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  1. #1
    Join Date
    Aug 2008
    Location
    Toronto, Ontario, Canada
    Posts
    147

    Help with air cleaner and thermostat choices

    I am replacing my furnace and air conditioner in the near future. I will be purchasing the following Goodman system:

    Furnace: 95%, variable DC fan, 2 stage, 90,000 btu Goodman GMV950905
    Air Conditioner: 2.5 ton, 14 SEER (15 SEER with coil and furnace combo), R410A, Goodman SSX140301 (ARI # 3000063)
    Humidifier: Currently installed Aprilaire 550
    Air Cleaner: Currently installed Electro Air SS-10 EAC (may be changed, see below)

    I still have questions on my air cleaner choice as well as on my thermostat after speaking to my chosen contractor (who I chose after considering referrals and quotes by several contractors).

    Air cleaner:
    I am not sure if my current Electronic Air Cleaner (EAC) is operating properly and what the airflow (cfm) of my new system will be to properly size a new filter system if needed.

    The previous home owner installed an Electro Air SS-10 EAC. Problem is this unit is rated for up to 1200 cfm air flow (recommended for a house up to 2400 sq ft). My house is 2800 sq ft with a 10 year old Bryant 80% furnace so this unit was under sized and I am not sure if it is operating properly.

    I cannot tell if it is absorbing as much dust as it should. The owner’s manual calls for monthly cleaning of the cells, but they did not seem to collect enough dirt to warrant this so I increased the time between cleanings. The last cleaning was after about 6 months of use and they only had a very fine film of dust on the plates. I imagine this is the amount of dust that would stick to any flat surface in the ducts due to the air flow around them. This is why I don’t think they are working properly.

    I contacted the manufacturer who said since the 2 lights on the panel turn on the unit is functioning, but the air may be flowing past the cells too quickly for them to properly clean the air because the EAC is undersized.

    Can anyone tell me what the airflow of the new furnace will be? The contractor told me the fan will be set for 1000 cfm for the a/c (400 cfm/ton) and that the air flow would be lower than that for heating. The furnace specs list air flows between 1458 to 2115 cfm for high stage heating so I am confused as to what the maximum air flow will be. This will determine if my existing EAC will work as well as if my return duct will need to be increased in size.

    Thermostat:
    I am lost of what to go for here. My contractor told me he will install anything I want but didn’t give great direction here.
    I read a little about the Honeywell IAQ that can control both temperature and humidity. Will this thermostat be able to control my Aprilaire 550 humidifier during heating season and be able to dehumidify using the a/c during cooling season?
    Any other recommendations? I want a programmable thermostat because I use that feature.


    Thanks,
    John

  2. #2
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    33,396
    I'd dispose of the EAC. Get a good media filter like an Airbear, Honeywell F150 or 200, or comparable Aprilaire. They do a good job without the up front expense, constant maintenance or repairs.

    With a 2.5 ton A/C, you'd move 1000 CFM. The furnace will need to move more in heat mode to keep from overheating. That is a good sized furnace for only needing a 2.5 ton A/C.

    The IAQ will handle a humidifier and work with the variable speed blower to slow down for extra dehumidification in the summer.

  3. #3
    Join Date
    Jan 2008
    Posts
    1,858
    Dump the EAC - go with 5" media air cleaner.
    "Hey Lama, hey, how about a little something, you know, for the effort." And he says, "there won't be any money, but when you die, on your deathbed, you will receive total consciousness." So I got that goin' for me, which is nice. - Carl Spackler

  4. #4
    Join Date
    Aug 2008
    Location
    Toronto, Ontario, Canada
    Posts
    147
    Thanks for the quick responses. I now have a few more questions!

    It sounds like EAC's are not looked upon very highly. I'll look into a media filter.

    How can I find what air flow the furnace will need for heating mode? This will determine the filter size I need and if I need to redo the duct work for the return drop since it is kind of small (I think it is around 9 X 21 off the top of my head). As I mentioned in my earlier post, the Goodman specs call for anywhere between 1458 cfm to 2115 cfm for high stage heating.

    Does anyone have any opinions of the media filters made by Five Seasons Comfort. I was told they manufacture the Goodman line of air cleaning equipment and filters as well as some of the other big names such as Carrier. It would be easy for me to get since my installer is a Goodman dealer.

    As for the furnace size compared to the A/C size, I'm located near Toronto, Canada and my heating needs are higher than cooling needs. The old A/C is a 3.0 ton, but I determined it was slightly oversized for the house after doing a heat gain calc.

  5. #5
    Join Date
    Aug 2008
    Location
    Toronto, Ontario, Canada
    Posts
    147
    I have talked to the contractor and got some answers.

    The furnace fan will push a maximum of 1458 CFM in heating mode. The installer recommended a 20" X 25" filter that is capable of up to 2000 CFM. He wasn't able to clearly answer my question with regards to the return duct size. Is the current 9" X 21" duct large enough to supply the required air flow?

  6. #6
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    21"x9" is too small for 1450CFM. You need at least 21"x12" better is 12"x24" or any combination to get to around 280-300 square inches of duct cross section.

    With a 20X25 filter, using a 12"x25" return makes the most sense.


    However, if your return is undersized, I would suspoect you supply is undersized as well. You want around 230-250 square inches of supply duct. Otherwise it will be noisy on high stage and the blower will consume more energy than it should.

    The ductwork is however adequate for you cooling needs.

    If you increase the supply duct size you may want to have dampers added to all the supply branches if possible (if you don't already). You may find you air balance will change when the trunk size is changed.

  7. #7
    Join Date
    Aug 2008
    Location
    Toronto, Ontario, Canada
    Posts
    147
    Thanks for the help motoguiy.

    I took some measurements and did some quick calculations.

    Total return duct area of 256 square inches (a 8"x16" return trunk runs to the front and back of the house from the return drop so I added it like two seperate 8"x16" ducts).

    Total supply duct area is 397 square inches (one 8"x14" trunk to the rear of the house, one 8"x16" trunk to the front of the house, and two 5" round ducts are all directly connected to the plenum).

    I was only looking at increasing the size of the return drop (currently 10"x21"), not the return trunks. This was to satisfy a larger media air filter which was to satisfy the 1450CFM that the furnace will push at high speed. As I mentioned before the contractor recommended a 20"x25" filter.

    Would increasing the return drop size do anything since it is already fairly close in cross sectional area to what I added up for the return trunks?
    Last edited by john_ertw; 04-22-2009 at 09:41 PM. Reason: clairify

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