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BTU and pipe diameters...

rsilvers Member Posts: 182
edited April 2022 in Oil Heating
How big a deal is it to leave a 3 foot section of newly-installed components that are 1-1/4 size for a system that has a 1-1/2 inch boiler loop to a hydro-seperator of 1-1/2 size and is 180,000 BTU? The output of the hydro-seperator would pass through this smaller section for a few feet before branching to eight 3/4 inch zones.

My understanding is that as long as you can flow 18 GPM, things are good. I used a head-loss calculator and it says that changing the pipe size down for 3 feet raises the water velocity by 1.5 fps and has a cost of 0.15 in head loss as a result. That will make the pump work a small amount harder, but the BTUs should still flow. Am I correct? I think its only about a 300 BTU reduction for a given pump watts, and slightly more watts will restore the BTUs.


  • Jamie Hall
    Jamie Hall Member Posts: 21,541
    Quite right. The BTU flow is mass only -- and since water is incompressible, the pipe size is actually not relevant to the BTUs. What is relevant in terms of size is that a smaller size will have higher velocity and hence higher head loss (and taken to an extreme, noise and erosion problems). But a short section makes no difference to speak of.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • hot_rod
    hot_rod Member Posts: 19,924
    You are below the 4 fps guideline, so no problem. It’s just adding a small amount of head.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • rsilvers
    rsilvers Member Posts: 182
    I decided to replace it. While it is fine, it's always going to bug me to go from 1.5 to 1.25 to 1.5 again. While I am at it, I will change the stainless steel nipples to brass to reduce the chance of galvanic corrosion.