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Furnace sizing

Weezbo Member Posts: 6,232
first start with Insulating everything the pipes, the walls, the ceiling ....seal the windows and doors... then seriously consider more insulation....

it sounds like the current furnace is kaput.. it also sounds like it was basically losing a lot of heat somewhere so, i would not suffer the delusion that the existing plenum and associated duct work is "Fine" ...it is not right to compare mice to kangaroos buh my sisters place has nat gas and is around 1600 sq ft and her gas bills are around 100 a month in those months... as a something to meet or exceed type gage...

you should look for adding more insulation. seriously...

the furnace ought to be better at delivering the heat than the one you had,maybe it was too big and the duct losses or stack losses were off the chart.

think of this a min...if you have a pipe running out of the furnace blasting heat into the atmosphere while it may be helping heat my home and all, it is a losing battle because i live thousands of miles away...now if half of the heat that is going out the pipe meant to deliver the heat is doing about the same thing...each and every time it goes into that mode of operation you have about 25% of the process actually delivered...that would be on or about 18-> 20 K btus...see where that is going? you might only have needed 21000 yet with the hidden "Features" of the heating system...you were not getting particularly comfortable ...

think about it...


  • Brian_58
    Brian_58 Member Posts: 14

    Right now I've got 800sf being heated by a 70,000btu 80% furnace. I live in Michigan and it gets cold here, having uninsulated walls doesn't help the problem. The last two winters my furnace kicks on and runs and runs and runs to the tune of a $200 gas bill in December, January, and February..... and I keep the temp at 64-67. This summer I'm finishing the bonus room upstairs which will make the entire house 1100sf. Now the fan motor is dead and I'm debating a couple things. Should I:

    1. Replace the motor, keep the furnace
    2. Replace the unit with a newer, same size, unit
    3. Replace the whole unit with a bigger unit.
    4. Replace the unit with comparable size but higher efficiency
    5. Move to a warmer state (this is my wife's solution)

    Thanks in advance for your feedback! Have a great weekend!
  • Jim Davis_7
    Jim Davis_7 Member Posts: 67

    Odds are that your furnace is only delivering about 50-60% of the BTU's it is rated. It is only capable of about 72% not 80%. I would think a 90% furnace the same size would work just fine and cut your bill 30-40%. These can be set up to operate near 90%. The contractor would have to be trained and know how to set up the furnace with a combustion analyzer or it may not operate any better than the one you have.

    A good contractor would do a Heat Loss & Gain on your house to assure the correct size and would also determine if your duct system is adequate.
  • Jed_2
    Jed_2 Member Posts: 781
    800 sq. ft.< 1100 sq. ft.

    70,000 BTU/H input or output? 80%>72% ; 50-60% - whatever.
    Uninsulated!!!!! Insulate, insulate, insulate! Stash away the savings for the 90%+ furnace Mr. Davis mentions, and then take your vacation to Forida at your leisure.

  • John@Reliable_14
    John@Reliable_14 Member Posts: 171
    I'm with Jed

    Insulate, insulate, insulate! First and use the savings for a new system. Or even better, insulate, them have a heat loss done for the whole house including new space. You will save allot of money even with blown-in stuff, compared to what you have now vs. just a new system. PS don't forget the ceilings, more important than the walls.
  • Kevin_54
    Kevin_54 Member Posts: 30

    When you say you will only get 50% to 60% of its rating do you mean the input BTU's or output BTU's.
  • Weezbo
    Weezbo Member Posts: 6,232
    Wifer smart *~/:)

    warmer is good :) i like heat :) sell quick !

    build an insulated geodesic dome with two hundred feet of above ground space ...= cut down on taxes :)

    build luxuriant 3ooo sq ft home below ground with GSHP , radiant heating and cooling, prismatic natural light collectors//solar collectors, pV ... put small barrel stove up stairs so everybody feels guilty that you live in a pinner house , with wood stove:)

    the neighbours will stay up night and day wondering how on earth you manage to pack so many people in there:)

    if they ask... just tell them you sleep standing up :)
  • Jim Davis_7
    Jim Davis_7 Member Posts: 67
    50% to 60% output based on input

    Most furnaces deliver between 50%-60% of their rated input to the plenum at factory specs because equipment is designed for worst case senario. The actual delivery to the living space would be even less than this depending on the duct system.
  • Greg_40
    Greg_40 Member Posts: 43

    First- weatherize & insulate your home, next have an accurate HVAC sizing done, discuss options with the HVAC dealer. 1) Lessens the energy impact your home has- good for everybody. Always attack the real problem first- if your home is a gas hog, put it on a diet. 2) Guessing what size of HVAC system a home needs is bad mojo- uncomfortable rooms, poor humidity regulation, wastes energy 3) It's like buying a car these days- HVAC equipment has options available that many consumers are unaware of- look into them to improve comfort, indoor air quality and efficiency levels.
  • Brian_58
    Brian_58 Member Posts: 14

    Good feedback, seems that replacing the motor is not a popular idea. As for moving, Weezbo, I live in Michigan which is about the worst housing market in the US right now.... selling is not an option.

    I KNOW insulation is a huge issue but cost is also one. I'm not keen on having holes cut on the inside of my house every 16" and I'm not a fan of taking off the siding to do it from the outside until I need new siding (I still have the original aluminum) but this is a consideration.

    As for heat loss and duct work, my house is fitted with 3 1/2" ducts..... the youngsters reading this will have no idea what I'm talking about; yes, I said 3 1/2" ducts. To replace them has been quoted several times and it takes me back to a cost issue. In a housing market as bad as this one I'll never get any return on the thousands it would cost to re-fit the entire house.

    So that's what brings me to the furnace issue because it is the only feasible and justifiable replacement cost. The one I have is shot so I have to get one anyhow. 70,000 is the input right now. Thanks for your help!
  • Weezbo
    Weezbo Member Posts: 6,232
    the lowest budget item is the fan motor.

    perhaps the plenum could discharge more heat from the heat exchanger by adding a larger insulated supply to the new room back cut the door,suffer a small hit in comfort in that room,and improve the overall efficiency of the system.. someone needs to be able to see with a wrench in their hands though.....

    it is unlikely the cost of operation will go down though....
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