How to avoid an ASHRAE 15 violation on your next VRF project

One thing that can easily be overlooked when designing a VRF system is a potential ASHRAE 15 violation. 

ASHRAE 15 is a code that limits the amount of refrigerant that could potentially leak into an occupied space and still be considered safe. 

Why is this such a big deal with VRF? 

First – one of the benefits of VRF is its zoning power; the ability to install individual fan coils in space, all connected to a single outdoor unit. 

This provides ultimate comfort and zoning control, but it could come at the cost of violating ASHRAE 15, particularly in the smallest zones.

Before moving on, let’s further define ASHRAE 15. 

As of this writing, ASHRAE 15 allows up to 26 lbs of R410a per 1,000 cubic feet of occupied space. This limit is reduced by 50%, to 13 lbs of R410a per 1,000 cubic feet, for buildings classified as institutional, which are spaces in which people could need assistance evacuating the building due to, but not limited to, health or age (think nursing home). 

Finally, if the system holds less than 6.6 lbs of R410a, then it is exempt. 


Let’s look at an example:

A designer selects a 30 ton VRF system to serve 9,000 square feet of offices. One zone, a small 10′ x 10′ office with 8′ ceilings, is designed with a high wall unit.

Based on ASHRAE 15, this office is allowed a potential leak of up to 26 lbs of R410a per 1,000 cubic feet

Assuming the system will contain 38 lbs of R410a, is this acceptable?

Use the following formula to determine the MINIMUM square footage of the smallest zone:

where RCL = refrigeration concentration limit 

Following this formula, where RCL = 26/1,000 for R410a, the calculation shows that the minimum square footage is 168.3 square feet. That means the SMALLEST zone with 8′ ceilings in this system must be equal to or greater than 169 square feet. This is a violation in our example since the office is only 100 square feet.

You can also use this handy chart by VRF Wizard to help calculate the minimum square footage of a zone. 

If you find that your design violates ASHRAE 15, there are a number of ways to fix this:

  • Increase the volume of space by either removing the drop ceiling, adding a transfer duct or installing an undercut door.
  • In an application with a drop ceiling, if the plenum space is used as a supply or return plenum and the connected HVAC system fan is always on, that space can be used in the volume calculation (assuming that the leaked refrigerant could dissipate into this plenum space)
  • Install a ducted unit in lieu of a high wall
  • Reduce the size of the refrigerant piping with a piping or system redesign
  • Install a dedicated single zone mini split

Do you need help confirming ASHRAE 15 compliance on a VRF project?

Give us a call at 816-842-5400 or send us an email at

We would love to walk a job with you or help you layout your VRF project to make sure it is ASHRAE 15 compliant! Sources:

Brad Telker
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