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  1. #1
    I got a quote from a well qualified dealer for a Rheem RGFD modulating furnace. It is replacing a Bryant #320AAZ042080AAKA 2-stage (79K/55K BTU out). He calculated (rule-of-thumb) that we need about 77K for our 2200 sq ft house. As such, he wasn't comfortable with the 75K Rheem, so bumped it up to the 90K BTU (85K out?). With the modulating (3-stage for our zoned system), I'm not too worried about the extra heating as it will likely run at 40-65% of capacity most of the time, but am concerned about the increase in blower size.

    My Bryant lists a 1/2 HP blower. I don't have airflow specs for that motor, but if anyone does, I'd appreciate the numbers. The 75K Rheem also has a 1/2HP blower and lists that at 1200 CFM max, so I'm guessing that's the ballpark I'm in with the Bryant. The 90K Rheem jumps to a 1HP blower, which lists a max of 2000 CFM and min of 1200 CFM. That's considerably more than I have now, which I know has benefits, but I'm concerned about the additional noise due to increaseed air velocity.

    In my system, the 2 main zones each have about 225 sq. in (total area) of round ducts running to various registers from main trunk lines. At 800 CFM (guess at low speed of current blower), that gives an outlet velocity of about 500 ft/min if only one zone is open and 250 ft/min if both are open. At 1200 CFM (guess at high speed of current blower), 770 ft/min for 1 zone or 385 ft/min for both. At 2000 CFM, I get 1280 ft/min for 1 zone and 640 ft/min for both.

    So, even at my minimum speeds (1200 vs 800 CFM), my velocity increases 50%. Will this be noticable and are these speeds considered acceptable?

    I raised this question with the dealer, and he wasn't concerned. We like how quiet the air delivery is with our current system and I want to make sure we don't do something we'll regret.



  2. #2
    Join Date
    Apr 2004
    First off, if your load was 77k, thats the heating requirement. The 75k furnace represents the furnace input, not the output. 75k yeilds a total capacity of 72,000 output. The 90 puts out about 86,000. What we dont know is what size your cooling is. This is rather important. In a zoning system, you can generally get away with something between the peak load and the highest load at any given time. so the 75 might work ok. Again, at this point I'd be more concerned with the cooling size. a 75k furnace will drive up to a 3 ton. the 90 will drive a 3 ton up to 5 tons.

    I'm not sure he read you the specs correctly. The 2000 CFM would be the highest fan speed in cooling mode IF you select a 5 ton drive. In heating the max fan speed on the 09 is 1195 cfm. The constant fan speed is 1600 or 800 cfm which is an installer set up function. It's preset to 800 CFM.

  3. #3
    docholiday - Thanks for the reply. It's certainly possible that I've got it wrong, but I appreciate any help straigtening me out!

    I looked at my A/C unit and no cooling capacity was listed. From the model number (Bryant 594CN030-A), I can only assume that the "030" is 3 tons. Does anyone know for sure?

    Assuming I do have 3 tons of cooling, help me fill in the corresponding fan speeds. My dealer told me these are pre-set and can't be changed in the field.

    Cooling @ 3 tons = ________ CFM

    Heating @ 100% = ________CFM

    Heating @ 65% = ________CFM

    Heating @ 40% = ________CFM

    Fan only = 800 or 1600 CFM



  4. #4
    Join Date
    Mar 2005
    Suppy NC
    it is a 2.5 ton unit rated at 1000 cfm
    have him do a real load and not rule of thunb
    i am not up on the rheem product so i will leave that to doc

  5. #5
    Join Date
    Sep 2002
    South Dakota

    When all is said and done have him take actual velocity and CFM measurements. Actual measurements are the real proof of the pudding.

    Fewer than half of all technicians even have the instruments necessary or the knowledge of how to take airflow readings. The ability of a contractor to take readings is "one" of the indicators as to the quality of their service.

  6. #6
    Join Date
    Apr 2004
    For the 90K....

    Cooling @ 2.5 tons = 1100 CFM (See below)

    Heating @ 100% = 1195 CFM (I'll get back to this too)

    Heating @ 65% = ? CFM (you can figure this)

    Heating @ 40% = 515 CFM

    Fan only = 800 or 1600 CFM

    Ok, there you go, you can probably figure the 65% close. The machine will calibrate itself based on your duct system and can be altered from there by either manual inputs or by the calibration mode.

    What I wanted to point out is the Cooling speed is not always higher than the heating speed. Too many techs think "high is for cooling and low is for heating" Thats a dangerous rule of thumb.

    Personally I would think the 75k would be a right choice based on what you told me, The 90 cannot slow the blower below 1200 cfm for cooling without some fancy footwork by the installer. He can make it go as low as 1100. But then you would lose some of the dehumidification functions and latent performance would suffer. The bonus would be 500 cfm for fan only. High fire would have 995 cfm, 40% would run at a super quiet 430 cfm. I also suspect the 17" cabinet of the 75k would fit your coil better than the 21" wide 90k.

    Also as Norm mentioned, you are not going to overcome duct issues with this or any other furnace. Your system is zoned so I hope the guy knows what he is doing. I would consider locking out w2 unless all zones are calling.

