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Do You need to account for friction loss?

Ryan Cash

UAMCC Associate Member
Whether you run the All-In-One RoofTec exterior cleaning system, or any other pressure washer, you may have a few questions that are common in the industry:

What hose diameter do you need for your pressure washer?
Is Friction loss something I need to worry about?
Do I need a 3/8” or ½” hose for pressure washing?
Do I need to consider a larger water supply hose?
What size hose do I need to use to plumb my pressure washer?
I feel like I’m not getting enough water to my machine, what gives!?

These are all pretty common questions in the exterior cleaning industry. A lot of times we think that a larger hose is always better..but is it?

To answer these questions, we need to understand two different things

  • Flow Constriction/Restriction
  • Friction Loss

Assuming there are no air leaks in the system, the most common reason that your equipment is not putting out enough water is due to constrictions in the water flow.

While many may think this is due to the diameter of the hose, that may or may not always be the case.

In fact, you might be surprise at the flow rate you can get from various hose diameters (these numbers are based on running 75 Psi, roughly the average out of a home’s spigot...Flow rate will increase with increased PSI)
  • 1/4” – 2 GPM
  • 3/8” – 6 GPM
  • 1/2” – 9 GPM
  • 5/8” – 15 GPM


Reviewing these numbers will tell you the minimum pressure hose diameter you'll need to adequately run your pressure washer.

So knowing this, the next factor in deciding which hose to look at Friction Loss.

Friction loss is the resulting resistance as water (fluid) moves along the inside wall of either a hose, pipe, or hose fittings, resulting in a drop in PSI

Points to remember about friction loss:

  • Friction loss increases as GPM increases
  • Total friction loss varies with length -- the greater the length, the higher the friction loss.
  • Friction losses on reeled hose average about 21 percent more than for straight hose lays.
  • Friction loss is nearly independent of pressure.
  • Friction loss increases 4 times for each doubling of water flow. Reducing the diameter of a hose by 1/2 will increase the friction loss by a factor of 32 for the same flow.
  • To account for friction loss, the pressure at which the pump is working must be increased. The pump pressure must also be or decreased to compensate for the head loss or gain, to produce the desired nozzle pressure.
Now, while there is mathematical formula for friction loss…

FL = C * (Q / 100) ^2 * L / 100
FL = Friction loss in PSI
C = Friction loss Coefficient (From Table)
Q = Flow rate in GPM
L = Hose length​

It’s not going to be completely accurate (or even necessary to understand) because every hose is going to have a different “C” value based on the inner construction of the hose.

To get a rough estimate of friction loss as a result of your pressure hose, you can instead use this chart:

Friction loss.jpeg

So what’s the take away from this chart and why is any of this important?

Let's take a look at a real world example:

The Rooftec XCS500 is a 5.5 GPM machine. If you were to run that machine or any other that runs at 5.5 GPM, a 3/8 pressure hose is more than sufficient and there is no need to jump to a 1/2 pressure hose. Here's why...

  • As we saw in the water flow chart, a 3/8 hose will provide adequate water flow (GPM) and is capable of transferring more than the pump itself can put out.

  • By looking at the friction loss chart, we see that even at 300ft of hose you’ll only be dropping somewhere in the range of 500PSI from pump to gun.
    • To some, this friction loss may seem like a lot, but this can be overcome by understanding that most concrete only needs a max of 2000-2500 Psi to clean. So even a 2500 Psi machine can still adequately clean concrete with 300ft of 3/8 pressure hose.

Jumping to a 1/2" pressure hose with 5GPM machines may actually be more problematic. You’ll lose hose flexibility and gain weight, while receiving very little, if any, benefit.

However! If you’re jumping up to an 8 or 10 GPM machine, you will want to make a jump up to a larger diameter pressure hose as well. A 3/8 hose would not allow adequate water flow (GPM). With these larger pumps, switching to a 1/2 and would only experience around 300PSi of pressure loss with a 300 ft hose. So this, in theory, is the best hose to use.

So what about supply hoses and plumbing on the system itself?

For this, friction loss is a non-factor. When talking about the plumbing on the machine, at most you may have a couple feet of hose. Additional to that, PSI isn't a factor since the bulk of the plumbing is before the pump.

Constriction is the only factor here. With that said, there are two important factors in plumbing on your system.

  • Hose Diameter
  • Hose Fittings

As long as your hose is able to supply adequate water flow for your pump, your main concern will be the fittings.

Let’s say that you are running 1/2” hoses for all of your plumbing. While this may adequately supply the pump “in theory”, the reality is that the 1/2 hose may have to be connected to a 1/2 hose barb...which has a much smaller internal diameter than the 1/2 hose.

While a 1/2” hose may have an Internal diameter of 1/2”, a 1/2” hose barb will have an outside diameter of 1/2”. This allows the hose to fit snugly over the barb. Because of this, the inside diameter of the 1/2 hose barb will be much smaller. This will lead to water restriction and a loss of flow.

90 degree turns, excessive fittings and over complication are all things that will effect your supply water flow. This is a case where the more streamlined, the better.

On the supply side, it’s always best to go slightly larger than necessary, knowing that your smallest fitting is going to be the limit of your capabilities. If you pump has a 1/2 inlet, your smallest fitting should have an internal diameter of 1/2”. You can go larger, but there’s no perceived benefit, as your flow will be restricted at the pump regardless.

Here’s a good example:

Here is a 1/2 poly hose barb

hose barb 2.jpg hose barb 1.jpg

If you look at the image on the right, you’ll see that the inner diameter of this 1/2 hose barb is actually about 5/16. Much smaller than the 1/2 hose that it’s supplying water to. If you hook up to a 5 GPM pump with these fittings, you’d actually starve your pump from much needed water flow and potentially cause the pump to cavitate.

Similar flow restrictions can happen at check valves and other elbows and connections throughout the plumbing.

To resolve this, it’s best to choose fittings that have an inside diameter that meets the inlet size of the pump. In general most 5/8" fittings have an orifice large enough to fully supply a pump with a 1/2" inlet.

You can then either switch out all hoses for a 5/8 as well, or in most cases, you can just heat up the end of the 1/2 hose and stretch it over the 5/8 hose barb.

With the right plumbing, fittings, and pressure hose, your machine will run quietly with minimal vibration and give you years of life!