. This is because it is actually working on a centrifugal principle, which relies on creating flow through kinetic energy. This is why kinks, blockages etc. are so bad for it.


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