Hi,
I'm in the process of replacing a 25-year old Goodman furnace+A/C here, in NJ, and current specs are 80% 100,000 BTU heating and 3-ton A/C, for a 2000-2900 sqft house (depending on whether you count the finished but not heated basement or not).
The furnace is in the finished basement, ducts are running under the basement ceiling non-insulated, and up through various walls (some external) to the registers on the first and second floor. There are no registers in the basement, but the door to it is kept open at all times. Return plenum is only 25"x10", and there are two return grills in the house: one on the first floor and one on the second, each is about 13"x19".
Current system seems oversized, no matter how I slice the load calc, i.e. treat the basement as unconditioned and then take a hit on the duct load, or say that the basement belongs to conditioned space and then get penalized due to the increase of square footage. Since no contractor provided load calc of their own (some just chose to match current furnace, some lowered it to 80,000), I'm leaning based on the attached load calcs toward going for a 66,000 furnace and 2.5-3 A/C, but I am hitting some static pressure issues here.
With my currently running 4-speed Goodman, an anemometer and static pressure meter I was able to measure duct performance at various furnace speeds, and they're like this:
1300 CFM
Return: 0.20"
Supply: 0.18"
Furnace: 0.65"
Filter: 0.22"
Coil: 0.05"
1000 CFM
Return: 0.16"
Supply: 0.16"
Furnace: 0.54"
Filter: 0.18"
Coil: 0.05"
850 CFM
Return: 0.10"
Supply: 0.12"
Furnace: 0.34"
Filter: 0.12"
Coil: 0.03"
For some reason current coil registers very low pressure drop of 0.05" at 1300 CFM, whereas specs for all new replacement coils are generally around 0.23" for 17.5"-wide plenum. This makes me worried because replacing the coil will make furnace static at 1300 CFM even higher, i.e. 0.83".
There were multiple options thrown around from the contractors and my sides including 17.5"-wide, 21"-wide 70,000 and 90,000 BTU two-stage variable blower speed Carrier furnaces in an attempt to match load calc more accurately and perhaps lower airflow in order to avoid overloading ECM blower. But for some reason ALL of them will want to blow high heat at 1300 CFM, which, given the high I think will put the static pressure across the furnace with the new coil at 0.83".
I frankly don't quite understand why a 70,000 BTU furnace with 3 torches will want to blow high heat air at the same CFM as a 90,000 BTU furnace with 4 torches. And there is no way to adjust the heat blower, as per the furnace manual. But all of the furnaces will happily reduce blower rate for 1st stage heating.
I feel like I have couple of options here in order to deal with the static (duct changes aside):
1) Go for 70,000 BTU 21"-wide furnace and coil in order to reduce coil pressure penalty from 0.26" to 0.18". It will run at 1255/1160 high/low heat CFM, and will have a 1hp ECM, hopefully it won't burn as fast.
2) Pick a 90,000 BTU 17.5"-wide 1/2hp ECM furnace (a 21"-wide 90,000 BTU would have much higher non-adjustible high heat blower rates) and expect it to run mostly in first stage at 980 CFM vs 1200 CFM high)
Are there other ways to tackle the high pressures here (like find a furnace with PSC motor, find a furnace that will not want to blow 1200 CFM for 70,000 BTU)?
Are there drawbacks to running 90,000 BTU furnace in low speed heat all of the time in an attempt to reduce CFM (drawbacks like condensation, incomplete combustion, etc)?
Thanks!
I'm in the process of replacing a 25-year old Goodman furnace+A/C here, in NJ, and current specs are 80% 100,000 BTU heating and 3-ton A/C, for a 2000-2900 sqft house (depending on whether you count the finished but not heated basement or not).
The furnace is in the finished basement, ducts are running under the basement ceiling non-insulated, and up through various walls (some external) to the registers on the first and second floor. There are no registers in the basement, but the door to it is kept open at all times. Return plenum is only 25"x10", and there are two return grills in the house: one on the first floor and one on the second, each is about 13"x19".
Current system seems oversized, no matter how I slice the load calc, i.e. treat the basement as unconditioned and then take a hit on the duct load, or say that the basement belongs to conditioned space and then get penalized due to the increase of square footage. Since no contractor provided load calc of their own (some just chose to match current furnace, some lowered it to 80,000), I'm leaning based on the attached load calcs toward going for a 66,000 furnace and 2.5-3 A/C, but I am hitting some static pressure issues here.
With my currently running 4-speed Goodman, an anemometer and static pressure meter I was able to measure duct performance at various furnace speeds, and they're like this:
1300 CFM
Return: 0.20"
Supply: 0.18"
Furnace: 0.65"
Filter: 0.22"
Coil: 0.05"
1000 CFM
Return: 0.16"
Supply: 0.16"
Furnace: 0.54"
Filter: 0.18"
Coil: 0.05"
850 CFM
Return: 0.10"
Supply: 0.12"
Furnace: 0.34"
Filter: 0.12"
Coil: 0.03"
For some reason current coil registers very low pressure drop of 0.05" at 1300 CFM, whereas specs for all new replacement coils are generally around 0.23" for 17.5"-wide plenum. This makes me worried because replacing the coil will make furnace static at 1300 CFM even higher, i.e. 0.83".
There were multiple options thrown around from the contractors and my sides including 17.5"-wide, 21"-wide 70,000 and 90,000 BTU two-stage variable blower speed Carrier furnaces in an attempt to match load calc more accurately and perhaps lower airflow in order to avoid overloading ECM blower. But for some reason ALL of them will want to blow high heat at 1300 CFM, which, given the high I think will put the static pressure across the furnace with the new coil at 0.83".
I frankly don't quite understand why a 70,000 BTU furnace with 3 torches will want to blow high heat air at the same CFM as a 90,000 BTU furnace with 4 torches. And there is no way to adjust the heat blower, as per the furnace manual. But all of the furnaces will happily reduce blower rate for 1st stage heating.
I feel like I have couple of options here in order to deal with the static (duct changes aside):
1) Go for 70,000 BTU 21"-wide furnace and coil in order to reduce coil pressure penalty from 0.26" to 0.18". It will run at 1255/1160 high/low heat CFM, and will have a 1hp ECM, hopefully it won't burn as fast.
2) Pick a 90,000 BTU 17.5"-wide 1/2hp ECM furnace (a 21"-wide 90,000 BTU would have much higher non-adjustible high heat blower rates) and expect it to run mostly in first stage at 980 CFM vs 1200 CFM high)
Are there other ways to tackle the high pressures here (like find a furnace with PSC motor, find a furnace that will not want to blow 1200 CFM for 70,000 BTU)?
Are there drawbacks to running 90,000 BTU furnace in low speed heat all of the time in an attempt to reduce CFM (drawbacks like condensation, incomplete combustion, etc)?
Thanks!
source https://hvac-talk.com/vbb/threads/2243334-Replacing-system-help-me-pick-width-blower-and-BTU?goto=newpost
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