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  1. #66
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    Quote Originally Posted by crmont View Post
    I hate to agree with Ryan but after looking at the pictures a little closer, it appears that there is a 16x20x1 air filter in place and it might even be an "allergy free" type filter. The return duct is also cut in one side of the furnace which historically makes for low airflow. Also the indoor coil looks suspiciously small for a 15 seer 3.5 ton. Please post the model numbers of the indoor coil and the Lennox furnace so i can verify the match up. Finally, have the static pressure checked and compared to the blower data. If the blower data isn't available, the furnace model number will clarify. Thanks.
    I just remembered something. My son wants to be an HVAC tech when he gets older and he and I were watching the tech inspect the furnace and I noticed that the opening of the return plenum that's connected to the side of the furnace is actually cut like a circle that's the same size as the blower and not a cut rectangle that matches the size of the return plenum. If you know what I mean. The tech never mentioned anything about that. The new upstairs furnace does have a cut rectangle that is in the whole lower part of the side of the furnace that's connected to the return plenum.

  2. #67
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    May 2007
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    DC Metro Area (MD)
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    Quote Originally Posted by 545GAlady View Post
    I just remembered something. My son wants to be an HVAC tech when he gets older and he and I were watching the tech inspect the furnace and I noticed that the opening of the return plenum that's connected to the side of the furnace is actually cut like a circle that's the same size as the blower and not a cut rectangle that matches the size of the return plenum. If you know what I mean. The tech never mentioned anything about that. The new upstairs furnace does have a cut rectangle that is in the whole lower part of the side of the furnace that's connected to the return plenum.
    May or may not be an issue depending on the size of the "hole." Again, you need a tech that can quantify the actual CFM you are getting by properly measuring ESP. For a 3.5 ton unit, your target would be 1400 CFM. It's doubtful that the total filter surface area is being used with a setup like you describe. Also, it's hard to tell from the pictures, but the flex duct may be marginal for 3.5 ton (with a standard, non-variable PSC motor at least), especially if there are lots of bends. Can you give us sizes (in. dia) and approximate run lengths?

  3. #68
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    Sep 2012
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    Quote Originally Posted by RyanHughes View Post
    May or may not be an issue depending on the size of the "hole." Again, you need a tech that can quantify the actual CFM you are getting by properly measuring ESP. For a 3.5 ton unit, your target would be 1400 CFM. It's doubtful that the total filter surface area is being used with a setup like you describe. Also, it's hard to tell from the pictures, but the flex duct may be marginal for 3.5 ton (with a standard, non-variable PSC motor at least), especially if there are lots of bends. Can you give us sizes (in. dia) and approximate run lengths?
    Thanks for the input. I'll see if another tech can look at that. The whole system has 16 supplies and 7 returns. The 2 supplies in the main living room are 8 inch and the two supplies in the master bedroom are 6 inch. The master bedroom has 1 return. I will need to find out the run lengths of the ducts. The office has a 8 inch supply and 2 returns. The main hallway has two returns and the dining room has 2 returns as well. The other supplies are pretty much 6 inch (2 of the ones in the kitchen) and the rest are 4 inch (I think). The returns are all 8 inch. Before the heat pump was installed, the contractor did do a load calculation and said 3.5 tons was enough, which I already mentioned in my very first post in this thread. Hope this helps.

  4. #69
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    May 2007
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    Quote Originally Posted by 545GAlady View Post
    Thanks for the input. I'll see if another tech can look at that. The whole system has 16 supplies and 7 returns. The 2 supplies in the main living room are 8 inch and the two supplies in the master bedroom are 6 inch. The master bedroom has 1 return. I will need to find out the run lengths of the ducts. The office has a 8 inch supply and 2 returns. The main hallway has two returns and the dining room has 2 returns as well. The other supplies are pretty much 6 inch (2 of the ones in the kitchen) and the rest are 4 inch (I think). The returns are all 8 inch. Before the heat pump was installed, the contractor did do a load calculation and said 3.5 tons was enough, which I already mentioned in my very first post in this thread. Hope this helps.
    I was more curious of the size of the ducts coming off the supply and return plenums at the furnace, which I assume serve distribution boxes (?). Not atypical for new construction installations.

  5. #70
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    Sep 2012
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    Quote Originally Posted by RyanHughes View Post
    I was more curious of the size of the ducts coming off the supply and return plenums at the furnace, which I assume serve distribution boxes (?). Not atypical for new construction installations.
    Oh Ok. Sorry for not giving that info. I will measure the sizes coming off of the supply and the return plenums.

