jc-etech
01-28-2007, 07:45 PM
Hi all,
First off....warning long post!!!!
The bottom line question is whether to have the initial duct trunk sections modified to eliminate a restriction from the plenum to the ductwork. One contractor thinks it is necessary, others don't.
Second question is whether to use a 3 ton blower drive or a 5 ton blower drive.
Thanks in advance for all replies......JC
Continue reading for background info:
Having original 20 year old Bryant NG furnace & A/C replaced on our 2850 sq. ft. 2 story home in SW Ohio.
Original system is 120K Input furnace (75% eff ??) /42K (3.5t) AC (8 SEER ??) which still works, but I recently noticed heavy rust on/around tube headers at A-coil, so I know the A-coil is on death row (so to speak) and will only be a matter of time before a leak develops. 20 years old, builders grade system, I think I got my money's worth!
Had contractors in (6) and told them all the same thing:
1) matched 16 SEER system with 2 stage Copeland Scroll.
2) 95% eff furnace with V/S blower system
3) Existing system works GENERALLY okay, but 2nd story bedrooms are always slightly warm in summer, as we have resorted to ceiling fans and portable fans to assist.
4) Must do load calc and submit summary sheet along with quote.
5) Would REALLY like to cash in on the $300 A/C tax credit,
$150 furnace credit, and the $50 advanced math blower credit. Not however to the point of wasting money to get it, either now in the purchase price, or in the future in the form of energy costs from mismatching.
I have now narrowed the prospective pool down to 2, both quoting the same Lennox hardware:
XC16-036 outdoor unit
CX34-43C-6F cased A-coil with TXV
G61MPV-060-091 95% Eff. NG Furnace <-- Note 5 ton blower
Also:
Honeywell VisionPro IAQ stat w/outdoor & indoor remote sensors.
new filter rack, 20x20 filters
Adaptations of existing duct sys. to New Plenum as reqd.
Contractor #1- Believes duct mods are needed at plenum/ duct interface.
Contractor #2- While existing duct/plenum connections are not ideal, thinks that duct mods are overkill and that we would be wasting our money on something that we would never perceive as a real benefit.
Load calc info per Wrightsoft:
Cool Sens: 28K Latent: 5K Total: 33K at 95/75deg F.
Moisture grains: 34
Heating : 75K at -5/70 deg (Farenheit)
Airflow (cooling) 1319CFM
The Indoor equipment is down in the unfinished basement (which is only heated and cooled parasitically). The upflow furnace has the A-coil on top, return air is coming into the side of the blower cabinet, currently thru a 16x25x1 filter (3M, 1250 rated). All this sits roughly in the center of the house and has a pair of 14 X 8 metal ducts running lengthwise almost the entire 40ft of the long direction of the house, one duct forming the supply, the other the return. In addition to this, a single 6" round duct connects directly to the front of the plenum, 90degrees from the east and west 14 x 8 trunks mentioned before.
2/3rds of the upstairs takeoffs come off the east trunk, the downstairs takeoffs are somewhat more evenly distributed between east and west. The aforementioned 6" round coming direct off the plenum goes to a single register beside the front door. This area opens up to a 2 story atrium consisting of open stairs and open railing around the second floor balcony.
Returns are up near the ceiling in the interior wall of each room and they all seem to be drawing well. They use a metal-lined stud cavity/floor joist cavity as able to tie into the 14 x 8 return trunk. Supply registers are on the floor usually near windows on the outside walls. Most supply stacks are 5 x 12 going to the second floor. All ducts have manually adjustable dampers near the takeoff from the trunks. In addition most of the registers are adjustable.
In the summer, we will shut down most 1st floor registers and open up the 2nd floor ones. In winter, the opposite. We have never had a problem with insufficient heat, but in the summer as mentioned before, the upstairs never seems to be as cool as we would like. The system seems to blow a decent amount of air, and we wouldn't say it is clammy. Just warmer than we would like.
The changes proposed by contractor #1 involve enlarging the 1st 5 feet or so of the trunks with larger ducts (if memory serves me, 14 x 12 or so), where the ducts take-off from the plenums, as well as catching a few of the initial branch takeoffs . This would reduce the restriction that may be present at the plenum/trunk interface, thereby allowing the system to move a higher volume of air ultimately to the second floor with less static pressure.
I have tried to reconcile this in my mind and did a quick calculation:
Assumptions:
3 ton X 400 cfm/ton = 1200 cfm as an example from blower
If plenum is 19 x 19: 361 in^2 at 1200 cfm = velocity of 480fpm
Initial 6" round duct carries 110cfm at 550 fpm straight off of plenum, leaving 1200-110=1090 for the both trunks.
