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Trane has 80k BTU S9X2 furnaces in 4 or 5 "nominal tons." What does "nominal tons" refer to?

ac21
ac21 Member Posts: 12
Trane offers its 80k BTU S9X2 furnaces in both 4 and 5 "nominal tons."

Model #'s: S9X2B080U4PSB and S9X2C080U5PSB.

What is the "nominal tons" referring to? (I thought 1 "ton" = 12k BTUs.)

We currently have a 3 ton AC unit that we are thinking of replacing with a heat pump (to create a hybrid/dual fuel system). We rarely use AC and the 3 ton AC has been more than sufficient for our cooling needs.

If pairing an 80k BTU Trane S9X2 with a 3.5 ton Trane XL16i heat pump would the 4 ton version of the furnace be the most appropriate match? Is there any reason an HVAC contractor would recommend pairing it with the 5 ton version of that furnace?

Comments

  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 13,149
    Nominal tons refers to the nominal air flow the furnace is capapable of delivering. 1 ton of cooling needs 400-450 cubic ft of air movement (CFM) so a nominal 4 ton furnace will move about1600-1800cfm and a 5 ton will move 2000-2250 cfm. The furnace BTU expressed is the heating BTUs

    A higher heating BTU output furnace will handle more AC than a lower rated heating BTU furnace.

    The right way to size the furnace is to do a load calculation on the house for heating and cooling. Then pick the furnace that will supply the heating load and provide enough air flow for cooling.

    In the northeast the cooling load is usually about 1/2 the heating load but that is only a rough rule of thumb. The contractor should also check on the static pressure (resistance) of the supply and return ducts when making the selection
    ac21
  • JUGHNE
    JUGHNE Member Posts: 10,322
    Your heat pump size would be based on the cooling size.
    You have a 3 ton AC now that meets the needs of the house.

    As ED said, you need to do a load calculation on your house.
    The 80,000 input may be more than you need.

    Some blowers may not slow down enough to handle 3 ton AC/HP.

    Oversizing the heating furnace will cause short cycling of the burner,
    worse that is that oversizing the AC will also cause short cycling of the AC.

    This can give you a cold and clammy feeling in your house.
    This leads to a lower tstat setting to overcool as one attempts to dehumidify the air.

    AC control is strictly based on temp, the quicker the temp is satisfied the less humidity is removed.
    You want longer run times for comfort.

    Drying the air is what you want for comfort.
    ac21
  • ac21
    ac21 Member Posts: 12
    edited August 2022
    Thank you very much @JUGHNE and @EBEBRATT-Ed ! Today I used the 15 minute method of calculating design heating load described here: https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler

    The results suggest that an 80k BTU furnace is more than I need.

    I get an implied heat load of 37,056 BTU/hr at 65°F, and 27,778 at 60°F. With the ASHRAE sizing factor 1.4, I get 51,879 BTU/hr at 65°F, and 38,889 BTU/hr at 60°F.
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 13,149
    Your #s do not seem to ad up to me. I would look for a more accurate calculation.

    Oversized equipment means unhappy heating and cooling. Oversized is not good Bigger is not better in this instance
    ac21
  • ac21
    ac21 Member Posts: 12
    @EBEBRATT-Ed I found an error in my calculation at 60°F.

    I get an implied heat load of 37,056 BTU/hr at 65°F, and 40,811 at 60°F.

    With the ASHRAE sizing factor 1.4, I get 51,879 BTU/hr at 65°F, and 57,135 BTU/hr at 60°F.

    This method suggests a heating appliance with an output between 50,000-60,000 BTU/h would be okay.
    Hot_water_fan