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  1. #1
    Join Date
    Jun 2005
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    I posted a question to another thread a week or so ago, and the answers I received sparked the need for another question/remark.

    I moved into an 18 year old house a year and a half ago, and within the next year or so I will need to buy a new furnace and AC. Every once and a while I take some time to do some research, and this forum has been a gift!

    The only thing that I have decided on conclusively is that Iíll go with an 80% furnace. I live in Toronto, and the winters arenít that bad (although last winter was relentlessly cold!), my house is only 1200 sq ft, I keep the heat in the daytime at <70 as Iím busy all the time, and that warms me up, and like the house cold at night, so the t-stat goes down to 60. Based on that, I donít believe there would be any payback on a 90%. Please correct me if Iím wrong!

    Here is where Iím getting bogged down. Single stage, two stage or variable? While I recognize the benefits of the variable, Iíve pretty much counted that out. It should all come down to comfort, right? Last winter, while it was bitingly cold for long periods, I was quite comfortable with my 18 year old single stage. It does have a 2 speed blower on it though, circulating all the time on low, and I wonít give that up. As far as the 2 stage goes, it seems there are issues with your duct work. If you donít have a well designed system, (and after trolling this site, it seems that most of us donít!!) while operating in the first stage most of the time, you may not get enough heat pushed to your upper level. Right? Well, my office is upstairs. Not an option. Iíve had a contractor here trying to sell me a two stage. Did he look at my duct work? NO! In fact, due to the answers to some of my questions, he wonít be back. According to him, it is NEVER necessary to balance ducts, and when I asked about doing a load calc, he told he that that wasnít necessary either! Dave in NJ, kudos to you for being so honest with your client Linda. I hope she recognizes what a gift honesty is. Iím often wondering if contractors in general (there have been a few since becoming a HO) think everyone is uninformed or if Iím clueless because Iím a woman!

    SoÖif my ducts arenít that good, I should stick with the single stage, right? What if my ducts are well made? Iíve been comfortable with the single stage, and the advantages of the 2 stage are mostly comfort. Would there be any other advantages of going with a 2 stage?

    Oh, one last thing. From what I can tell, it looks like the single stage (Iím looking at Carrier, but it will be whatever that good contractor I eventually find installs!) have at least a 3 speed blower, that you can run all the time. Is that right?


  2. #2
    Join Date
    Mar 2004
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    The only thing I consider imperative here is that you recognize that whatever system you get, the output capacity of the new system should be no more than the output capacity of the old one. You are happy with it, after all.

    Since the old furnace would have been less than 80% efficient, that means that the burner input capacity of the new furnace should be *smaller* than the old one. If the old one didn't have to work very hard in very cold weather, you may want to look for less output capacity, too. Slow and steady is good for comfort and it's good for efficiency.

    I wouldn't rule out the high efficiency furnaces, though... energy costs are only going to rise, and if you haven't checked yet, you may find the price difference smaller than you expected. 90+% efficient furnaces can have a dedicated combustion air intake, which I think is a major comfort benefit in a cold climate. With an 80% furnace located in the house (not in an attic or garage), all the combustion air has to get sucked in through gaps, cracks, windows, etc. That air is going to be cold, dry, and will feel drafty.

    A two speed furnace can run low and slow (which is very comfortable), yet have some extra oomph so that it can heat up the house quickly when you ask it to. If you don't mind not being to heat the place up quickly (ie you will use a programmable thermostat and have a consistent schedule) then that doesn't matter. At that point you can get the same comfort level out of a single stage furnace that is sized with minimal excess capacity.

    Is this two storeys (I spelled it like that just for you) or one, detached, semi-detached? You're probably going to be in the market for the smallest capacity central furnaces, in the 40-50,000 BTU range, given the square footage and what would probably have been pretty darn tight construction.

  3. #3
    Join Date
    Jun 2005
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    Thread Starter
    Wyounger, thanks for your reply.

    My house is a 2 story(no need for creative spelling this way!) detached, with a full unfinished basement. The previous HO insulated the visible ducts in the basement, which the house inspector told me was a smart thing to do in an unfinished basement.

    I have a programmable t-stat, the kind starts your furnace/AC in advance so the house is the temp you want it at, at the time you set the t-stat at. Yikes, does that make sense?

    My current furnace is 60,000 btu input and 54,000 btu output, and you are saying the new one should be smaller than this, because it will be more efficient. The above mentioned contractor that I had here told me I need a 75,000! Question, when a contractor throws out the BTU number, are they usually referring to input or output? Or is that a dumb question?


  4. #4
    Join Date
    Jun 2005
    SW Wisconsin
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    As a general rule they would be stating the "Input" to the furnace, however, they should state both "Input and Output to your conditioned space," and explain both to the HO.

    The BTUH Output is what counts when you are trying to meet the heat/loss.

    Within design limitations they should begin to engineer all natural gas furnaces so that you could set the Btuh output to whatever automatic variable Btu/hr scale you wanted.

    These days it is relative easy to computerize the control of all phases of HVAC/R, to optimize both efficiency of operation and comfort zone performance. Additionally, it should not be that costly to do so.
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