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  1. #1

    Question about adding return air?

    Hi guys. What a great site!
    I just ran across this forum while researching a question that just came up after having my furnace checked for the year.
    This spring we bought a 7 y.o. 2600 sq. ft. ranch with a full basement in se lower Illinois. It has a Heil 125k btu, DC90 furnace with AC. Im am not sure what brand the AC is.
    We had a local company come out to do the service. The company came very well recomended by a couple neighbors.
    I was there during the service and the technician gave me the following recomendations and I was hoping for someone to let me know if the suggestions are good ones:

    1) The static pressure is supposed to be .5, it checks .59 The serviceman recomended adding additional return air. Each room, except the kitchen and bathrooms already has a return air vent high on the wall. Each of the grills measure app. 8 by 16 inches. We have (6) total returns in the house with 26 registers that the heat comes out from.
    He said he wouldnt recomend adding return air to the bathroom or kitchen because of odors. He said there are a couple options, A) enlarge the exisiting return openings in the wall and use the next size larger grill. B) add a return grill directly below one of the existing returns, about a foot off the floor. He said if I didnt add the additional return air it would shorten the life of my furnace. Is this true? What option would you suggest?

    2) Our house is L shaped and one room (daughters bedroom) farthest from the furnace is about 3 degrees colder than the other rooms. He said zone heating was about my only option. Is there another option?
    The strange thing is that during the summer, when the ac was running, the room was the same temp as the rest of the house. My daughter didnt start to complain about the temperature untill the cold weather arrived.

  2. #2
    Join Date
    Nov 2000
    Location
    Eastern PA
    Posts
    68,940
    There are many ways to reduce static pressure in a system. Enlarging the return area is only going to be affective if the returna area is the problem.

    Other possible issues could be;

    • Undersized or too dense of an air filter.
    • Ducting too small.
    • Too few supply outlets for the system.
    • Furnace blower set too high.
    • Dirty indoor coil.


    Just be certain that your contractor has addressed all possible reasons for the high static.
    Government is a disease...
    ...masquerading as its own cure…
    Ecclesiastes 10:2 NIV


  3. #3
    Quote Originally Posted by RoBoTeq View Post
    There are many ways to reduce static pressure in a system. Enlarging the return area is only going to be affective if the returna area is the problem.

    Other possible issues could be;

    • Undersized or too dense of an air filter.
    • Ducting too small.
    • Too few supply outlets for the system.
    • Furnace blower set too high.
    • Dirty indoor coil.

    Just be certain that your contractor has addressed all possible reasons for the high static.
    Thank you for the quick response RoBoTeq.

    1) The furnace has (2) new 1x25x25 filters installed in the shape of a V in the cold air return. Merv 8.
    2) Not sure on the ducting, the serviceman commented that the original contractor did a good job on the install. All the seams and joints are sealed. They are not insulated though.
    3) I have 26 supply floor vents throughout the house. Good/Bad?
    4) The worksheet he left has the "tap speed" for heat= medium hi and AC= low. "Heat Rise" =60. "Supply temperature above coil" = 130. "Static Pressure"= .59 "Gas Pressure" =3.5
    5) He took off an access panel to check the "a coil" and said it looked good. Is this the coil you are refering to?

    He drilled a half dozen small holes all over the furnace and ducts. He covered them with foil looking tape.

    Let me know if I left something out.

  4. #4
    Join Date
    Jun 2007
    Location
    Metro St. Louis
    Posts
    181
    Quote Originally Posted by Scotty1965 View Post
    Thank you for the quick response RoBoTeq.

    1) The furnace has (2) new 1x25x25 filters installed in the shape of a V in the cold air return. Merv 8.
    2) Not sure on the ducting, the serviceman commented that the original contractor did a good job on the install. All the seams and joints are sealed. They are not insulated though.
    3) I have 26 supply floor vents throughout the house. Good/Bad?
    4) The worksheet he left has the "tap speed" for heat= medium hi and AC= low. "Heat Rise" =60. "Supply temperature above coil" = 130. "Static Pressure"= .59 "Gas Pressure" =3.5
    5) He took off an access panel to check the "a coil" and said it looked good. Is this the coil you are refering to?

    He drilled a half dozen small holes all over the furnace and ducts. He covered them with foil looking tape.

    Let me know if I left something out.
    Sounds like you got a good company there. They did a lot of checking and measurements, and then provided you with the actual data.

    Your issue might be due to overall furnace sizing. At 135K BTU, that's a lot of heat. The cycles are likely pretty short. The ductwork doesn't get a chance to warm up, resulting in duct losses. So, the first few registers get 125* air. The ones at the end of the line are getting somewhat cooler air.
    Worked fine in the summer because your A/C was better sized (speed tap=low), so the A/C ran for longer times.

    You've got .59" static pressure? I'm not sure that's too bad. If your furnace is oversized, a "correctly" sized one would require less airflow, and therefore, a lower static pressure.

    Call your contractor back and see if they can do a flow test on a couple of your registers. Make sure that you're not losing your airflow before it gets to your daughter's room. And have them check the temperature outputs at different registers.

    Overall, I'm not sure your SP is a problem. Getting the BTUs to your farthest room is a problem. You're either losing the airflow or the temperature. Either one can be fixed.

    Post the size of your A/C. Is your 2600 sqft on the main level, or is it 1300 sqft for each level?

  5. #5
    Waterloo, thank you for the reply.
    Our home is a 2600sq. ft. ranch with a full basement.
    The 2600 sq. ft. on the main floor is completely finished. The basement is unfinished. The furnace is 125k btu.
    I agree. I am working with a good company. They left me a worksheet with all the items they checked.
    When the service man was at the house and I asked about my daughters room, he checked the temperature coming out of the 2 floor registers in her room. They were both about 102 degrees. The warmest temperature from a floor register was from our dining room at 128 degrees.
    It has been about 35 degrees during the day and about 20 degrees at night. When the furnace runs, it runs for about 8 minutes. It runs between 2-3 times in an hour.
    How can I get the temperature higher at the 2 registers in my daughters room?

  6. #6
    Join Date
    Jun 2007
    Location
    Metro St. Louis
    Posts
    181
    Wow. I would guess that the output temperatures decrease on each register as you move away from the furnace.

    In the summer, your basement is probably 2-5 degrees cooler than your upstairs. The A/C provides about a 20* drop in air temp. Therefore, the difference between the air inside the ducts vs the air outside the ducts is only 15 degrees.

    In the winter, this difference is closer to 65 degrees (65 degree basement, 130 degree air in duct). And since metal is a decent thermal conductor, you are losing a lot of heat to the ductwork and the basement. The longer the air has to be in the ductwork, the more opportunities it will have to transfer this heat to the basement.

    In my opinion (I'm not a pro!), you either need to get a smaller furnace, reduce the firing rate on your existing system, or insulate your ducts.

    The smaller furnace would run longer, allowing the ducts to heat up and reduce losses. I'm not sure it's even possible to reduce the firing rate on a furnace - call your pro and see. For either option, you'll want to get a full Manual J (Heat Load calculation) performed.

    Insulating your trunk lines would probably be the cheapest option. You'll probably want to do this anyways just to even out the temps.

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