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sizing buffer tank for multiple sources

I've wondered this for a while and Hydronic Design Studio's buffer tank simulator does not allow for multiple heat sources.

So, how does one size a buffer tank for a multi-stage GSHP situation? A local engineering outfit that designed a couple of our recent large systems seems to size them quite large and i'm not sure why (I suppose I could ask them ;), example: 3X 5 ton GSHP's have 2X 120 gal. buffers.
As the system got closer to design conditions you would be using more heat and whatever heat pumps we're running would be getting longer run times, so I don't see how you could size it as one source. Also we are using tekmar staging controls that manage stage delay & vary the differntial depending on system load etc. which would also reduce the size of buffer required I would think.

Any thoughts on this?


  • kimko
    kimko Member Posts: 2
    buffer tank

    Purpose of a buffer tank is to prevent the 1st stage of the heat pump from short cycling. If the loads (zones) start to drop off you could have a situation where the rated GPM of the heatpump(5ton)are not met.Your job was probably calculated like this

    5ton= 60,000btuh
    30 deg temp rise
    Hope this helps
  • staging

    Hmmm, thanks for that info, interesting take on it. Our buffer tanks are piped like oversized low loss headers, so the heat pumps would never be lacking flow, also our staging control can have different rotation settings like first on last off etc. I still can't see a short-cycling condition occurring with a buffer tank sized for just the first stage.

    I did size one like this recently: a two compressor 6 ton GSHP w/30 gal. buffer, system is trv'd panel rads in basement & ducted fan-coil on main floor. I've been monitoring it for 3 months now and compressors get good long run & off times (5-6 minutes +) even in the milder weather so I'm not sure what to think.

  • bump!;0)
This discussion has been closed.