Decided on new furnace, have some questions
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  1. #1
    Join Date
    Sep 2012
    Posts
    373

    Decided on new furnace, have some questions

    As I talked about my basement system in my last thread, we decided to wait on doing anything to it for a few more years. A Contractor came yesterday to quote us on a new furnace for our first floor to replace our current Lennox G40. He did a manual j which turned up to 3.62. This means that the current heat pump (3.5 ton Trane XR15) is sized properly. This contractor is from the same company who installed our upstairs Lennox sl280v and they are a Premier Lennox Dealer. They are one of the top rated companies in my area with five-star reviews. Here's the estimate:

    Install Lennox SL280UH090V48B 90,000 BTU 2-stage variable speed gas furnace.
    Reconnect/reinstall flue pipe.
    New drain pan with float switch.
    Sealing of all ductwork

    He inspected ALL of the ductwork and surprisingly said that the ductwork is all fine. It just needs some major stretching out. He also said system is short on return air, but unfortunately, another return can't be added, unless it was put in the basement. Is variable speed a wrong choice when there's not enough return air?

    I'm also wondering if the cfm for when the heat pump is in heat mode is the same for the gas heat on first stage. After looking at the manual online, The default heating speed is 1035 on low fire and 1135 on high fire. It can be adjusted to a higher speed or lower speed. The heat pump needs 1400 cfm since its a 3.5 ton. Does the cfm for the heat pump in heat mode the same as the furnace in heat mode? The outdoor temp lock out is set at 40*F, just an FYI. Thanks in advance.

  2. #2
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    33,833
    Shy on return air means the motor works harder, uses more juice, lowers SEER and HSPF. Can be noisy.

    Heat pump fan speed is cooling speed and set independently of gas heat speed.

  3. #3
    Join Date
    Aug 2001
    Location
    Pavilion, NY
    Posts
    2,095
    Insufficient return air should be corrected no matter what motor is used. Low airflow especially in a heat pump application is bad bad news. When the heat pump is running you will need 1400cfm and your heating airflows listed will be what the unit operates at when the furnace only is operating.
    ...

  4. #4
    Join Date
    Sep 2012
    Posts
    373
    I appreciate the information, thanks. I will call the contractor back tomorrow and discuss it with him. Is it OK if the new return was placed in the basement? The contractor mentioned that it won't effect the basement system.

    So when the heat pump is in heating mode the cfm will be the same as cooling mode, correct? And the cfm will be different when only the furnace is running? With the short return I'm guessing the system is only running a little bit below 1400. A new return should correct this issue.

  5. #5
    Join Date
    Aug 2001
    Location
    Pavilion, NY
    Posts
    2,095
    Well,, is your basement part of the heated space? Do you have any appliances vented into a chimney? Pulling air from a basement when it is not being supplied air will cause a negative pressure and could backdraft appliances into the space. This of course could be an unsafe condition.
    ...

  6. #6
    Join Date
    Sep 2012
    Posts
    373
    Yes, the basement is heated, but the basement has its own system that is not gas. The appliances are seperated in their own little room with a louvered door. Our basement is large (2000 sq. ft. finished) and the return will be placed FAR away from the utility room.

  7. #7
    Join Date
    Sep 2012
    Posts
    373
    The supply air needs to match the return air, so having one return in the basement that's not close to the supplies is ok?

  8. #8
    Join Date
    Jul 2006
    Location
    Virginia
    Posts
    4,604
    depends onhow short on return air . some times you can enlarge all accessable duct and decrease the static pressure enough to get by .what size is the return
    We really need change now

  9. #9
    Join Date
    Sep 2012
    Posts
    373
    Quote Originally Posted by catmanacman View Post
    depends onhow short on return air . some times you can enlarge all accessable duct and decrease the static pressure enough to get by .what size is the return
    We have 3 ducts coming off of the return box. Two of them are 10 inch, and the other is 12 inch. We have a total of seven 8 inch returns, with three coming off the 12 inch duct, two coming off the 10 inch duct, and two coming off the other 10 inch.

  10. #10
    Join Date
    Sep 2012
    Posts
    373
    Another thing... the contractor never mentioned what the actual size of the new return will be. After giving some information in my last post about the current returns, what size will the new return have to be?

  11. #11
    Join Date
    Sep 2012
    Posts
    373
    Update... New furnace was installed a few hours ago. So far I'm very pleased with the performance of it. The installers worked hard and cleaned up after themselves, but during the install an issue showed up.

    What happened was when the installers took the new furnace out of the box, there was no front panel. As of now the furnace operates with no main panel (there is a separate panel for the blower compartment and it's installed) and the owner of the hvac company is coming on Monday to install the main panel and will apologize for the problem. Other than that I'm extremely satisfied with the performance of the new unit. Thanks for all your advice and recommendations!

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