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Manual J follow up question

Gwisejr | Posted in General Questions on

Good morning, 

I have a follow up question from my earlier questiona about Manual J

The HVAC Vendor finally recalculated with all the correct parameters and we went from needing 3 Tons on main and 2 tons on 2’nd floor to 1.5 Tons on main and 1.1 Tons on 2’nd floor.

The question I have now is concerning the ducting. They had already installed the ducting and the incorrect units. How likely is it that the ducting will need to be changed?  What info do I need to ask them to provide in order to confirm that the ducting does or does not need to be modified?


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  1. Expert Member
    Peter Engle | | #1

    If the ducting was properly designed for the larger systems, you shouldn't have a problem. Lower flow rates than designed conditions seldom cause problems. The most likely difference with be a quieter system with less wind noise. It is possible that the registers won't "throw" the air far enough with lower velocities, but that can be adjusted for, if necessary, with selection of register types/sizes.

  2. Gwisejr | | #2

    Thanks Peter, I was hoping that would be the case. It was hard enough to get them to correct the inputs for the manual J and most of all to get them to use the 1% mean temp instead of the 0.4% mean.

  3. Gwisejr | | #3

    Well, it seems that the sticking point might be that in their original manual J, they did not orient the house correctly and they basically said that they oriented it to maximize the load. Looking at the differences between the first and the most recent calculations, I would think that they would need to revisit the ducting. Im seeing some large changes in the per room CFM requirements . Not sure what percentage change would necessitate a reconfiguraton of the ducting.

    Any insight would be appreceiated.


  4. Jon_R | | #4

    You need to review the before and after Manual D data.

    I'm interested in how you plan to re-balance the system to achieve the vastly different heating/cooling CFM numbers every time you switch between heating and cooling.

  5. Gwisejr | | #5

    I was also wondering that. I've never seen any sort of balancing system in any of the houses that i've lived in that would account for different heating and cooling loads. And I would assume that every house is going to have some rooms there there is a large difference. Especially the kitchen area I would think..

  6. Expert Member
    Peter Engle | | #6

    You're probably not going to rebalance the system for heating and cooling, especially if it is zoned separately for the first and second floors. With the doors open, you won't notice that much difference in most houses. Some rooms might be a little warmer/cooler in one season or another. If necessary, you can adjust the dampers at the registers to tweak the temperatures seasonally.

    As to whether any adjustment to the ducting is necessary, that requires a Manual-D calculation, as mentioned above. With generally oversized ducts, you might get lucky and only dampers in the ducts are needed to throttle down any seriously oversized ones. These would be adjusted once during commissioning and then left that way.

  7. Jon_R | | #7

    An open door will move about 400 btu/hr with a 1C difference. This is rarely enough to fix the duct imbalance problem.

    If you want more comfort, you need a zoned system with a separate thermostat in most rooms. This is easy with hydronic systems.

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