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Choosing a new furnace; single-stage vs multi-stage

the_donut
the_donut Member Posts: 374
I’m looking to replace my 30 year old international comfort 80% natural gas 80,000 btu and older 2.5 ton ac in Lafayette Indiana spring after next. Converted my 200 sq ft 3 season room into a proper addition. Brings total square foot to 1600.

Wondering if it’s worth getting variable ac and multistage furnace. Did a quick manual j and attached results. Seems pretty close to what I’e experienced. Furnace runs about 90% duty cycle at -10*F. Wondering what load calc you guys use. I will be moving utilities to new room (about 5’) and replacing 12” flexi return in attic with galvanized. Also plan on moving condenser to north side instead of southwest. Lineset run would be about 30’ with 4’ height increase.

Comments

  • HVACNUT
    HVACNUT Member Posts: 4,594
    Looks like a 55K heat loss and a 3 ton A/C.
    You'll probably be hard pressed to find a single stage 55K BTU furnace so a 2 stage would be nice and the 2nd stage will probably never kick in.
    Also look at 2 stage A/C
    Get a good steam humidifier, Merv 11 filter and UV light.
    You'll need a 2H 2C thermostat with humidity control.
    SuperTech
  • the_donut
    the_donut Member Posts: 374
    Any brand or model recommendations? Any advantage from 2 stage to variable?
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 12,004
    Anything variable speed watch out. You loose a blower motor and big $$$$to replace it. Not worth it for the fuel savings as far as I am concerned
    the_donutSuperTech
  • GW
    GW Member Posts: 4,539
    I like two-stage furnace when the ductwork is looking a bit small. I’m guessing you are aware that a high-efficiency needs more duct volume then your current furnace...if you plan on keeping the same BTU. I almost always drop the BTUs. I like variable speed ac when the ducts are newer. Just do a new 2 stage ac.
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    [email protected]
  • the_donut
    the_donut Member Posts: 374
    It’s been a while since I have been in the crawl. I want to say ductwork is 16x24 down for supply with 12x24 on main return and 3x 6x14 jumpers that feed in to return. I think it is a 18” return at main and drops down to 12” then hits air handler. I was going to replace with solid same size as return junction in hall, at least 24x24 in size.
  • GW
    GW Member Posts: 4,539
    Ok it's hard for me to see the nuances and variables, getting a good local guy is the best route. In general, the two-stage isn't 'that much more' money
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    [email protected]
  • John Mills_5
    John Mills_5 Member Posts: 935
    I would not have a furnace without 2 stage and variable speed. I'd say every brand would have a 60K or so 95% in either single stage or 2 stage and variable speed motor. The furnace size seems about right for 1600 sq ft here but 3 ton seems high unless you have excessive windows. In our climate and electric rates, we really can't justify going over 13 SEER (which would likely be 14 SEER with high efficiency motor.) The 17 SEER 2 stage and the 20 SEER variable speed will be in the scrapyard well before you see any ROI. I wouldn't do variable speed cooling with floor vents without zoning. If you are filling the duct system with 25% capacity airflow, the cold air will hug the floor. No velocity to throw it up.
  • the_donut
    the_donut Member Posts: 374
    Will the variable speed motor play nice with a single stage air conditioner. Biggest problem I have is 2 bedrooms have 2 windows and exterior walls while every other room has 1 exterior wall. These 2 beds are difficult to balance.

    Eventually figured out return was leaking in attic, near filter housing, and bypass for humidifier. After sealing the joints and adding a damper in the bypass loop the house heats and cools much more evenly. Will low speeds cause balance issues. House is a rectangle roughly 32x50. Furnace is located in opposite corner from problem bedrooms.

    One supply trunk feeds east along south 50’ of house. This trunk tees about 8’ in and runs to north end feeding 3 14x4 registers in great room and 1 14x4 in north bed. South trunk feeds 2 12x2 in bathrooms and 90s at end of house to feed 2 14x4, one per bedroom.

    A 3 season room was converted on west end of house, 21’ s to n, and 7’ e to w. This room has no registers or returns. 2 of the 14x4 registers are within 1’ of room.
  • John Mills_5
    John Mills_5 Member Posts: 935
    VS blower is fine with a single stage A/C.