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Primary delta t 40 degrees?

McSteamer22 Member Posts: 23
We all know most mod con boilers have a primary delta t of 40° Because it’s close to 200k btu of output but with only 1” tappings,  I’ve read pri/sec pumping, classic hydronics, searched many Other materials and I’m having a hard time seeing it.  Does anyone have any detailed explanation or simple one for that matter on where the 40° of shedding is taking place?
thanks in advance 


  • Jamie Hall
    Jamie Hall Member Posts: 15,934
    Presumably you are talking about primary/secondary here? If so, the secondary (building loop) is taking water from the primary loop at some reasonable temperature from the boiler -- but not as high a temperature as the boiler water, and running it though the secondary loop. When it comes back it will be at some lower temperature (one usually shoots for 20 degrees, but it doesn't really matter). Some of that water is recirculated in the secondary loop. Some of it is mixed back into the boiler loop. Depending on the relative flows and the relative temperatures, you can get almost any delta T you want in the boiler loop.

    200,000 BTUh at a delta T of 40 is only 10 gallons per minute, which isn't that much for a short bit of 1 inch pipe to take.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    EBEBRATT-Ed Member Posts: 9,086
    Mod cons usually run with a high delta. On most of them the Heat exchangers are very restrictive on the water side and it take a lot lager circulator to run with a lower TD
  • McSteamer22
    McSteamer22 Member Posts: 23
    Thanks @Jamie Hall
    i get how the primary has the luxury of not needing as much flow as your system delta t, I know gpm = delta t x 500 all day I guess I was Just looking For some concrete math.
    it seems that because the primary is  “injecting” that you could just take your System delta t and multiply it by 2.
    I appreciate all of your input, I work with Adam who you had spoke with a few weeks ago about crossovers traps and other components. (Somehow you have become the new dan 🤫)
  • SuperJ
    SuperJ Member Posts: 605
    All things being equal, it's better to have a lower entering water temp and high delta T, than low delta and a hot return. The low delta is generally achieved by over-pumping the primary which means the cold return gets diluted with hot supply and mixed upwards. This kills efficiency. Better to under-pump the primary and dilute the hot supply down to the desired temperature.

    Modern modcons (Like some Lochinvar, and IBC) have variable speed primary pumps that track the boiler modulation %(load), this should help to match primary and secondary flow, especially when used in conjunction with a system (secondary supply) temp sensor for control.

    The number 1 mistake (beside fundamentals) that I see on modcon installs is often related to overpumping. The primary flow should be less than the secondary, better to raise the delta and keep the flow low. (max delta and min flow requirements need to be respected).

    Commercially a lot of the high mass boilers I work on don't have min flow or max delta limitations any more. I often see 80F return with up to 170F output in cascade systems.
  • hot_rod
    hot_rod Member Posts: 14,797
    The manufacturers typically show the acceptable delta across the HX in their installation manual, 40° would be on the high side. I know many will also show the flow rate and sometimes even the circulator required to assure that flowrate and deltas. They will also have a sensor in S&R that can check and know that delta and respond it it gets too wide, basically modulates the burner.
    It is a balance between efficiency and stress to the HX.
    And of course you still need to provided the required SWT to the heat emitters within that delta :) It's great that the boiler might leverage an 80 120F range, but if the load requires 120F??
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
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