The Importance of Pressure (PSI) vs Flow (GPM)
When choosing a new pressure washing set-up it's important to consider both the flow and pressure rating of the machine. The machine pictured above is a 3500 PSI 8 GPM Hot Water trailer unit that we custom built. It's a great unit for commercial work such as washing buildings and doing concrete work especially when paired with a surface cleaner. However, this unit would be way too much and completely unnecessary for washing cars. First, let's talk about how both PSI and GPM are both important then what recommendations we have for different job requirements. As always, you can contact us and we will be happy to make recommendations specific to your application.
Pressure Rating (PSI)
Pressure washers are rated on pressure by Pounds per Square Inch (PSI). The amount of water pressure needed to adequately clean will greatly vary based upon the job. You need enough pressure to essentially break the bond between the substance you are trying to remove and the surface. To equate this to something easier to understand think of a mechanical situation. If you have a bolt that you are trying to remove and it's barely tightened, you can use a smaller wrench because it doesn't take a lot of torque to remove it. However if you're trying to take off a bolt that is rusted and stuck, you're going to need a much bigger wrench to create more torque to remove the stubborn bolt. The same theory applies to pressure washing. If you're simply trying to wash dust or sand off of a surface you won't need a unit with a very high pressure rating because it doesn't take much o break that bond. If you're washing equipment with difficult to remove dirt and grime it may require more pressure to get the substance to release from the surface. Always keep your job application in mind when determining how much pressure you will need to adequately do a job.
Flow (GPM)
Pressure washers are rated on flow by Gallons Per Minute (GPM). The amount of water you need to adequately pressure wash will vary based upon the job just as the PSI will. You need enough flow to adequately rinse the dirt and grime away. If you have a machine with a very high pressure but very little flow you may be able to get the grime off the surface but end up with a big mess because there isn't enough flow to rinse the grime away. If you're washing cars you won't have to worry about flow quite as much as you do if you're cleaning driveways. When cleaning cars, you get to rinse from top to bottom and it's a little easier because gravity helps to rinse all the dirt downhill. Even with car washing you will want an adequate flow to make sure everything is properly rinsed. If you plan on cleaning driveways using a Surface Cleaner, getting a machine with adequate flow is essential. Surface cleaners will allow you to utilize the pressure of your machine and remove dirt and grime from concrete. The trick becomes getting all of the dirt and grime rinsed away- this is where the flow can make a huge difference.
When considering GPM, always consider your water source! If you plan on relying on water from a water hose on a residential home, you need to know if you will have adequate water supply. You can test this by timing how long it takes to fill a 5 Gallon Bucket from a water hose. To supply a 4 Gallon Per Minute machine, we recommend that you be able to fill a 5 gallon bucket using just the house in about a minute. If you choose a machine that has a higher GPM rating and you don't have enough water coming into the machine then it will not run properly. In our area in North Georgia, we recommend that anyone using a machine rated at 5 GPM or higher have a water tank to supply water to the unit. We have had customers in other parts of the country that didn't have adequate water supply for 4 GPM machines so this may vary based on your location. If you have any questions about this, please contact us!
Other Considerations:
It is always important to consider factors other than Pressure and Flow. You can learn more about these in our other articles
- Direct Drive vs. Belt Drive (Important when considering usage)
- Hot Water vs. Cold Water (Important when considering what substance you are trying to remove)
- Attachments
- Portability (Do you plan on loading & unloading the machine or mounting it on a trailer?)
Our Recommendations Based on Job Application:
Please note these are our general recommendations. We always encourage customers to contact us before purchasing to make sure that they purchase equipment that will work best for the job in mind.
Light Duty Usage (Around the House- Use a Few Times a Year)
This unit by Mi-T-M is an economical Light Duty pressure washer made for the homeowner who wants a unit to use around the house on occasion. It is rated at 2400 PSI 2.4 GPM This is an adequate pressure and flow for completing small jobs such as washing a car, deck or rinsing a home. It wouldn't be adequate for using a driveway surface cleaner or for using jobs that require the machine to run for long hours frequently. It is great for light homeowner use. We have been happy with Mi-T-M product performance.
Medium Duty Usage (Homeowners who Frequently Use Their Machine)
This unit by Mi-T-M is great for Medium Duty usage. It is rated at 3000 PSI and 2.4GPM. It features a heavier duty pump than the unit above. The crankshaft driven pump on this unit is built to run for longer hours and to last longer than an axial pump. This pump is great to use for washing a home, driveway and cars. The higher quality pump will make it last much longer. You could use this unit with a small surface cleaner. It would be fine to use to clean a few houses on the side as a side job. It wouldn't be recommended for all day every day use but you would see a great life out of this unit at medium usage.
Contractor Grade (Using the Machine a few times a week on Houses, Driveways)
This unit by Pressure-Pro is great for a Contractor Grade Usage. It is Rated at 4000 PSI and 4 GPM. It features a Honda Engine and a heavy duty General Pump. You can use this pressure washer for any average cold water job. This unit works great with a surface cleaner. We recommend the General 18" Surface Cleaner (Greasable Version). It has the right balance of pressure and flow to clean most houses and driveways. It is built with long-lasting components. There are only a few reasons why you would need to go with a larger unit. One reason is if you are running your pressure washer all day every day- in which case we would recommend going with a Belt Drive Unit. Another reason is if you are cleaning surfaces that require you to remove difficult to remove substances such as oil in grease- in which case you will need hot water. The third reason is if you are doing very large jobs such as large concrete areas- in which case we would recommend going with a higher flow unit so that you could power a larger surface cleaner such as a Big Guy 28". Larger flow units can also offer set ups that allow you to run 2 guns at the same time. If none of those reasons apply to you, we highly recommend this unit for an average contractor use.
Contractor Grade- Heavier Usage (You use your machine all day, most days)
This unit by Pressure-Pro is great for heavy usage. It is very similar to the E4040HG except for the fact that it is belt driven. Belt driven pumps turn at approximately 1/2 the RPMs of a Direct Drive Pump. So, Belt Drive Pumps are designed to run longer and last longer. It is Rated at 4000 PSI and 4 GPM. It features a Honda Engine and a heavy duty General Pump. You can use this pressure washer for any average cold water job. This unit works great with a surface cleaner. We recommend the General 18" Surface Cleaner (Greasable Version). It has the right balance of pressure and flow to clean most houses and driveways. It is built with long-lasting components. A lot of our local pressure washing companies use this unit to wash residential homes and driveways. It is a unit that holds up well to heavy usage.
