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New install Furnace

larrylwilllarrylwill Member Posts: 5
Im going to a friends next month to help them fix up the 1/2 finished house the bought.

They have a propane furnace thats now sitting in the Living room with the top ducts just open at the top, there are 3 6" duct outputs. The house is small about 920sf. They use a pellet stove for heat this will be supplemental. I plan to move the furnace to a attached room off the kitchen, where the washing machine and dryer will be relocated.

         The Living room dining room kitchen is about 400 sq ft.. I plan to drop 2 4" flex vents off one 6" run in the LR dining and one off another 6" in the kitchen and  carry that run to the Bedroom and drop another 4". I plan to use the 3rd 6" for the bathroom 50sf and back bedroom which is about 130sf. The return will be over the kitchen wall which does not go to the celing. I will add a drop celing in the living, dining room. They will come in overhead. I will be using 2-4" flex drops off each hard 6" run.

Please critique and/or critize (constructive please).


  • heatpro02920heatpro02920 Member Posts: 991
    edited April 2014
    what size furnace

    whats the cfm on the furnace?

    is this house down to the studs?

    I wouldnt run a central return, I would run the largest flex duct you can fit for the supplys {minimum of 6" flex} and then run 75-80% to each room for a return, on the opposite side of the room {in other words, 2-6" supplys with an 8" return for that room}... Make sure you are taking outdoor combustion air if this is going in a laundry and seal your returns with ductmate...

    you can use airtec mv diffusers with 6" flex...
  • BillWBillW Member Posts: 187
    edited April 2014
    Furnace location

    It is not a good idea to put any combustion appliance like a furnace or boiler in the same space as the laundry machines. The detergent dust, chlorine bleach and the dryer lint will cause damage to the burner and controls, or cause a fire. The dryer and the furnace will "fight" each other for combustion air, possibly pulling carbon monoxide back into the house with potentially deadly results. Find another place for the laundry, and save yourself a lot of trouble.
  • TechmanTechman Member Posts: 2,144
    edited April 2014
    Flex size

    Where did you come up with the 4" flex? How much CFM is going thru the 4"? The 6"? What is the CFM of the furnace? No AC in the future?
  • IronmanIronman Member Posts: 3,542
    4 inch flex - never

    A 4inch flex line will carry about 40 cfm at normal static pressure. That's 240 cfm for 6 of them - well below the minimum cfm that even the smallest (40k btu) furnace would require.

    We never use 4" flex on duct system except maybe a short run to a 3x5' half bath with no exterior exposure.

    4" flex is for bath exhaust fans.
    Bob Boan

    You can choose to do what you want, but you cannot choose the consequences.
  • M LaneM Lane Member Posts: 123

    * As previously said, keep the unit as far from the laundry area as you can get. Lint and dust will be a constant source of problems.

    * Place the registers over windows, to wash the air over them.

    * Ditto on the 4" duct. Don't.

    * Some units are multi-directional, meaning they be installed horizantal; thus allowing more options for location like an attic.
  • RJRJ Member Posts: 484

    I recommend a manual damper installed on the supply plenum for each run for balancing purposes,   you can buy a ductulator on line to help you size all duct.
  • larrylwilllarrylwill Member Posts: 5
    New install Furnace

    Sorry Guys my emails from tis forum was going into googles spam box.


    furnace is in another state, I do not know the size, I saw it but didnt

    look at the specs. The heat with pellets and would only use this for

    emergency heat, maybe will run the fan to circulate the heat. No air

    condition, they have a roll around. They don't have the money to install

    anything else,

    There are 3 6" hard ducts comming out the top, thats about all I know untill I get up there end of May.

     I could drop 6" flex off the 6" hard duct. I planed to use vents with closers.


    never concidered the washing machine problem since every house I owned

    with oil or gas heat had the furnace in the same room. I could isolate

    the furnace with a wall and put a small outside vent to allow air into

    the furnace area. I would also run the retuen right through the wall at

    the furnace since its about where I drew it, I don't have the time to

    run a 8" or bigger return from the other side of the house.

    Yes I planed to place them over windows where possible.
  • larrylwilllarrylwill Member Posts: 5
    New install Furnace

    I found I had a picture of the furnace.
  • heatpro02920heatpro02920 Member Posts: 991
    edited April 2014
    Thats a small 90+

    I wouldn't worry about the air trouble, it gets its combustion air from outside, problem solved, if you are worried about negative pressure, seal the ducts well, vent the dryer properly and use a small door vent...

    Run at least 4- 6" vents and if you are using a central return run it so it is directly over their pellet stove, so they can run "fan only" and circulate the warm air from the pellet stove throughout the property... If you can get 12" flex for a return and maybe another small return outside the laundry room, you will be more than good...

    stay away from 4" duct, make sure you have enough cfm to cool the chamber, you dont want to choke that thing to death, you will make short work of the heat exchangers with insufficient airflow...
  • TechmanTechman Member Posts: 2,144

    Those supply ducts look more like 8". If you are running returns from any bedrooms the return should be larger than the supply. KindaSorta, 8" supply, 10" return or if supply is 100 the return would be about 120
  • RJRJ Member Posts: 484

    I would install these dampers on the plenum ahead of the runs     easy install
  • larrylwilllarrylwill Member Posts: 5
    New install Furnace

    I had them measure them and they were slightly under 6".

    As for the return I have never lived in a house with more thna 1 return including the last hoouse I built, They installed only 1 return in a 3 dtory 2500sf house. This house had a new heat pump installed some years ago and they put 1 return in the hall, My friend had a new install 2 years ago, 1 return.
  • larrylwilllarrylwill Member Posts: 5

    Why would you use this kind of damper over open/close vents?
  • heatpro02920heatpro02920 Member Posts: 991
    edited April 2014
    central return

    When I built my house {4300 sq ft} I have a return in every room, my bedroom has 2 returns... returns are your friend the more the better..

    I am on an install now, old construction/remodel- I have 6 and 7" supplies and 8" returns throughout the house, 2- 98% furnaces variable / modulating ect with dual air exchangers, and dual steam humidifiers , I will use a central return on small houses {or zones} with open layouts, or where cost is an issue, but for a decent FWA system, independant returns is the way to go...

    As far as the dampers go, they arent that much more cost wise, but you will need to do all home runs to use them for each diffuser, the mv's I linked are not as loud other on the market, and if you size the duct correctly balancing shouldn't be a huge issue, its a small area, air flow will be key..
  • RJRJ Member Posts: 484

    Better to balance air flow at the plenum    using vents or diffusers for balancing can cause velocity noise. although most residential installs don't use these, I would put them in if I was doing a new install.
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