Nothing ever happens in a timely fashion with my projects. Heat and ac are no exception. A friend and I hung a Goodman furnace and coil assembly horizonally from the lvl's this weekend. The garage is fully insulated including the windows and doors. The duct run will run approximately 4' and make a left turn. That run will be approximately 34' long. My thoughts initailly were to start with 14" round (tapered from the rectangular outlet) and step it down at each of 3 louver vents to 8" for the last run to the 4th louver vent. Those 4 vents are spaced roughly 7' apart and blowing across a 21' span. I'm now thinking that the 4' run could be tapered down to a 12" elbow and then dropped to the 10" straight run of 34'. The vents could be damped to equalize air flow without restriction overall unit flow. Does that sound feasible to the knowledgable persons here? I'd like to get the heat up and running within the month as the winter sets in.
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Single diameter straight duct okay?
- Thread starter ezzzzzzz
- Start date
pseudorealityx
Well-known member
What size unit is this?
mrobins297aaa
Well-known member
if know the cfm of the unit you can use the following to size the ductwork:
0-100cfm = 6"
100-250 cfm = 8"
250-450 cfm = 10"
450-750 cfm = 12"
750-1100 cfm =14"
0-100cfm = 6"
100-250 cfm = 8"
250-450 cfm = 10"
450-750 cfm = 12"
750-1100 cfm =14"
mrobins297aaa
Well-known member
also if you know cooling capacity in tons, the industry standard is 400 cfm per ton, so if you have a 2 ton unit it will require 800 cfm +-, 3 ton would be 1200 cfm and so on.
you could use that to size the ductwork.
you could use that to size the ductwork.
Ohmthis
Well-known member
Post the entire model number of your furnace. We can use it to find the blower speeds and use them to help you. Are you planning on having A/C?
see attached pdf. If you post you complete model number, I can double check my numbers to make sure this still works.
top is what I would recommend. bottom would be the minimum of what I would recommend. These sizes are based on the fan static pressure with a typical rise in temp at start up.
going to small on your supply duct will kill your fan prematurely with the back pressure. Do you have a supply grille in mind? Is this layout what you were wanting, left then straight?
top is what I would recommend. bottom would be the minimum of what I would recommend. These sizes are based on the fan static pressure with a typical rise in temp at start up.
going to small on your supply duct will kill your fan prematurely with the back pressure. Do you have a supply grille in mind? Is this layout what you were wanting, left then straight?
Attachments
see attached pdf. If you post you complete model number, I can double check my numbers to make sure this still works.
top is what I would recommend. bottom would be the minimum of what I would recommend. These sizes are based on the fan static pressure with a typical rise in temp at start up.
going to small on your supply duct will kill your fan prematurely with the back pressure. Do you have a supply grille in mind? Is this layout what you were wanting, left then straight?
Reviving this thread. The unit is a GMH950453BXAE. The a/c is 1.5 ton (GSX130181 condenser and CHPF1824A6 air coil). I believe that 1200 cfm would be well on the high side and 800 cfm might be close to ideal. The layout above from mrgm was my original thought. Now I'm leaning to rectangular duct. Either way I'm now looking for the best deal on ductwork. A dollar saved is always spent somewhere else in my household.
OccupantRJ
Well-known member
Reviving this thread. The unit is a GMH950453BXAE. The a/c is 1.5 ton (GSX130181 condenser and CHPF1824A6 air coil). I believe that 1200 cfm would be well on the high side and 800 cfm might be close to ideal. The layout above from mrgm was my original thought. Now I'm leaning to rectangular duct. Either way I'm now looking for the best deal on ductwork. A dollar saved is always spent somewhere else in my household.
I would go with round ductwork, as I have done three times in the past. It is fairly inexpensive and easy to work with, and sections are easily purchased as needed. I started with 14 inches and dropped 2 inches in diameter for each pair of 6" outlets, for a total of 10 on an 1100 cfm unit. Duct sealer was used on all joints.
PWC Repair
Well-known member
1200 cfm is the maximum that furnace is designed to flow. That's 3 tons of air movement when the blower is on high. It should be set up to run on low speed for heating which would be closer to 800 cfm. I believe if you just ran 12" round pipe halfway and then reduced to 10" for the other 2 outlets it would work just fine since you're not flowing through much restriction at all. The supply registers will also need to be able to handle over 200 cfm each, which won't be a problem for a 220v type supply grille. The round pipe will be the cheapest and easiest to assemble. Since the ductwork will be in the "conditioned" area it won't need to be insulated but I'd do it anyway with insulation sleeves. If you know anybody that works for a heat and air company or church, school, handyman, city, county, etc you can get a better deal at the HVAC supply house. There's a good chance they have an account or will recognize your buddy and give you the discount.