  7. #7
    Join Date
    Jun 2006
    If your existing 79K does or did an adequate heating job, then the 75K unit should meet the maximum heating load requirements, since it is much more efficient. Moreover, too much heating capacity is not only more costly and more inefficient, but may lead to duct issues since they were sized for the 79K output, not 90K
    I would verify the heating load by calculation or test* and have the esp and the flowrate of the existing ductwork tested at the high RPM to determine the ductwork adequacy BEFORE signing any contract.
    *For a given outside temperature, say T0 and a setpoint thermostat temperature, Ti and a design ouside temperature of say Td,the coldest anticpated otside temp, you should get an ideal duty cycle of about
    which I call f. Now if the actual fraction you measure is x,i.e.
    (furnace "on" time)/(total time of cycle). Note furnace "on " time means time of firing the furnace only.
    Now your heat load is approximately
    Q*x/f multiplied by the efficiency
    where Symbol* means multiply and Q is your furnace input heating rate.

  8. #8
    Doc - Thanks for the great feedback. I rechecked the literature and found the specs you reference. The difference between 995 & 1195 CFM (high) and 430 and 515 CFM (low), is less of a concern than I had originally thought, but I will still follow through with my dealer. The big difference to me is the 500 vs 800 CFM for fan only operation. The 90K running at 800 CFM in fan-only is in the same ballpark as the 65% heating fan speed for that same unit...seems high to me. Can I assume from the info you provided that the 75K unit (-07) has selectable fan-only speeds also (500 and 1000 CFM)?

    Also, are there guidelines for recommended air velocity in ductwork and vents? I hear alot of talk about CFM and over/undersized vents, but not much about air velocity. I have to believe there's some science behind proper duct sizing. My background tells me that the ducts should be sized for an air velocity range (feet per minute). Here's my take: a furnace stage needs a certain airflow (CFM - it's a fixed size box) to maintain an acceptable temperature rise across the heat exchangers. That airflow then flows into and is distributed among the various ducts. The cross-sectional area of those ducts will dictate the resulting air velocities. The smaller the duct, the higher the velocity and vice versa. The higher the velocity, the greater the pressure drop and more noise will be created. Thus, I would think guidelines for air velocity would be established from which ducts can be sized to handle certain blower air flows (CFM). If anyone can fill me in to let me judge where I will end up with either unit, I would appreciate it.

  9. #9
    Join Date
    Nov 2006
    Hamilton, Ontario

    duct velocities

    Maddog-Air velocity (cfm) is one of the driving factor in duct design. For residential duct work:

    Supply Return

    Main duct (max, fpm) 900 700

    Branch duct (max, fpm) 750 650

    It is a challenge keeping the velocities below these limits. Typically anything over 90 cfm to a room requires two supplies.

  10. #10
    Join Date
    Nov 2006
    Hamilton, Ontario

    last post

    Guess I should have previewed that before I posted it.
    The chart I had got compressed when it was posted basically though the numbers there are correct.

    Supply-max velocity in trunks and branches is 900 and 750 cfm. For returns around 700 for the main duct, and 650 for a branch is a good general rule of thumb.

  11. #11
    Join Date
    Mar 2004
    Airflow isn't the only question here.

    Even with a modulating furnace, having lots of excess capacity is not a good thing. They can only modulate down to 40% of their rated *burner* capacity; below that level they have to cycle on and off like everything else. You get your peak efficiency (and comfort) during long, steady runtimes, though, not by cycling on and off at your minimum capacity. By getting the smallest unit that will do the job, you are building a system that can spend more time in the zone where the furnace can actually modulate, and less time where it cycles on and off like a regular furnace.

    The 75k input unit can go down to a minimum output of 28,200 BTU; the 90k can go down to 33,840.

    If you were happy with your old furnace's ability to keep up in your coldest weather, though, even the 75k unit is too big. Your old furnace's output was 55,000 BTU on high and probably 35,500 BTU on low (two stage furnaces usually are 65% of full capacity on low stage). If you get the 75k input mod, you will have lots more capacity for the coldest weather, but the minimum output isn't much lower. So whenever your old furnace was cycling on and off, a 75k mod will generally also be cycling on and off. So there wouldn't be tons of effect from the modulating feature.

    The 60k input modulating furnace has the same full-power output (55,800 BTU) as your old furnace. But it can modulate all the way down to 22,560 BTU, about 1/3 lower than your old furnace can. That means that it will be able to do the quiet/slow/steady thing (the specialty of properly sized modulating furnaces) on milder days than your current furnace can.

  12. #12
    Join Date
    Oct 2006
    Emerald Coast, FL 30.1N 85.8W
    I haven't seen one comment on the total system pressure.

    Check whether your ducts will handle the extra velocity pressure, along with the static pressure of the AHU.

  13. #13
    Originally posted by wyounger
    Your old furnace's output was 55,000 BTU on high and probably 35,500 BTU on low (two stage furnaces usually are 65% of full capacity on low stage).
    I think you misread my initial post. My current furnace is 79,000 BTU output on high and 55,000 BTU output on low. Even with that, your point is valid that with the same size system (75K input in this case), you get a much lower, and more efficient output with the low end of the modulation. For a 75K Rheem, the low output is 28,200 BTU verus my current 55,000.

    Originally posted by dan_001978
    Supply-max velocity in trunks and branches is 900 and 750 cfm. For returns around 700 for the main duct, and 650 for a branch is a good general rule of thumb
    Dan- Thanks for the great information. My rough calcs show for the 90K (1195 CFM max) about 765 ft/min in the branches if only one zone is calling. For the 75K (995 CFM), that goes down to about 640 ft/min. Those are both pretty close to your guidelines, but the 75K puts me in better shape.

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