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Hi .i have a ranch just under 1000 sq.ft with a 75000 btu furnace (york diamond 90). I had a guy come out and check it cause it wasnt working right. It kept shutting off. This was due to not enough cold air return. I was getting a limit switch open diagnostic code. The main cold air return trunk is buckling bad cause it is so starved for air. So to fix the problem temporarily I keep my basement door open so the air can get to the unit. There is an opening in one of the (I think 8x12) return ducts that is sucking the air in from upstairs. On top of that he says my unit isnt putting out half the btus that it is rated for. So he looked at the ductwork. There are (2) 6" tees coming off the plenum on one side that are reduced to 5" supply lines branching off to 4 rooms. On the other side there is a 8x10 duct feeding 4 more 5" supply lines (2 to the upstairs and 2 open blowing into the basement). He said he could help fix the btus problem by separating the lines and making them their own individual lines instead of being in tees. So he would cut into the plenum and redo some of the branches. Not sure exactly how but it was something like that. He said this would avoid a major reconstruction of ductwork (which should be 6"). He also said he could shut down one of the burners on the furnace reducing the btus and he had done that before. He would do it for $150. that seemed reasonable to me. I have no clue about this stuff. I would just like some input/comments on this job and if I should do it? I dont want my furnace to die early. It is only a few years old. Obviously a moron did the ductwork (previous owner). If I am posting to the wrong thread please let me know and I'll repost it somewhere else. Thanks a million!
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It sounds like this guy is being fair and honest with you. Keeping in mind that a furnace blower is like a water pump it is much easier to push water than to pull it. A blower is the same way. So if he has 4-6" pipes on one side for hot air and
the 8x 10 with 4-5" on the other side you can probably produce enough static
pressure to make the hot air work, as long as you get the return air up to the proper size. The minimum return should be at least 168 SQ". You said your return is 8x12= 96 SQ", so you have to get that up.
When you plug off one burner you will be putting undue expansion and stress
on the heat exchanger. This is not a good idea. If you get the return air up and change the hot like you are saying, then check the gas pressure to make sure you are not over firing it. Most furnaces are make to operate at about a 70 degree temperature rise across the heat exchanger. So when you get done check the temperature rise and see how close you are.. If you have to you can up the blower speed to develope a little more static pressure. Paul -
Thanks a lot for the reply. Actually, I didnt mention it but....My main trunk for the return is 8x23 where it comes off the furnace then it tees off near the ceiling to a perpindicular 8x12 duct. One end is open and the other end is going up to the living area. The end that is not open has a big hole cut in the top of the duct and it is using the joist cavities as a return from the living area over the basement. And all the lines get reduced to 5" at the registers if they off 6"lines.Comment
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I can not see your system. so thats why i told you how to check it out. Someone can run a 8X 24 duct for return air, but if there is only 100 SQ" ofOriginally posted by chrisc View Post
area going into that 8x24 duct then the 8x24 is not doing you much good. Start at the far end and start adding it up. Like i said you should have at least 170 SQ", 200 would be better. If you have to bring some down the stairway, then do so or reduct it.
I think you told me that some of your return air round runs were 5" round. A 5" round pipe if good to return about 60CFM of air And i am guessing that your
low cfm on your furnace would be at least 700 to 800 cfm of air depending on what blower you have in it. later paulbmComment
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