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VFD needed for 3-way valve system?

Our new HVAC system's chilled water system called for VFD on the chilled water pump. Our contractor, balancer, and control guy have never seen a VFD on a system with 3-way valves at the air handlers. In fact, the balancer needed all the pump had, just to get the right numbers. Was that expen$ive VFD necessary?

Comments

  • tim smithtim smith Posts: 2,275Member
    If all valves are 3 way, I don't see a reason for vfd

    That's my take on it. I would have thought it to be better to be 2 way valves w/ vfd pump control for energy use but 3 way works.
  • Brad White_185Brad White_185 Posts: 265Member
    No Three Ways About It, Tim is Correct.

    Using 2-way valves are the essential ingredient to using a VFD. 3-way valves in that application are bypass valves and do not back-pressure the differential between supply and return. Thus the VFD has nothing to respond to. (Usually differential pressure about 3/4 distant from the pumps.)

    In fact, so many 3-way valve setups make it worse than nothing- many do not have a balancing valve in the bypass port. This passes more water to the return during bypass mode and can raise the return pressure because the coil is no longer in the picture. Double-negative reason not to use 3-way valves.

    It MAY (just MAY) be possible to disconnect the bypass port and use the body as a 2-way. You can ask your ATC contractor or whomever supplied that valve if that can work. That is the "poor man's approach" to show a benefit but the stroke aspects and Cv ratings may not be ideal. If not doing this method you have to replace the valves with 2-ways.

    The pump HP savings can be calculated and demonstrated. It also will help with your available pump capacity due to diversity. Right now every node needs total flow, bypassed or not. At least 2-ways will allow a backing-off.

    If there are a handful of valves, changing them out is worthwhile. I save the use of 3-way valves only where budget and system size/flow rate do not justify VF control and/or a DP bypass valve.
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