Each trunk must carry 1090/2= 545cfm.
If trunk is 14 x 8: 112 in^2 at 545cfm = velocity of 699fpm.
Therefore anything over 1200 cfm will result in greater than 699fpm at each trunk/plenum interface. This also incorrectly assumes that the two trunks can be balanced without incurring any loss of comfort in the upstairs (again, remember that 2/3rds of the second floor runs originate from the east trunk).
At this point, even though only one contractor recommended the duct changes, I am starting to believe they are appropriate.
I wholeheartedly concede that this is beyond my field of expertise (I design/program/debug special purpose automated machinery for a living), and I may be all wet with my numbers.
I expected more load calcs to be performed, but out of the 6, only 3 included them, and of these, only two did what I would have deemed acceptable. Forget about getting a full blown manual D!!
As you can see, I wasn't EVEN trying to cut corners.
Second question if you're still reading this :)
Both vendors quoted the 5 ton blower drive furnace, Model G61MPV-060-091. This is partly because of my quest to get the $150 tax credit for 95% efficiency. For some reason, Lennox doesn't make a 'G61MPV-036C-091'. The closest thing is the 'G61MPV-036C-090', which falls a couple tenths of a percentage point short of the magic 95% number.
Pro's for the 5 ton blower drive:
At the reduced speeds in this application, the wattage required by the motor will be well below the maximums it is capable of, thereby increased life expectancy. Since it is electronically commutated, the usual loss of efficiency doesn't seem to apply here.
Cons for the 5 ton blower drive:
At the lower cooling stage, the lowest setting for the blower is 870CFM on low cool. Since 2 tons of cooling X 400 CFM/ton = 800CFM, this would seem to be more airflow than ideal for dehumidification. Maybe this could be remedied by utilizing some way to drive the blower with a PWM signal as used by a Harmony II or CCB1 control. If I remember correctly, this allows the blower speed to be reduced to at least 50% High cool speed.
Pro's for the 3 ton blower drive:
More appropriate airflow settings are easily attainable without additional hardware for low cool. Also the ECM motor is less expensive should it need to be replaced.
Con's for the 3 ton blower drive:
Motor is being driven 'harder', and less headroom may be available if/when necessary to push air into second floor (not saying this is the correct way to go about this).
Again, if you're this far down, I thank you for your input!!
-JC
First off....warning long post!!!!
The bottom line question is whether to have the initial duct trunk sections modified to eliminate a restriction from the plenum to the ductwork. One contractor thinks it is necessary, others don't.
Second question is whether to use a 3 ton blower drive or a 5 ton blower drive.
Thanks in advance for all replies......JC
Continue reading for background info:
Having original 20 year old Bryant NG furnace & A/C replaced on our 2850 sq. ft. 2 story home in SW Ohio.
Original system is 120K Input furnace (75% eff ??) /42K (3.5t) AC (8 SEER ??) which still works, but I recently noticed heavy rust on/around tube headers at A-coil, so I know the A-coil is on death row (so to speak) and will only be a matter of time before a leak develops. 20 years old, builders grade system, I think I got my money's worth!
Had contractors in (6) and told them all the same thing:
1) matched 16 SEER system with 2 stage Copeland Scroll.
2) 95% eff furnace with V/S blower system
3) Existing system works GENERALLY okay, but 2nd story bedrooms are always slightly warm in summer, as we have resorted to ceiling fans and portable fans to assist.
4) Must do load calc and submit summary sheet along with quote.
5) Would REALLY like to cash in on the $300 A/C tax credit,
$150 furnace credit, and the $50 advanced math blower credit. Not however to the point of wasting money to get it, either now in the purchase price, or in the future in the form of energy costs from mismatching.
I have now narrowed the prospective pool down to 2, both quoting the same Lennox hardware:
XC16-036 outdoor unit
CX34-43C-6F cased A-coil with TXV
G61MPV-060-091 95% Eff. NG Furnace <-- Note 5 ton blower
Also:
Honeywell VisionPro IAQ stat w/outdoor & indoor remote sensors.
new filter rack, 20x20 filters
Adaptations of existing duct sys. to New Plenum as reqd.
Contractor #1- Believes duct mods are needed at plenum/ duct interface.
Contractor #2- While existing duct/plenum connections are not ideal, thinks that duct mods are overkill and that we would be wasting our money on something that we would never perceive as a real benefit.
Load calc info per Wrightsoft:
Cool Sens: 28K Latent: 5K Total: 33K at 95/75deg F.
