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Does an 850-square-foot house have to have return air ducts?

Brett Obraza | Posted in General Questions on

I live on a slab one story home. I currently have an old forced air gas furnace that I would like to get replaced as well as install central air conditioning. I had a local contractor come out and quote a complete system and was told I would need all new ducts because I didn’t have a return air system. The furnace currently is in a central closet with a large air grate on the side letting air back to the furnace. It is right in the middle of the house and I usually leave all doors to all bedrooms open. He quoted me $12000 for the whole job.

Can I install a new high efficiency furnace with my current setup with a large central air return to the furnace closet and no return ducts?

Sorry for the long post but I can not afford the cost the hvac company is giving me, and would like to upgrade the heating/cooling system.

Thanks for any help.

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Replies

  1. Charlie Sullivan | | #1

    No apology needed for the long post. To help us answer, please also let us know:
    1) Your location or climate zone.
    2) Fuel used for your present furnace (natural gas?)
    3) Where the ducts are now.
    4) Some idea of the age of your house and how good the insulation and air sealing are.

    One piece of standard advice is that if the contractor didn't look at the insulation and do a room-by-room heat load calculation (sometimes called a "manual J" calculation"), the system he is proposing is likely oversized and thus overpriced and will not work well.

    My hunch is that there are a lot of good and cheaper options you can consider, but some more information would help.

  2. GBA Editor
    Martin Holladay | | #2

    Brett,
    A well-designed forced air system needs a return duct (or a dedicated return air pathway) from every room in the house. So your contractor is right.

    Many contractors cut corners, however, and you have been living in such a house -- a house with a single central return. If you close your bedroom doors, pressure imbalances can force air into cracks in your ceiling and walls, raising your energy bills and potentially causing moisture problems.

    For more information on this issue, see Return-Air Problems.

  3. Brett Obraza | | #3

    I live in Des Moines, IA. We see 90 degree plus summers with high humidity and also get single digit winter days regularly. It is a Natural gas furnace. The ducts are installed in a line down the center inside wall of the house with small branches to each room. The house was built in 1952 and has below average insulation, but I have gutted a couple exterior walls for rot repair and reinsulated those. I also plan on opening up the rest of the exterior walls to reinsulate, as well as adding some more loose fill to the attic.

    The contractor went through and measured all the windows but did not look in the attic.

    Thanks again. I appreciate the help.

  4. Brett Obraza | | #4

    Also the big problem is I do not have any closets big enough to house the furnace with return ducts. The contractor suggested putting it in the attic, but would be limited to 80 percent efficient do to condensation?

  5. GBA Editor
    Martin Holladay | | #5

    Brett,
    You shouldn't put a furnace in the attic unless you are prepared to convert your vented unconditioned attic to an unvented conditioned attic.

    Here is a link to an article with more information: Creating a Conditioned Attic.

  6. Brett Obraza | | #6

    OK Thanks for the response. So my best best would be to get a high effiency furnace without return ducts because my closets aren't big enough for a return system? Anything I can do to help the circulation in the house?

    Thanks a lot.

  7. Expert Member
    Dana Dorsett | | #7

    You can utilize partition walls as jump-ducts for the return paths and still preserve privacy by putting a grille near the floor on one side of the wall, and near the ceiling on the other. It doesn't need dedicated return ducts, just a sufficiently low-impedance return path. Jump-ducts and door cuts qualify.

    For an 850' house in US climate zone 5A Des Moines almost any furnace is going to be ridiculously oversized. You can probably use the same supply ducts with a 1.5 ton ducted mini-split heat pump, and it should cost less than half that $12K figure. You could also install a hydronic baseboard system running of a hot water heater for less money than that. (But that wouldn't air-condition.) It helps to have done the heat load analyisis before picking any equipment.

    Even a fairly leaky 850' house would have a heat load of ~20,000 BTU @ -4F (the 99% outside design temp in Des Moines: http://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/Outdoor_Design_Conditions_508.pdf ) A fairly tightened up 2x4 framed house with low-E replacement windows or low-E storms will come in under 15,000 BTU/hr.

    If you have a fuel use history it's fairly simple arithmetic to put an upper-bound on the heat load, using the nameplate efficiency of the existing equipment to make it your measuring instrument. If you have mid to late winter gas bills withe EXACT meter reading dates and the amount of gas use, with a ZIP code (for the closest weather station data) it's a 5 minute calculation (it takes more time to type it into the forum than it does to run the calc.) Don't use a billing periods when you turned the thermostat to 50F and spent the week in Belize, eh? :-) Only use billing periods when it was occupied, and only wintertime bills, since that minimized the errors from other gas use like hot water, and solar gains.

  8. Charlie Sullivan | | #8

    Dana's suggests are good. More on the different options for providing return air paths without ducts:

    https://www.greenbuildingadvisor.com/blogs/dept/green-communities/return-sender-hvac-return-pathway-options

  9. Brett Obraza | | #9

    Thanks to everyone for the helpful comments. I really appreciate everyones help. This gives me a lot of really good options to consider.

    Thanks again, and have a good day.

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