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Latching circuit for buffer tank

I have a 3-zone system with a buffer tank. There's a high-limit aquastat in the tank with a 180* set point. As currently wired, the boiler and the primary (boiler) circulator run when any zone calls for heat AND the tank is below set point (minus differential). The problem is that I have a small bathroom zone with frequent and short off/on heat calls, and if it's the only zone calling and the tank isn't up to set point, the boiler is on/off cycle quickly (the heat calls are a lot shorter than the time it takes to bring the tank up to set point).

I want to set it up so that if the boiler comes on it stays on until the tank is up to set point regardless of heat calls, but won't come on again if there are no heat calls, even if the tank falls below set point. I guess I need a latching mechanism, which I've designed using a DPST relay and a 24VAC transformer (see attached sketch - hopefully it's legible). In the sketch the boiler and primary circulator are one logical unit, and this circuit is designed to turn that unit on/off.

My question is, is there some off-the-shelf controller for this, or is there a better way to accomplish this?



  • Ironman
    Ironman Member Posts: 6,702
    How about some more info? What type of boiler and controls? What kind of emitters? How is the tank piped into the system?

    Keeping the tank at 180* would not be the most efficient method.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • dtrani
    dtrani Member Posts: 25
    I don't want to keep the tank at 180. The idea is that if I'm going to bring the boiler online, keep it online until the tank is at 180 so that the next series of short heat calls are satisfied by the tank. If there is no later heat call, then let it the tank drop. I just don't want to short-cycle the boiler while the tank is hot but not up to set point.

    It's an iron boiler piped pri/sec with the tank as the separator. There's mix of radiant floor, panel, and baseboard.
  • hot_rod
    hot_rod Member Posts: 15,410
    A cast iron boiler, or some non condensing type?
    Is the tank maintained at a certain temperature, then loads pulled off that tank? That is typically the best way to use a buffer. It sounds like your tank only get input when a zone calls, maybe piped in series?
    So temperatures vary based on loads and run time of the boiler?

    Here are the most common ways boilers are piped to buffer tanks. It shows a 10 gpm boiler flow with an 8 gpm load.
    The best way to pull off the loads is through an outdoor reset control, you will maximize the storage.

    Also a link to some other ideas for piping and controlling buffer tanks.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • dtrani
    dtrani Member Posts: 25
    edited February 2016
    Thanks. Useful info. My system is a "4-pipe configuration" with a cast-iron boiler.

    If I have a sustained call for heat from a small zone, it works great and I have long burns to heat up the water and long off cycles while the heat in the water is used. And if the tank is already up to temperature, the tank satisfies any short calls. But, if the tank isn't up to temp and I have short calls, the boiler fires for each call until the tank gets to temp. That's what I'm trying to avoid. I have a small bathroom that's kept warm all night while the rest of the place is set back, and this causes lots of short cycles during the night.

    I've run this system rewired so that the tank is always kept at set point, and that works great in terms of eliminating short-cycling, but since I have long weekday and overnight setbacks, I lose a lot to standby loss.

    I want to run so that every time the boiler fires it stays on until the tank comes up to temp, but the tank temp is allowed to fall if there are no calls to heat.

  • hot_rod
    hot_rod Member Posts: 15,410
    I don't see how your plan would reduce standby loss that much? Could you better insulate to reduce tank loss. Most insulated tanks lose less than. 1 degree per hour in indirect tanks
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • bob_46
    bob_46 Member Posts: 813
    I think your logic is valid as far as reducing short cycles. I think your schematic is good. I would go for it, worse case scenario you're out the cost of a transformer and relay. Let us know if you are pleased with the result.
  • VelvetFoot
    VelvetFoot Member Posts: 48
    I just logged on to ask a question, but I saw this so I thought I'd chime in.

    I have a latching setup with my pellet boiler. A pellet boiler likes to run for a while, hence the buffer tank. I didn't really look at your diagram-my mind isn't flexible right now-but I turn my tank on when the top aquastat is 140, or whatever you want. The heat call is latched by a relay that gets its signal from a boiler electrical component, and which keeps the call for heat contacts closed. When the boiler's aquastat is satisfied, the relay de-energizes and the call for heat contacts open. The tank is then ready to supply loads until it's aquastat drops below 140 again.

    You could also use a second sensor down towards the bottom of the tank to shut off the boiler and keep it latched until it's satisfied.

    See page 40 in that idronics doc. I used Ranco aquastats, which only have one sensor so you have to use two if you're going to use two sensors. I recently discovered that Honeywell has a line of controllers that have two sensors - the T775 line I guess you'd call them.
  • njtommy
    njtommy Member Posts: 1,105
    The Honeywell T775 controls are great we use them all the time at work for steam to hot water bundles/ out door reset control.
  • dtrani
    dtrani Member Posts: 25
    Thanks for the input. I built this a couple weeks ago with a simple xformer and DPST relay and it works just as planned. The relay is really loud however, and I was planning to find something quieter, but the T775 looks very interesting. I'm guessing it can do what I'm looking for but I'll have to read the details.
  • riverwood
    riverwood Member Posts: 1
    I don't think ppl quite grasped your situation. I used to install heating systems & you'd be surprised how little depth plumbers and companies have into these situations. I've been messing around(at home) with multiple buffers with various inputs and it's an art to get this right and what you're doing should be a significant improvement(I'm about to do same). Maintaining a buffer in the 'classic' way make no sense at all. Boiler cycles pretty quickly, making it pointless. Also, I'm converting it to a 2 pipe system. How did you get on with your relay and did you get a DPST latching relay? If you have a link for the relay it'd be great thanks.
  • STEVEusaPA
    STEVEusaPA Member Posts: 5,454
    I don't think the original poster is still monitoring this post from 5 1/2 years ago.
    EBEBRATT-Ed Member Posts: 11,161

    The op probably has a new system by now LOL
  • JakeCK
    JakeCK Member Posts: 526
    edited October 2021
    Halloween is less than a day away so we should all expect some level of necromancy. ☠️