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Primary/Secondary zone differential bypass valve

JamesP_VTJamesP_VT Member Posts: 5
Recently had a new gas boiler (GV90+) installed, including all new zone controls. Heat is baseboard hot water running 3 zones plus DHW, all using zone valves and the primary circulator pump in the boiler. Control is using a Taco 4 zone controller with priority. Everything seems to be setup and sized appropriately. One part of the install that looks wrong to me is the system is setup with a primary/secondary loop. (I think i have the terminology correct) The primary loop feeds directly back to the boiler (using 3/4 pipe which matches the zones, 1" feed/return lines). I understand primary/secondary is normal, but since the system is using zone valves instead of circulators it looks like there should be a differential pressure bypass valve, but instead the installer put a ball valve in the secondary loop and suggested putting it in a partially closed position. I asked him about it and he said something to the effect that it's not really needed and he likes to use as few parts that could fail as possible. How wrong is this setup, and how much could it be hurting my efficiency? The boiler does seem to short-cycle quite a bit, (on for 3-5 minutes, off for 3-5 minutes even when the two larger zones are calling for heat, and i'm wondering how much the incorrect primary loop is contributing to this. I'm going to start measuring return temps with the valve closed and partially open to see how much of an impact it has. Also, the boiler already has a secondary pump to protect the heat exchanger, so is a primary/secondary loop even necessary?


  • EdTheHeaterManEdTheHeaterMan Member Posts: 572
    edited January 14
    @JamesP_VT said: Also, the boiler already has a secondary pump to protect the heat exchanger, so is a primary/secondary loop even necessary?

    Probably not. Using the "System" circulator on the GV boiler may be OK as long as there is enough flow through the system to keep the boiler from short cycling. Sounds like there is not enough flow from the system to keep the boiler from overheating and cycling on the limit.

    Pictures will help with the rest of the diagnosis.
  • hot_rodhot_rod Member Posts: 13,318
    Need to see a pic or drawing.

    A delta P circulator is ideal for zone valves and eliminated the need fora PAB
    Rule of thumb 4 Zone valves or more need bypass valve
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • EdTheHeaterManEdTheHeaterMan Member Posts: 572
    This is the diagram if you are using the boiler's primary circ as the "system" circulator... the main pump for the zones.

    there are no additional circulators in this diagram.

    This is the diagram you should have if you are using Primary Secondary piping Notice the additional circulator.

    The circulators are renamed here to: "System" Circulator is the one in the diagram. The primary circulator in the boiler is no longer the system circ. Now it is the primary loop circulator. The other circulator in the boiler still remains as the bypass circulator.

  • EdTheHeaterManEdTheHeaterMan Member Posts: 572
    Your boiler may be oversized. some guys do that because you are making hot water, but that is usually not needed with a priority control. Measure the radiation, (If baseboards the length of the aluminum elements only, not the amount of cover!

    if cast iron, there are charts available.
  • JamesP_VTJamesP_VT Member Posts: 5
    edited January 14

    The first diagram is basically what I have, but there is no pressure differential bypass, instead it's a ball valve. I made some measurements today, and I'm getting just under 20F drop when 2 main zones are on, and the bypass ball valve is closed. When I open the ball valve a bit, return from the zones is still close to the same drop, but the return to the heater is much warmer, due to the flow going directly back to the boiler (less than 10F drop). It seems like when any zone is open, essentially all the flow out of the boiler should be going through the zones, instead some is going directly back to the boiler. That just seems wrong. But how much trouble does that cause? Basically I'm trying to know how much I should insist on a pressure differential bypass instead of the ball valve. Overall I'm happy with the work the installer did, and I trust he did what he thought was correct. I'm no plumber, so that's why I'm here trying to find out if this is a reasonable design choice, or just plain wrong. Photos of system attached. Ball valve bypass is at bottom right of photo near boiler (bypass in closed position for testing}.
  • PC7060PC7060 Member Posts: 9
    edited May 20
    I realize I'm coming to the party rather late, but I recommend you talk to the tech support folks for the GV90. I did a system evaluation for the GV90 a year or so ago and the technical support guy was very clear the a bypass was required to ensure the return water to the boiler never went below 140 degree F to prevent the boiler crash/reset.

    From my limited analysis, the GV90 is a tweeked up version of the classic CI boiler and cannot tolerate temperatures below the condensation level of the applicable fuel (NG / LP).

    @JamesP_VT - "I should insist on a pressure differential bypass instead of the ball valve"

    The GV90 installers manual provide several configurations piping diagram including the ball valve feedback control you have. An better configuration uses a digitally controlled valve to maximize output of hot water to the secondary loop while keeping the return water above 140 DF.

    Most of the diagrams in manual were pretty fuzzy as the pictures provided by @EdTheHeaterMan show. I was able to get a copy of the manual with clean images if you are interested.
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