Why Size Matters When It Comes to Master Mixing Valves

by JRSBlogWriterMay 29, 2018

Despite What You’ve Heard, There is Only One Way to Size Your Master Mixing Valve

sign small big choose wisely

You might think when selecting a master mixing valve, that you can’t go wrong by going big. However, Joe Penza, product specialist for Acorn Controls strongly advises against that philosophy. While oversizing isn’t the major issue it once was due to Acorn Controls MV17 master mixer’s outstanding low flow performance, an oversized valve costs more—sometimes a lot more. Repair parts will be more expensive, and you will spend more time replacing them.

Even if you feel you’ve never had a problem when you’ve gone big, you are in fact forfeiting some level of low flow performance possibly to the extent that it will make a difference in bather comfort, or worse, bather safety. The larger valve may need servicing sooner because its sensor has to work harder dealing with flow rates that are better suited to a smaller valve. Not to mention the damage to your reputation should the building owner discover that a smaller valve would have met their needs without having to deal with all the above issues.

The ONLY WAY to size your master mixing valve is to perform calculations using site and design data and then, use a valve’s flow coefficient (aka Cv) to select the right valve. This will save you money and a few headaches in the long run.

The flow rate through a master mixing valve occurs only when there’s a higher inlet pressure than outlet pressure, no matter how small or large the pressure difference.  This differential has only one ideal term, PSID, meaning Pounds per Square Inch Differential.  We also like to call it “available system pressure differential”.  If you call it “loss (or drop) through the valve” (acceptable jargon for most mixing valve manufacturers), it could imply that you believe the valve should be sized based on the pressure you can afford to lose through the valve at peak demand. 

That well-intended but misplaced belief that you need to minimize loss through the valve has led design engineers to oversize their valve selection for their application.

To size your mixing valve correctly you need to know key data points such as:

  • Type of Project
  • Master Mixer Setpoint Temp
  • Minimum CW Temp
  • Fixture Outlet Temp.
  • Inlet pressure at the valve
  • Height of Riser to Farthest Fixture
  • Horizontal Distance from Riser to Farthest Fixture
  • Usage Factor (if the number of fixture units per Hunter's Curve is less than 50)
  • Fixture Count
  • Hunter’s Curve Fixtures Units
  • CV (or flow coefficient) required to meet peak demand
  • GPM required for peak demand, based on Hunter’s

With that information in hand, you must now spend hours doing calculations to determine the proper size mixing valve. Just kidding. We took care of all the tedious work and created a calculator (it works on your mobile device so you can use it right on the job site!) that runs the numbers for you. All you need to do is plug in the data.

However, for our representatives wanting to understand the magic behind our master mixing valve calculator, you can download a guide (restricted access) written by Joe Penza. This guide details the steps taken to size a valve correctly using the example of a nursing home. Joe even compared how his proper calculations stacked up against his “best guess,” and we don’t think anyone would be quick to second-guess Joe’s best guess. The results surprised everyone…everyone except Joe.

To download the guide simply use the Rep Login area on the MGO site and look for the guide under *Downloads >> Sales Support and Contacts or under Downloads >> Tools and Calculators.

If you have questions or would like more information on any of the Acorn Controls products, please contact us.