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347347 Member Posts: 127
I'm installing an HTP EFTU85 (heating only) for a garage radiant floor. The total load is about 10,345 BTU's. I'm connecting it to an 18 gallon boiler buddy I had laying around to try and keep from short cycling.
It looks like there is no need for a primary circulator between the buffer tank and boiler, is that correct in my thinking? The boiler has a built in circulator that seems to do that job if I read the manual properly.

My second question is, What type of control do I install in the buffer tank to use all of the hot water before the boiler turns to replenish it?
Thanks for your advice.


  • fenkelfenkel Member Posts: 128
    Boiler does gave an internal pump, which should move supply water to buffer tank..
    No control is needed on buffer tank... boilers internal return and supply temp probes will aid in when the boiler will fire back up...

  • IronmanIronman Member Posts: 6,176
    A great boiler. We've recently switched from the UFT to these.

    You've got a few available scenarios depending on some variables. If the loops are 1/2" and under 300', you can probably get by using the internal circ to pump the system too (some math required). The circ has the same curve as a 007. With a slab, you've got a lot of mass so the buffer may not be necessary but wouldn't hurt. It you use it, and if the pump is sufficient for the entire system and boiler, then you could pipe the tank in series (2 pipe) and you wouldn't need a system circ. If you need a system circ, then use the tank as a hydraulic separator (4 pipe) and use the optional system sensor.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • 347347 Member Posts: 127
    Thank you both.
    I have five 1/2" loop and if I remember correctly they are right around 200'. I have to dig out my radiant design paper work (its been installed for five years now).
    Bob, I don't understand the "in series" part.
    I have the buffer mostly piped (system side) so I will be using it.
    I having a mental block on the sensor. I know Fenkel said to user the boiler internal sensors but I would like to draw out all the heat out of the buffer before it turn the boiler on to recharge it. I hope that makes sense.
  • 347347 Member Posts: 127
    Just looked my notes on the design. I use 4 1/2" loop approximately 150' long (I had small rolls laying around).
  • fenkelfenkel Member Posts: 128
    Look for a guide from caleffi on buffer tanks...the purpose of a buffer tank is to limit  short cycling.... the buffer  tank will involve a process called stratification, basically layers of different water temps..
    The top of tank will be hot, the middle might be warm and bottom will be colder than the middle of the tank...
      rarely will you be able to use all the heat from buffer tank before the boiler would restart...

  • 347347 Member Posts: 127
    I'll look it up.
    Thank you
  • IronmanIronman Member Posts: 6,176
    edited January 13
    "In series" meaning you only use 2pipes on the tank, one in and one out. As in, water comes in the bottom from the boiler and goes out of the top to the system.

    This is the simplest method and all that's really needed for a small single zone job like yours. It will also only need the boiler circ instead of a second.

    The system sensor in an option part that would tell the boiler the water temp coming out of the buffer tank to the system. It's not necessary if you pipe the tank in series as I described above. If you pipe the tank with 4 pipes, you're gonna need the sensor and an additional circ.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • 347347 Member Posts: 127
    Got it.
    Thank you
  • hot_rodhot_rod Member Posts: 14,425
    Here is my prefered way to pipe a small system buffer, this is a two pipe hydraulic separator piping. It allows the boiler to flow directly to the load on a design condition, or the system circ just pulls from the heated tank. It gives you the best use of the tank when it is needed and bypasses when it is not needed. It does require a second system circulator, and it does assure the boiler always has adequate flow with the internal circulator. AND it does require more piping.

    The system sensor from the boiler becomes the control and call for heat to the boiler.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
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