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Math for short cycling blues

hot_rodhot_rod Member Posts: 11,378
It comes up often on how to determine cycle time on zoned or micro zoned systems. Here is some number crunching for both one speed, and modulating heat sources. Also a 2 pipe buffer tank option.
Bob "hot rod" Rohr
trainer for Caleffi NA
The magic is in hydronics, and hydronics is in me
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Comments

  • SuperJSuperJ Member Posts: 478
    Great post!

    One point to consider is the where you measure tank temperature changes things quite a bit. The top will hit setpoint quickly but if the sensor is located down on the side there will be way more hot water in the tank.

    Do think it is stressful to a PEX pipe/copper to run large temp swings?
    My office is in the basement so I can see and hear the PEX in my ceiling expand as the system heats up, so I've settled on a 25f swing. I'm worried about rubbing being and issue over years of expansion and contraction cycles, so I try and keep it hot once it's hot.
    I've got a recipe for short cycling, but it does ok, 100kbtu 34gal Polaris serving a bunch of panel rads (less than 20kbtu of rads) with TRVs. Currently running 2 cycles per hour with a 25f swing and an aggressive ODR curve.
  • BrewbeerBrewbeer Member Posts: 576
    edited January 8
    SuperJ said:


    One point to consider is the where you measure tank temperature changes things quite a bit. The top will hit setpoint quickly but if the sensor is located down on the side there will be way more hot water in the tank.

    This.

    I have a 4-pipe 30-gallon buffer, and found that with the system sensor positioned in the middle of the tank in the sensor well, the boiler would "porpoise", basically, the boiler speeds up while heating the water above the buffer tank's sensor well. Once that hotter water gets to the sensor, the boiler immediately slows down to minimum fire. The results is the boiler goes through a speeding up and slowing down cycle, which repeats over and over without maintaining a constant temperature. This issue was solved by strapping the system sensor to the piping on the output side of the top of the buffer.

    Hydronics inspired homeowner with self-designed high efficiency low temperature baseboard system and professionally installed mod-con boiler with indirect DHW. My system design thread: http://forum.heatinghelp.com/discussion/154385
    System Photo: https://us.v-cdn.net/5021738/uploads/FileUpload/79/451e1f19a1e5b345e0951fbe1ff6ca.jpg
  • hot_rodhot_rod Member Posts: 11,378
    SuperJ said:

    Great post!



    One point to consider is the where you measure tank temperature changes things quite a bit. The top will hit setpoint quickly but if the sensor is located down on the side there will be way more hot water in the tank.



    Do think it is stressful to a PEX pipe/copper to run large temp swings?

    My office is in the basement so I can see and hear the PEX in my ceiling expand as the system heats up, so I've settled on a 25f swing. I'm worried about rubbing being and issue over years of expansion and contraction cycles, so I try and keep it hot once it's hot.

    I've got a recipe for short cycling, but it does ok, 100kbtu 34gal Polaris serving a bunch of panel rads (less than 20kbtu of rads) with TRVs. Currently running 2 cycles per hour with a 25f swing and an aggressive ODR curve.

    The tank will stratify when there is no flow going thru it, when heat call and boiler loading stops. When pumps are flowing, moving flow top to bottom you should not see a lot of stratification.

    Stratifying is good as it increases storage, but balance that against a mod con running out of condensing mode to load a tank to 160- 180.

    Those high temperature tanks are more common on wood, pellet and non-con boilers, run 'em hot, load the tank as high as possible and leverage a 100° ∆ in some cases.

    I think pex is ok to 180 for extended periods, but expansion movement does become an issue. Insulating the tube and putting hangers around the insulation is one good solution. I like the PAP or FostaPex best for this distribution lines.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • hot_rodhot_rod Member Posts: 11,378
    Brewbeer said:

    SuperJ said:


    One point to consider is the where you measure tank temperature changes things quite a bit. The top will hit setpoint quickly but if the sensor is located down on the side there will be way more hot water in the tank.

    This.

    I have a 4-pipe 30-gallon buffer, and found that with the system sensor positioned in the middle of the tank in the sensor well, the boiler would "porpoise", basically, the boiler speeds up while heating the water above the buffer tank's sensor well. Once that hotter water gets to the sensor, the boiler immediately slows down to minimum fire. The results is the boiler goes through a speeding up and slowing down cycle, which repeats over and over without maintaining a constant temperature. This issue was solved by strapping the system sensor to the piping on the output side of the top of the buffer.

    Good observation and fix. Yes you do need to adjust sensing points specific to your use and application. I remember seeing a Euro tank with a few tracks down the tank so you could slide sensors to multiple locations.

    I like the 2 or 3 pipe connections best of the various pipings I have tried.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • STEVEusaPASTEVEusaPA Member Posts: 2,852
    Great info as always. I love when I get a new Idronics in the mail.
    How are you making up the generously sized short headers?
    And generally how long (short), what diameter?
    I was thinking 3x3x (whatever needed) reducing tee, short piece of 3" copper, then 3X 1 1/2" reducer on each end to 1 1/2 male adapter into the tank.
    steve
  • hot_rodhot_rod Member Posts: 11,378

    Great info as always. I love when I get a new Idronics in the mail.
    How are you making up the generously sized short headers?
    And generally how long (short), what diameter?
    I was thinking 3x3x (whatever needed) reducing tee, short piece of 3" copper, then 3X 1 1/2" reducer on each end to 1 1/2 male adapter into the tank.

    In a perfect world, know the actual GPM possible in that section and size pipe for 2 fps velocity. The same concept used in primary secondary sizing or hydro seps really. No harm in oversizing of course.
    There are some brands of buffers with 2 and even 3" ports. Or one of theses "increaser" options. That is a 3/4 mip x 1-1/4 copper adapter I use on regular 3/4 HW tanks.

    The brass one is actually from an old Munchkin :) Looks like you are getting 1-1/4 connections, while the headers are actually 1".
    Screen Shot 2019-01-08 at 1.29.37 PM.png
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    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
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