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Old 12-27-2007, 08:30 PM   #1
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new duct design in old house


hi guys first day in the forum . i like what you guys are doin here . anyways im 22 and just bought my first house here in providence rhode island. i want to install a new furnace and the neccesary ductwork that goes with it, i will only be installing the ductwork ads i know somebody that will do my furnace. i would love to do it myself and i have been online for the past three weeks trying to absorb as much information about hvac as possible. i've read the alpinehomeair pdf file like 10 times already . i have a two story single family with a very inefficient steam radiator system which is straining my budget tremendously. i drew a picture of what i plan to do in the basement and i included the drawing in the attachment. what i want to do is come off the plenum and go about two feet and then go to the attic so i can feed the seconf floor from ceiling register.the second floor will need 700 cfm since all the rooms come to about 700 square feet. i dont know what size duct i have to come out of the plenum to go to the attic and then branch off about (5) 6 inch round ducts.. fr the first floor i want to come out from where the furnace will be about 3 ffet and have a trunk line TEE to the right to feed two rooms on the right and to the left two feed two rooms on the left . i just want you guys that have some more knowledge to just look at the drawing and make suggestions. sorry for the long story
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Old 12-27-2007, 08:33 PM   #2
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on and by the way i just want to get the SUPPLY designed first i will focus on the return once im done with the supply
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Old 12-27-2007, 10:13 PM   #3
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The only way to design ductwork properly is to do a heat loss /heat gain and then design the ductwork accordingly.
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Old 12-28-2007, 08:08 AM   #4
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alright i did a detailed heat loss and heat gain calculation yesterday. i bought a program called hvac-calc residential 4.0 and i have my results for my house. i did a whole house calculation. for the first floor which has 1,023
square feet i have a total of 10,143 btu gain and 33,146 btu lost. for the second floor with 816 suare feet i have a 16,587 but gain and 34,303 heat loss btu. this was taking in account rooms,floor,walls,windows,doors,duct exposure,ceiling,roof,insulation and everything else. what do i do now with this information in regards to my orinal design for each rooms cfm need? thanks in advance
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Old 12-28-2007, 03:19 PM   #5
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Your manual j will tell you how many CFM are needed for each room. Now you need to do a Manual D to design your duct to get the proper CFM to each room.
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Old 12-28-2007, 03:37 PM   #6
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ok before i did the heat loss and gain for my house i did a cfm calculation per square foot of each room. now how do i incorporate the heat loss to the original equation of the cfm ... i hope im not loosing anybody with my question. lets say for my living room i had a 15*14 room totaling 210 so i would need 210 cfm but in that same room the heat loss is 6,804 btus . so now how would that change the already number of 210 cfm?
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Old 12-28-2007, 03:51 PM   #7
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In your hvac-calc there is a cfm per room calculater. Don't use CFM per square foot calculator as it sounds like a rule of thumb deal. In your program the calculator is in one of the upper pull down menu's.
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Old 12-30-2007, 04:01 PM   #8
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You can not design the ductwork until you know what size equipment is needed.you have 67,000 btu heat loss and 26,500 heat loss (2.5 tons/30,000btu= 1000cfm). You would select a 75,000 btu range and what was the total sensible load ?match the equipment to the sensible load to find your total cooling size Mfg specs sheetwill tell you this . could be 3 ton or 1200 cfmnot sure,. Once you know the tons cooling then you know the CFM total for house. The program will brak down cfm per area for. have you though about zoneing.
P.S double check your load calculation to bee safe.
good luck
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Old 12-30-2007, 04:36 PM   #9
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coolmen thanks for the reply this is exsactly what i wanted to know now. my total heat loss is 67,449 but/h and my sensible gain is 26,730 bth/h so i dont know if i need a 80,000 btu furnace or be safe with a 90,000 but i dont want to oversize as i read that is the biggest mistake. so i now need to find out which equipment i need and what is the total cfm of that equipment so i can layout the dctwork design. one question is the cfm of a furnace referred to as how many tons it moves? and multiply that by 400 ? thanks again guys
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Old 12-30-2007, 06:19 PM   #10
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70 to 80,000 btu is plenty I personally would install my 92% high effiancy furnace 70,000 btu. I would get a 2 stage gas valve to enhance the comfort level.The furnace needs to be able to move (1200 cfm for the cooling side). as for the sensible load my 3 ton condenser matched with my indoor evaporater coil will show me a sensible of around 26,000 sensible BINGO.
Keep in mind that RETURN air Is very emportant for your 2nd flooor
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Old 12-30-2007, 11:25 PM   #11
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gfhdfh
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Old 12-30-2007, 11:28 PM   #12
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thanks. what do you guys think about goodman furnaces and ac. i know they are not the elite in hvac but as long as it will do the job i will be happy and for the price that they are goin in internet deals i am tempted. should i get one with a 3 ton blower for 1200 cfm or 3.5 or 4 ton? and about the return air. is there a problem if i have my supply and return grills in the ceiling in the second floor since i wont be able to put them anywhere else?
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Old 12-31-2007, 08:14 AM   #13
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There isn't an issue with having ceiling supply and returns. As for Goodman, buy anything else, the furnaces are overly basic but the a/c is the cheapest built piece of junk out there. It's like buying a compressor in a tin can.
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Old 12-31-2007, 09:10 AM   #14
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3.5 tons would be a little oversized but will work. do not go with a 4 tons.the hvac calc 4.0 load program i use just about every other day on every one of my jobs. what was your indoor design temp? The problem i see often with oversized equipment is the larger the unit the more airflow is needed. Ive seen systems that need only 2 tons but baught a 4 ton on the enternet, it sounded like a jet engine was running in his house .horrible.As for basements I try to get the return low on the wall by the floor to help pull the cool air off the floor if possible. as mike said ceiling supply/returns are ok and done all the time. furnace may be capable of going from 1.5tons to 3 tons and probably from 3 ton up to 5 tons. the larger furnace will be wider and a little more money
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Old 12-31-2007, 10:38 AM   #15
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Remember that buying equipment on the internet, most will have no warranty. Go to Goodmans, Lennox, Rheems sites and read the disclaimers.
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