Variable Refrigerant Flow Systems

February 29, 2012

Variable Refrigerant Flow Systems

Variable Refrigerant Flow (VRF) systems or Variable Refrigerant Volume (VRV) systems are a new kind of air conditioning in the US. They were originally developed in Japan in the early 80’s, but have only recently started to be used here. The systems take advantage of high tech controls to allow more than one hotel room to be connected to a single condenser mounted on the roof. The VRF units are very efficient and quiet, making them the most comfortable hotel room temperature control you could ever imagine.

But there is one catch. Multiple hotel rooms are connected to a single condenser and they share a common piping system for the refrigerant. If a leak occurs in a single unit, all the refrigerant in the system goes into one room. The resulting concentration of refrigerant can be lethal.

This danger has prompted many hotel chains, including Marriott and Hilton, to ban these systems from use in their hotels. There is an exception to this rule if costly refrigerant leak detection alarms are added to the rooms.

In this comic, the refrigerant began filling the room when the VRF unit was struck with the throwing star. The leak detection alarm might have saved the ninjas had it not been cut in half. Unfortunately, there is no NEMA rating that corresponds to “provides protection against ninja sword fights.”

Mark Robison, PE


  1. In Europe we have a norm EN378, that establish the limit of gas concentration, and has to be verified for every room, so if a leak occurs it is not fatal.

    In pratice, for a hotel we get a lot of systems connected to just one centralized control (BMS).

    By the way, I really enjoy your cartoons, keep up with the good work.


    Ricardo Perry

  2. Mark Robison says:

    Ricardo – Thank you for the comment and kind words about the comic.

    Regarding VRV systems, I had dinner last night with Rick Frey, Senior Director of Hilton Engineering Support, and I learned more about this issue. The present Hilton standards call for the volume calculation to include the volume from the floor up to the height of the bed, only. Not the entire room volume. This severely limits the number of fan coils per condenser.

    Rick has been doing further research and has concluded that the refrigerant gas mix well enough that the entire volume should be the calculation basis. He indicated that up to 16 guestrooms can be served by a single condenser, typically. I have not confirmed this calculation, but this is good news for the hotel industry.

  3. Bob says:

    I think some of your facts may be incorrect. There is a new Marriott in Atlanta touting their VRV system (link below.) If they have truly decided to ban the system since then, could you possibly forward me the info? Interesting topic and refrigerant concentration is certainly something to be aware of. Good to hear that they take the safety of residents seriously.

  4. Mark Robison says:

    Bob, thank you for the link to the Atlanta project. What has happened is that Marriott has been going through a sercies of time periods where they either allowed or dis-allowed VRV. The time period when the Atlanta hotel was built was during one of the periods when VRV was being allowed. It is only within the past year that a new interpretation was released that restricted VRV in Marriott projects subject to either a limited number of guestrooms on a refrigerant loop, or refrigerant detectors and alarms in every guestroom. If you wish further clarification on this you might want to contact Larry Frey at Marriott Corporate Engineering. Hope this helps. Regards – Mark

  5. Rick Frey says:

    You misunderstood what I said about gas mixing. Our testing indicated that 13 lbs of refrigerant released into a standard sized hotel room dropped the oxygen level below the 19% that ASHRAE indicates as a safe level. I never intended to indicate that our calculation requirements should be modified and the test results actually verified that our standards are appropriate.

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