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Controlling Parallel Circs with VFDs

Zman Member Posts: 7,589
Many of the commercial heating systems I encounter are large zoned systems with 3 phase circs with VFDs piped in parallel. The usual control strategy uses supply and returns transducers to help maintain constant pressure on the system. Most of the systems are configured lead/lag to keep the circ run time equal and the lead (active) circ is usually running at between 40%-80% depending on how many zone valves are open.

Lately, I have been hearing energy gurus tout the benefits of running both circs all the time at a lower percentage. I have to admit, "I don't get it".

Any thoughts?

"If you can't explain it simply, you don't understand it well enough"
Albert Einstein


    EBEBRATT-Ed Member Posts: 15,767
    No. I don't get that either. I would assume motor efficiency drops off when running at low speed. I don't see what that would accomplish
  • Jamie Hall
    Jamie Hall Member Posts: 23,567
    I don't get it either -- but not for quite the same reasons. If you have two pumps -- or whatever -- and particularly if they are lead/lag, which they should be -- I'd prefer to run one of them up to near full power -- that 80% isn't a bad figure for that -- and then bring the other one on. At that point there should be, in my mind, a very clear indication somewhere that the lag pump (or whatever) has turned on. Why? So somebody finds out that the lead pump didn't have the capacity.

    This may be quite innocent -- simply greater demand. It may also be anything but innocent. The lead pump has gone paws up on you. It's nice to know that before the lag pump also fails...

    Now I'll admit that most of my work along those lines was in sewage and water supply. And in those applications -- except for single residences -- there was always a lead and a lag pump, with the lag pump set to come on if the lead pump couldn't keep up. And there was ALWAYS a prominent alarm when the lag pump was running, so someone could come and fix the lead pump before you got sewage all over the street...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • hot_rod
    hot_rod Member Posts: 22,396
    Larger pumps usually show the BEP on the curve charts. With that point you could determine the wire to water efficiency, using the formula in this article.


    Unless the drives change something, I would think running the pumps as close as possible to BEP is the most efficient operation, energy consumption wise. I think the only way around that would to be able to modify the impeller as it is spinning :). Back in the day they would "trim" the impellers to get the pump to match the load as accurately as possible.

    Hydraulically speaking, if the pumps can constantly adjust to loads where the balancing valves are not "shedding" pump head that would be ideal, or not needing balance valves at all. Balancing devices are parasitic, they cost you pumping energy.

    Take a look at what diverter tees, a throttling device, cost you in EL, for example!
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • ScottSecor
    ScottSecor Member Posts: 863
    I feel like I'm sitting in one of Siggy's classes again (scratching my head, kicking myself for not getting that engineering degree).

    I've ben working with a rep that sometimes suggests two relatively large variable speed circ ECM pumps with parallel piping. If memory serves, the system requirements were roughly 250gpm at 50 feet head.
    For us this technique worked best as secondary pumps with heating systems that have changing demand (Danfoss valves or zone valves) and some sort of outdoor reset. We typically only run one pump at times with light loads. When the demand increases the second pump turns on and the pumps continue to vary their speed. The pumps communicate with each other, all sensors are built into the pumps no external sensors required.

    I believe the idea is to have the single pump run at it's most efficient point on the curve, then have the second pump kick in when the load cannot be met. I guess you could call it lead/lag. Keep in mind that the systems that we've done this with require the second pump. On almost every older system, the pumps (and often boilers) were redundant.
  • offdutytech
    offdutytech Member Posts: 133
    When I worked for Siemens doing DDC we called this demand flow. Primary used in chilled water plant optimization. Indeed the pump curves were looked at to verify the most efficient point at which pumps would operate. This was only part of it. Delta T and several other factors come into play with this like electrical rates, etc. One thing in this that often gets over looked is things like transducers, flow meters, etc over time those sensors need to be calibrated and or verified for accuracy. One of the biggest issues are pressure transducers, If not installed properly they get air bound or clogged with system sediment. The later is more problematic with a phased project, especially if an addition or remodel is done to the system such as a university or hospital.