  6. #71
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    I'm probably wrong about the sizes, but here's the results:

    Supply plenum:10 or 12 inch trunk (I think), 10 inch trunk, two 8 inch runs, and two 4 inch runs.
    Return plenum: Two 8 inch runs and a 12 inch trunk.

    I'll have the tech look at the ducts to properly measure them.

  7. #72
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    Sep 2012
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    And I had a mistake on the number of supply vents. There's actually 17 supplies NOT 16.

  8. #73
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    Quote Originally Posted by 545GAlady View Post
    I'm probably wrong about the sizes, but here's the results:

    Supply plenum:10 or 12 inch trunk (I think), 10 inch trunk, two 8 inch runs, and two 4 inch runs.
    Return plenum: Two 8 inch runs and a 12 inch trunk.

    I'll have the tech look at the ducts to properly measure them.
    I'll assume you measured them properly, and offer you my opinion that you are certainly short on return. With a standard blower, a conservative estimate would be 450-500 cfm through the 12" flex return and 150-200 cfm through the 8" flex returns, assuming average length runs without sharp bends. Definitely low for a 3.5 ton system, so I do think you are either low on airflow, overcharged, or both. I would definitely recommend ensuring properly airflow and then going back to have the system properly charged per subcooling. Without the proper airflow, you aren't getting full capacity or efficiency. A 3.5 ton system doesn't necessarily need 1400 cfm, but I would at least aim for 1225 CFM (350 cfm/ton). Your furnace should have a blower chart showing the airflow provided at various external static pressures. Since it's a standard, non variable-speed blower, it probably has a maximum static of 0.5" or less (at which it is capable of delivering rated airflow), which you may be over already. There is no set amount of airflow that one size duct can handle, so it is important that you get the static pressure checked (for example, I have a 6" flex return in my basement -- short, straight flex run right off the return plenum pulling 200-250 cfm verified using a balometer).
    Last edited by RyanHughes; 10-03-2012 at 11:37 PM.

  9. #74
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    Sep 2012
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    Quote Originally Posted by RyanHughes View Post
    I'll assume you measured them properly, and offer you my opinion that you are certainly short on return. With a standard blower, a conservative estimate would be 450-500 cfm through the 12" flex return and 150-200 cfm through the 8" flex returns, assuming runs that aren't too long and without sharp bends. Definitely low for a 3.5 ton system, so I do think you are either low on airflow, overcharged, or both. I would definitely recommend ensuring properly airflow and then going back to have the system properly charged per subcooling. Without the proper airflow, you aren't getting full capacity or efficiency. A 3.5 ton system doesn't necessarily need 1400 cfm, but I would at least aim for 1225 CFM (350 cfm/ton). Your furnace should have a blower chart showing the airflow provided at various external static pressures. Since it's a standard, non variable-speed blower, it probably has a maximum static of 0.5" or less, which you may be over already.
    Thanks. The amount of returns does sound low to me. Whenever the system is on, the air going through the returns is pretty loud. We had the same problem with our old upstairs furnace, it also was short on return. Since we upgraded to a variable speed furnace upstairs, a new 10 inch return was added and now the cfm is balanced and everything is satisfied with that system.

  10. #75
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    May 2007
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    Quote Originally Posted by 545GAlady View Post
    Thanks. The amount of returns does sound low to me. Whenever the system is on, the air going through the returns is pretty loud. We had the same problem with our old upstairs furnace, it also was short on return. Since we upgraded to a variable speed furnace upstairs, a new 10 inch return was added and now the cfm is balanced and everything is satisfied with that system.
    Variable speed blowers are able to maintain proper airflow up to 0.8-1.0" static pressure, although I personally prefer to have the ductwork sized such that the static does not exceed 0.6" in residential. It was likely smart and necessary to add the additional return. Loud returns are symptomatic of high-velocity, high-static duct conditions.

  11. #76
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    Sep 2012
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    Quote Originally Posted by RyanHughes View Post
    Variable speed blowers are able to maintain proper airflow up to 0.8-1.0" static pressure, although I personally prefer to have the ductwork sized such that the static does not exceed 0.6" in residential. It was likely smart and necessary to add the additional return. Loud returns are symptomatic of high-velocity, high-static duct conditions.
    Before our new upstairs furnace was installed, the contractor inspected all of the ductwork to see if it was sized properly. It turned out that all we needed was a new return. Our upstairs system is smaller than our main level system. Thanks for the input about loud returns.

  12. #77
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    Sep 2012
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    All 7 returns are noisy, but the supplies are not.

  13. #78
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    I have no idea where the tech would actually put a new return.

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