Moisture grains: 34
Heating : 75K at -5/70 deg (Farenheit)
Airflow (cooling) 1319CFM
The Indoor equipment is down in the unfinished basement (which is only heated and cooled parasitically). The upflow furnace has the A-coil on top, return air is coming into the side of the blower cabinet, currently thru a 16x25x1 filter (3M, 1250 rated). All this sits roughly in the center of the house and has a pair of 14 X 8 metal ducts running lengthwise almost the entire 40ft of the long direction of the house, one duct forming the supply, the other the return. In addition to this, a single 6" round duct connects directly to the front of the plenum, 90degrees from the east and west 14 x 8 trunks mentioned before.
2/3rds of the upstairs takeoffs come off the east trunk, the downstairs takeoffs are somewhat more evenly distributed between east and west. The aforementioned 6" round coming direct off the plenum goes to a single register beside the front door. This area opens up to a 2 story atrium consisting of open stairs and open railing around the second floor balcony.
Returns are up near the ceiling in the interior wall of each room and they all seem to be drawing well. They use a metal-lined stud cavity/floor joist cavity as able to tie into the 14 x 8 return trunk. Supply registers are on the floor usually near windows on the outside walls. Most supply stacks are 5 x 12 going to the second floor. All ducts have manually adjustable dampers near the takeoff from the trunks. In addition most of the registers are adjustable.
In the summer, we will shut down most 1st floor registers and open up the 2nd floor ones. In winter, the opposite. We have never had a problem with insufficient heat, but in the summer as mentioned before, the upstairs never seems to be as cool as we would like. The system seems to blow a decent amount of air, and we wouldn't say it is clammy. Just warmer than we would like.
The changes proposed by contractor #1 involve enlarging the 1st 5 feet or so of the trunks with larger ducts (if memory serves me, 14 x 12 or so), where the ducts take-off from the plenums, as well as catching a few of the initial branch takeoffs . This would reduce the restriction that may be present at the plenum/trunk interface, thereby allowing the system to move a higher volume of air ultimately to the second floor with less static pressure.
I have tried to reconcile this in my mind and did a quick calculation:
Assumptions:
3 ton X 400 cfm/ton = 1200 cfm as an example from blower
If plenum is 19 x 19: 361 in^2 at 1200 cfm = velocity of 480fpm
Initial 6" round duct carries 110cfm at 550 fpm straight off of plenum, leaving 1200-110=1090 for the both trunks.
Each trunk must carry 1090/2= 545cfm.
If trunk is 14 x 8: 112 in^2 at 545cfm = velocity of 699fpm.
Therefore anything over 1200 cfm will result in greater than 699fpm at each trunk/plenum interface. This also incorrectly assumes that the two trunks can be balanced without incurring any loss of comfort in the upstairs (again, remember that 2/3rds of the second floor runs originate from the east trunk).
At this point, even though only one contractor recommended the duct changes, I am starting to believe they are appropriate.
I wholeheartedly concede that this is beyond my field of expertise (I design/program/debug special purpose automated machinery for a living), and I may be all wet with my numbers.
I expected more load calcs to be performed, but out of the 6, only 3 included them, and of these, only two did what I would have deemed acceptable. Forget about getting a full blown manual D!!
As you can see, I wasn't EVEN trying to cut corners.
Second question if you're still reading this :)
Both vendors quoted the 5 ton blower drive furnace, Model G61MPV-060-091. This is partly because of my quest to get the $150 tax credit for 95% efficiency. For some reason, Lennox doesn't make a 'G61MPV-036C-091'. The closest thing is the 'G61MPV-036C-090', which falls a couple tenths of a percentage point short of the magic 95% number.
Pro's for the 5 ton blower drive:
At the reduced speeds in this application, the wattage required by the motor will be well below the maximums it is capable of, thereby increased life expectancy. Since it is electronically commutated, the usual loss of efficiency doesn't seem to apply here.
Cons for the 5 ton blower drive:
At the lower cooling stage, the lowest setting for the blower is 870CFM on low cool. Since 2 tons of cooling X 400 CFM/ton = 800CFM, this would seem to be more airflow than ideal for dehumidification. Maybe this could be remedied by utilizing some way to drive the blower with a PWM signal as used by a Harmony II or CCB1 control. If I remember correctly, this allows the blower speed to be reduced to at least 50% High cool speed.
Pro's for the 3 ton blower drive:
More appropriate airflow settings are easily attainable without additional hardware for low cool. Also the ECM motor is less expensive should it need to be replaced.
Con's for the 3 ton blower drive:
Motor is being driven 'harder', and less headroom may be available if/when necessary to push air into second floor (not saying this is the correct way to go about this).
Again, if you're this far down, I thank you for your input!!
-JC