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flow rates, heat extraction

hot_rod Member Posts: 22,158
First off their output claims tend to be a bit lofty. For some reason they all spec 1" pex regardless of the claimed BTU output??

You are far from moving 160K through 1" pex. Even with a 40 degree delta t you would need 1.25 tube.

About 100- 120K at a 20 degree delta T would be more realistic. Do they really need 160K? Is that the building load from a current and accurate heat loas calc? I'd start with a new load calc and size from there.

Also the temperature drop from OWF to the building can be huge if the tubes were not insulated in a good water proofed sleeve. Just to beware :)

hot rod
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream


  • [Deleted User]
    flow rates, heat extraction

    I may take a job working on a outdoor wood boiler, The unit is 110 feet from the house. It is connected to a 1 1/4 e-z header in the bacement. It's 1 in. pex to and from the unit to the house. It has a 0011 at the wood boiler. They want to put a 160,000 btu heat exchanger in the plennum of a old hot air furnace an use the fan to get heat to the house. Am I wrong to assume that I would need 16 gpm of 180 degree water to this heat exchanger to get 160,000 btu's to the house? The e-z header has 3/4 in. nipples. 3/4 pex can only handle 8 gpm? Also sence the pex to and from the wood boiler is only 1 in. can I assume if 1 in. pex has a max flow rate of 12 gpm we can only get 120,000 btu's total to the house? this is an open system not under pressure.
  • Brad White
    Brad White Member Posts: 2,398

    There is no magic to the 1 GPM = 10,000 BTU rule. That is for a 20 degree delta-T.

    Sounds like you have the option of adding whatever coil (heat exchanger) you like, so select it for the capacity you want at the temperature and delta-T that you want.

    In other words, the 160 MBH listed capacity might well be at 180F in, 160F out and 16.0 GPM. It may well be only one or two rows.

    A deeper coil (more rows in airflow direction- could be 3, 4, 5, 6-rows for heating), lower face velocity and full face circuiting can get you what you want and can operate at lower or higher flow rates, (P/S), lower inlet temperatures and wider delta-T's.

    I would select the coil to work starting with the flow rate you can deliver (if piping seems to be the limitation here).

    I would also probably select the coil to accept 140F entering water which will allow you to run the boiler above dewpoint (if a concern) and have enough delta to allow a mix-down for the coil.

    If you limit yourself to 12 GPM delivery, that is just under a 27 degree drop. I would go for a 30-35 degree drop to be safe (9-11.0 GPM roughly) on the boiler circuit and set it up primary-secondary if condensing at the boiler is a concern, as stated. Run the boiler at the temperature you want and mix-down for the furnace coil.

    I like TSI Series 58 coils but there are others which can be custom or semi-custom made.

    The entire system should be checked for thermal balance, specifically that your boiler water, coil water, entering air and coil leaving air (air flow rate in CFM is a factor here) are all in harmony.

    Sounds like a lot but is entirely and routinely workable. BTU's are BTU's. You can deliver them at a variety of temperatures.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
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