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temp controller for system pump reverse indirect radiant floor heating

Please advise with any tips, I'm in the design stage of my project would like to use a reverse indirect water heater as a buffer tank for a mod con boiler off the tank we have some radiators on one zone remaining zones are low temp staple up radiant with transfer plates I want the buffer tank to help prevent short cycling when call for heat comes from my smaller zones at the same time i want to keep water in the tank @ min 140F due to the fact that this is also my indirect what im looking for is that the system pump should turn off when water in the indirect hits below 140F this will allow boiler to recover and bring temp up any ideas on controller for such a config.


  • mattyc
    mattyc Member Posts: 44
    I would think you could wire in a multi-zone switching relay and make your pump from your boiler to the buffertank the priority zone. The other zones will sip heat from the buffer tank, then when the tank gets too cold, the priority circulator kicks on to re-heat the tank as the other circulators will turn off. Hope this helps
    HVACNUT Member Posts: 5,248
    Man that was some sentence. 
    If you shut off the system circulator but leave the boiler circulator running, its going to short cycle anyway, no? The boiler probably holds less than a gallon and if there's no load...
    Typically a reverse indirect is set to the same temperature as the domestic limit setting (180°) on the boiler. Then use mixing valves to temper down. Using the tank for both a buffer and domestic, how will you incorporate ODR with a domestic override?
    Does the boiler not offer 3 separate circuit programs?  
  • Zman
    Zman Member Posts: 7,467
    I don't think reverse indirects are a very good fit with mod/con boilers with systems that use low temp emitters like in-floor radiant and cast iron radiators. You lose the efficiency advantage of the mod/con by running it at 140+ temps. You would then need to mix down the water temps to the floor.

    A much simpler setup would use a conventional indirect and small buffer tank.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • hot_rod
    hot_rod Member Posts: 19,858
    I’d look at the performance table for the indirect to see how low you can run it and get adequate DHW, it’s the return temperature that keeps the mod con efficient in condensing mode

    If you get the reverse with the most amount of coil that is your best chance. TurboMax reps have a software to run calcs, know your incoming water temperature, boiler btu and how many gpm will be adequate
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • ambercontractors
    ambercontractors Member Posts: 12
    Shkoyach to all great forum btw 
  • Eyshr
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • Dave Carpentier
    Dave Carpentier Member Posts: 545
    edited July 2022
    Have you already picked a boiler ? Some (most ?) non-combi boilers have some type of DHW setup where you can send the DHW temperature water via a DHW circ out to an indirect. When DHW isnt calling, then the System circ (and lower system temp water) pushes out to wherever (buffer or not). The boiler doesnt need a separate outlet, as the flow direction is based on which circ is running from a common node.
    Im not doing indirect in my new install (yet), but it looks like the Lochinvar Epic would do this function quite well.
    You would need a separate buffer tank, instead of trying to double-duty the indirect.
    Im using a sideport 12 gallon regular water heater for my buffer.

    30+ yrs in telecom outside plant.
    Currently in building maintenance.
  • mattmia2
    mattmia2 Member Posts: 8,382
    Note that any well insulated electric water heater or steel buffer tank will work, it is a closed system so it won't rust like a water heater heating domestic hot water will.
  • ambercontractors
    ambercontractors Member Posts: 12
    Thank you all for the information and advise.
    I want to go with an indirect water heater for the DHW, because according to my calc the domestic load for hot water can exceed 10 GPM on the hot water for an 45 to 1hr straight when the whole fam is home that is heavy duty stuff. Turbomax does have some options so I am considering that, at the same time generally the DHW load is minimal and gen my main loads through the week will be the radiant heating. I have 4 zones and they are only servicing portions of the house (main floor, master suite), that being the case and due to the fact my boiler room is very small (literally dont have room for buffer and for indirect) I would like to go with using the indirect as buffer. The main concern, is as follows when my zones are calling i want to let them use up as much as possible the water from indirect before my boiler has to kick on, therefore i plan to wire aquastat to boiler swicth. But when and only when my domestic HW is being used I want to stop the system pump and only have the boiler pump on till the the indirect is brought up to temp. Im considering running a second set of leads from same aquastat through flow switch on the hot side of the near piping and then to priority on a taco box or such this will activate priority only when domestic water is actually being used. (Pheeeew that was long)
  • hot_rod
    hot_rod Member Posts: 19,858
    A 20 gpm DHW load from a reverse indirect will probably require 200,000 btu/hr or more depending on temperature rise. They store a small amount on the DHW side and behave more like an instantaneous once you start pulling. They are small gallon size, 29- 40 gallon on residential models, you size around the amount of copper coil inside, 3 ir 4 loops for example. A 23 would be 20 gallons 3 coils 24 would have 4 coils

    The tanks need to run hot 180F or higher for high performance. Best to use their software and define both sides, boiler and DHW
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • ambercontractors
    ambercontractors Member Posts: 12
    yes agreed i did that calc my boiler is 200K BTU so im within their parameters, thank you
  • ambercontractors
    ambercontractors Member Posts: 12
    Hey everyone!
    I had an interesting question, as i said i want to use a reverse indirect DHW with 2 pipe config. as my buffer tank and my hydraulic separation. From what i've seen the way to encourage the SH to take water from the buffer tank first is to install a pressure differential valve with a pressure drop of 2 psi on the boiler side. The only valves that i was able to find online were max 1-1/4" acc. to my load calc i would like to keep the piping sized to 1-1/2" i would like to know if i install 2 spring check valves in series would i accomplish the same thing each spring check is rated at 1 psi wouldnt this accomplish the same thing as pressure differential?