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rooftop a/c blower question

I would stay with 3 phase and also the belt drive option is a plus in my book. What is the tonnage on this equipment? Can the roof support the weight? Just a few Q you should ask yourself before proceeding. 3 phase cost less to operate and equipment cost is also less especially if you are at 5 tons or higher.

Mike T.


  • ddenny
    ddenny Member Posts: 75
    rooftop a/c blower question

    I'm looking to replace a rooftop unit installed on the ground with one that I want to put on the roof. the present a/c unit on the ground has a 3 phase belt driven indoor blower motor. I'd like the new one to have a direct drive motor so I would'nt have to worry about worn loose or broken belts. question is when would I need to choose a belt driven blower over a direct drive. thankyou
  • Brad White_9
    Brad White_9 Member Posts: 2,440
    Direct Drive versus Belt Drive

    In general but not as an absolute, direct drives tend to be smaller in size/capacity and belt drives tend toward the larger. Yes, there is considerable overlap.

    Belt drives give the manufacturer and installer (and specifying engineer for that matter) leeway into getting a standard product with room to adjust for field conditions. Fewer items to stock, units can have wider ranges for the change in belts and sheaves.

    Your field conditions pose challenges sometimes and this is where belt drives shine. What on paper calculated out to 1.25 inches external static becomes 1.75 inches due to some field conditions and extra fittings- with a belt drive, you have some recourse, within motor limitations of course. Speed change is your variable.

    With a direct drive fan, you have what you have. Absent a wider performance range of selection and a variable speed drive you may be sunk. You may have to change out a motor.

    So the question becomes, how confident are you in your external static pressure calculation (if new work going in)?

    In your case, a pre-construction balancning survey can eliminate the guesswork. A profile of airflow at a certain external static pressure will put you in an excellent place for new equipment selection. Then it is a matter of calculating the difference in ductwork to and from the new location.

    A variable speed drive on a direct drive system, even of a constant volume system, can compensate for filter loading and initial balancing setup quite nicely! You have also eliminated belt/drive losses.

    There are more considerations than the above, but these are the major ones I see going in.

  • ddenny
    ddenny Member Posts: 75

    the size of the unit is 5 tons. the diner where it's going has a wooden roof which may be why they put it on the ground in the first place. but they also atached what used to be the cargo part of a box truck attached it to the main part of the building and made it into a walk in box. that's where I was planning on putting the new rooftop which would mean extending the ductwork. the added ductwork made me wonder if I should go with the belt drive. plus the fact that the unit there now on the ground has a belt driven motor. the reason for wanting one on the roof is becausse the owner wants to use the area where the old one sits for a "take out" window.
  • Brad White
    Brad White Member Posts: 2,398

    is a company you may want to check out for aluminum-based air handlers. If weight is an issue I would start there. You may still have to reinforce, but the less weight the better.
    "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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