Ryan Cash

New Member
How do I know how many GPMs my machine is actually putting out? Do I need a buffer tank for my machine? Is there a way to measure the chemical draw rate of my machine?

These are more of the questions I see a lot in groups from contractors who are fairly new to the industry. The good news is that all three of these questions can be answered using one simple tool! A 5 Gallon Bucket!

Let’s break these down one at a time.

1. How do I know how many GPMs my machine is actually putting out?

When you first purchase a new machine, and you’re looking at adding a surface cleaner, it’s a good idea to see how much your pump is actually putting out. Pressure pumps are rated by the manufacturer to show the maximum GPM a machine can put out in an ideal situation. The more plumbing, fittings, parts, and pressure hose length you use, the lower your actual GPM output will be.

To test your machine output, hook up to a water supply and turn on your machine. Start a timer and spray the water (directly from the hose, no spray tool) into your 5 gallon bucket. After a minute, turn off the machine and see how full the bucket it (if you have a higher flow machine, set your timer for 30 seconds and double the results).​
Once this is complete, you can repeat the test using different hose lengths and diameters to see how much your pressure hose effects your output.​

2. Do I need a buffer tank for my machine?

This will vary quite a bit based on where you’re working and what your machine is rated at. The simple answer to this is: You need a buffer tank if your water supply is less than the water output of your machine.​

To check this, perform the same bucket test but using the water supply rather than your machine. Start a minute timer and see how full your 5 gallon bucket is after filling it directly from the spigot.

If your water supply is greater than your machine output, no buffer tank is needed. If your machine puts out more water than the supply, a buffer tank will be necessary.​
In general, contractors running 4GPM machines or less will not typically need a buffer tank at all. Once you get upwards towards 5 GPM, you’ll need a small buffer tank. Running an 8+ GPM machine will normally require a 100+ gallon tank.
The key here is the difference between input and output.​

For example: Let’s look at the
Rooftec XCS500. This machine puts out roughly 5 GPM from the nozzle at full pressure and flow and has a 12 gallon buffer tank. Here’s the math to take into consideration:

  • If your water supply is 5+ GPM, you can run the machine as long as you’d like with no problems. Most residential water is greater than this, so in most cases the buffer tank isn’t fully used and the supply keeps up.
  • If your water supply is 4 GPM (This is considered low flow), then the machine will net (-1) GPM..4 in, 5 out. So the buffer tank would drain 1 gallon every minute. This would give you 12 minutes of cleaning time. However, every time you let off the trigger, the tank refills at 4 gpm. So if you’re moving around the home and letting off the trigger occasionally, even 4 GPM can be more than acceptable for getting a job done.
  • Anything below 3 GPM from the spigot would be considered very low flow and would need to be considered with taking/pricing a job. This output flow would net you at least (-2) GPM when running the machine and would drain the buffer tank in 6 minutes or less. For soft wash application, this is still adequate, as you’re off the trigger a lot as you move around the home, but such low flow rates can lead to a lot of waiting for the buffer tank to fill.

3. Is there a way to measure the chemical draw rate of my machine?

Draw rate of chemicals can be checked using the same bucket testing methods. The Rooftec XCS500 uses a flow meter for surfactant so there is a visible readout on how much surfactant you’re actually using, however with most machines this isn’t an option.

To test draw rates of chemicals in general, you’ll need a couple buckets.​

  1. Fill your “chemical” bucket with 5 gallons of water and place your output hose in the empty 5 gallon bucket.
  2. Turn on your machine and fill the output bucket.
  3. Empty the output bucket and fill it one more time so that you’ve sprayed a total of 10 gallons
    1. It is much easier math if you use 10 gallons
    2. the larger sample gives you a more accurate draw rate as well
  4. Look at the "chemical" bucket and note how much water was pulled from it
  5. Use this number to create a draw ratio

For an example, let’s say you drew 1 gallon from your chem bucket. This means that you have 1 gallon of “chem” and 9 gallons of water at output, so you’re drawing chem at a 1:9 ratio, or 10%.

For a deeper look into chem percentages and draw rates, check out this guide: Bleach Strength and Draw Rates

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