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How to temper a buffer tank in multi-temp system?

I'm currently roughing in for a multi-temp, multi-boiler residential install. We've been advised to wire the indirect HWT as a CH call, so that it'll still heat if the master boiler goes down(NTI TFT boilers; only heat a DHW call with the master). The system has 3 fan coils and 7+ manifolds of micro-zoned in-floor. The in-floor will be pulling from a buffer tank.

My question is, what is the best way to temper SWT down for the in-floor? Injection pump the boiler side of the buffer? Injection pump the micro-zoned, in-floor side of the buffer? OR, ditch "tank stratification" and mix the buffer tank?

I'm going to try to post a diagram of what I was thinking(mix the buffer tank). I haven't posted a photo to a forum in 15+ years.


  • GroundUp
    GroundUp Member Posts: 1,907
    Mixing valve(s) on the buffer tank outlet before the radiant zones is the most obvious solution
  • hot_rod
    hot_rod Member Posts: 22,118
    What is your primary goal with the buffer? handle the micro zones, extend run cycle, store enough to not fire boilers?

    The buffer could run at whatever the load is requiring via outdoor reset, or it could just maintain at 180, to cover DHW. The minimum tank temperature would depend on what those fan coils were sized for, SWT. Possibly 130- 140 minimum SWT?

    To maximize the mod cons it would be nice to size everything for 120 SWT at design. IF you need 180, you lose your condensing efficiencies.

    A two pipe buffer looks like this. When all loads are calling probably little if any flow into the boiler, it all goes to loads.

    As loads drop off the tank gets recharged until it reaches setpoint. On the next heat call flow is from the hot buffer to the load, boilers are controlled from the tank sensor. You are basically building a large water content boiler(s)

    Low temperature loads could be by a manual mixer that floats with ODR.
    A thermostatic valve.
    A motorized mixer
    injection mixing, not so common anymore.

    If you are going to run the system on ODR, a manually set mixer is super simple.
    Depending on the size of the low temperature loads and if you run the tank at setpoint 180, a thermostatic, or motorized mixer on it's own ODR curve.

    So some pros and cons
    A tank at high temperature give you the most drawdown with firing boilers, but cost you boiler efficiency, may as well use a cast boiler if that is the intent :) It would also allow DHW at any point in the heating cycle, assuming the boilers are sized for all loads running?
    If the boilers are sized just for heating on design day, you want to prioritize DHW call.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream