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Thread: Suggestions for new heat/ac in older slab home.

  1. #1
    DIY Junior Member Brainfogbetty's Avatar
    Join Date
    Jan 2013
    SE MI

    Default Suggestions for new heat/ac in older slab home.

    We are planning on getting a new furnace and central air for our home. Our current heat system is 25+ yrs old and not functioning well.
    The ducts are in the slab and currently only a few rooms have heat blowing through the vents. It's a cinderblock with brick veneer ranch type home that used to have an attached garage, which was converted into a 20 x20 bedroom with a full bath. This room has it's own separate wall heater and it does have
    a very small crawl space, but I don't think it's accessible. We want the new heat/ac for the whole house.

    I know we will most likely have to seal up the old vents. Will we have to have the new vents in the ceiling? and if so, I'm worried that the
    heat won't reach the floor, which will be cold because of the cement slab. Can new ducts be put in the walls and if so, how will that affect
    the cost? Can the new system be put in the attic? The current furnace is in a closet in my kitchen.

    I would be so grateful if someone could suggest some options for me so I have some knowledge before I call out a company. My husband saw
    a heat/ac unit that was one unit and we wondered how something like that would work?

    Thanks so much for any input.


  2. #2
    In the trades Dana's Avatar
    Join Date
    Jan 2009


    Ducts & HVAC systems in the attic are a generally lousy idea, since that pokes big holes in the insulation & pressure boundaries of the house, and puts the ducts in a cold space in winter and a hotter-than-outdoors place during the summer, which would add a lot to the heating & cooling bills (and system size.) If your ceiling heights can take it, running any new ducts in at the ceiling, under the attic insulation is a far better approach.

    If the budget is big enough and you can hit at least R50, you might consider insulating at the roof deck, sealing off any roof venting works, in which case putting it all in the attic is fine. You need the R50+ to avoid ice damming issues in an unvented roof design in your area, and it typically involves quite a bit of spray foam insulation (or rigid foam on top of the roof deck) to do it in a way that won't rot out your roof deck over time.

    What are your current wall/attic/floor insulation type & values? How about the BTU in/out ratings on the current furnace (and it's fuel type)? House size? How much fuel do you burn through in a heating season?

    From a heating point of view you'd be better off with a hydronic (pumped hot water) boiler, which is both more efficient & more comfortable, and doesn't take up a lot of room space, but it's more expensive than ducted hot air. Sizing both the boiler and radiation right is critical, and is typically oversized unless designed by a true professional. Heated floors are the total lap of luxury, but are probably a budget-buster.

  3. #3
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    New England


    If you have decent ceiling heights and are willing to make a lot of modifications, hydronic in or on the slab is really nice, but it would be costly to install since you'd have to raise most everything close to 2" and redo all of the flooring, and anything else that couldn't stand that increase. Installing radiant panels would be less of an issue and can provide really comfortable heat. For a/c, depending on the layout, a mini-split with an assortment of heads might work out, or a high velocity duct system which is quite small in diameter, but uses difuser heads and high velocity to distribute the air.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  4. #4
    Master Hot Water Mpls,MN BadgerBoilerMN's Avatar
    Join Date
    Mar 2011


    Here in Minneapolis we are currently working on a brick ranch home built in 1955 and started by spraying 2# foam under the deck. With a hot roof and plenty of room for the low velocity warm air we will finish mechanical systems including ERV and finish with a foot of cellulose over all for an R50. If the main floor were on a slab we would retrofit a radiant ceiling panel and keep both slab and ceiling within a couple of degrees of one another with an ambient temperature a couple of degrees lower still.

    In most modest sized bungalows in most climates a condensing water heater with intelligent sub-system for radiant panels will serve a the combi roll of space and domestic hot water heating.

    Of course if the space will allow, a multi-head mini-split would be the better bargain perhaps, but you will miss the warm floors.

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