1 Stage or 2 Stage Furnace/AC Unit?

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Old 02-17-15, 06:50 PM
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1 Stage or 2 Stage Furnace/AC Unit?

Im looking to replace a furnace and ac unit. the house is a 2 flat, built in the 1920s, and about 1600sq ft in chicago. My first estimate said it would be better a 2 stage unit and the 2nd estimate said 1 stage unit would be better because a 2 stage wouldn't get the heat all the way up to the 2nd floor and would take longer to heat up.
So im wondering if that is true. below are the options i have. any advice would be helpful.

1st Estimate
Lenox el296v
Lenox xc-13

2nd Estimate
Carrier 59sp5
Carrier 24abb3
 
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Old 02-17-15, 07:05 PM
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The person that told you a two-stage unit wouldn't get heat to the second floor is an idiot. Get some more estimates.

In truth, with the wide fluctuations in temperature that you experience in Chicago the two-stage would be a far better choice.
 
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Old 02-18-15, 02:45 AM
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2-stage is better when it comes to comfort. Airflow and heat output are lower, but the furnace runs for longer and heats more evenly/quietly.


to benefit for 2-stage, it has to be sized properly and controlled by a two-stage thermostat.
 
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Old 02-18-15, 12:22 PM
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I know, for a fact, two separate systems, one for each floor, will cost more to install but you will be far more comfortable. Another benefit is if one goes down you will still have at least some heat or cooling.
 
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Old 02-18-15, 01:11 PM
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I have more to add but I am going to wait and see if Evelyn is going to return.
 
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Old 02-18-15, 04:39 PM
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I know, for a fact, two separate systems, one for each floor, will cost more to install but you will be far more comfortable. Another benefit is if one goes down you will still have at least some heat or cooling.
That's just a patch for poor duct design and balancing. Most systems aren't designed right or balanced.
 
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Old 02-18-15, 07:29 PM
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Hi, thank you for the advice. Yes, the 2nd estimate said something about losing oxygen as it goes to the 2nd floor. I also would like to know if Lennox or Carrier company would be a better option. Any information would be appreciated. thankyou
 
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Old 02-18-15, 09:12 PM
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That's just a patch for poor duct design and balancing. Most systems aren't designed right or balanced.
Muggle,
I beg to differ. Most 2 story houses need less heat & more cooling upstairs than down. Unless you re-balance seasonally, most people are going to be uncomfotable sometime. At least that's how it is around here.
 
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Old 02-18-15, 10:34 PM
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I'll try to incorporate all the comments into my answer.

Grady is correct that two furnaces will be better able to control the comfort level on each floor BUT retrofitting two furnaces where there is now only one would be extremely expensive. Even in new construction two furnaces are going to be expensive.

Muggle is correct that a properly designed, installed and balanced duct system will achieve a high level of comfort in most two-level homes. The problem is that (unfortunately) few homes have properly designed, installed and balanced ductwork.

I sincerely doubt that you are considering major changes in the ductwork (really expensive) so the question is...what is best for YOUR home.

In the 1920s houses were built without insulation and "leaky" as it was thought that it was more healthful to have a lot of outside air coming into the house. Energy was relatively cheap back then and the cost to heat a house without insulation or air sealing was not all that much. To counteract these hugely inefficient building practices large ducts and huge furnaces were installed. Of course today that simply is unacceptable and now we want as much air sealing and insulation as possible in our homes.

So the first set of questions, is your house "as originally built" or have there been any energy saving retrofits installed? Things like new double (or triple) pane windows or even just storm windows. Has any insulation been added and if so what kind, where and how much? Has there been any kind of air sealing performed? Also, the biggie, does the house have its original furnace or has it been replaced? If replaced, about what year was it done? AND, what is the BTUs/hour rating of the existing furnace?

Next, what BTUs/hour size of furnace are these contractor/estimators quoting for your house? Are they sizing these furnaces the same as the existing furnace? This is important because the chances are almost 100% that the existing furnace is way larger than necessary, possibly as much as two or three times larger than necessary. Are you planning on making any more energy saving improvements on the house? If so, then these improvements MUST be considered when sizing the new furnace or you will get a furnace vastly oversized. An oversized furnace will have a higher capital cost as well as having higher operating costs. Those higher costs are nothing but pure waste.

If you are contemplating additional energy saving work, that work should be done prior to or in conjunction with the new furnace installation if at all possible. If it is not possible then you should still size the new furnace to operate with the new improvements. For sizing, the industry standard is doing what is called a Manual (book) J heat loss/heat gain calculation. Heat loss for the furnace and heat gain for the cooling. This calculation is run on a computer and takes into consideration the construction of the building, the amount of window and door space, the building orientation, the prevailing winds of the area as well as solar gain, the type and amount of insulation and the "leakiness" of the building. There are no short cuts or rules of thumb that can replace the Manual J calculation. Unfortunately, few companies will take the time to do a Manual J on an initial estimate but you should INSIST that one be done before making a determination of the size of the furnace AND that the proper sized furnace be installed rather than the generic size used for the initial estimate.

Note that the Manual J takes into consideration the local temperatures as recorded over several decades to determine the worst case heating (or cooling) requirements. Sizing to the Manual J calculation will get you a furnace that will run continuously during these "design" cold temperatures. That means that whenever the temperatures are higher than the "design temperature" the furnace will cycle on and off to maintain the desired indoor temperature. Another way of looking at it is that at any temperature higher than "design" the furnace is oversized. Now comes the beauty of two-stage furnaces. At design temperature the furnace will be operating on the second stage continuously. As the outside temperature rise the second stage will cycle off leaving only the first stage to maintain the heat to the desired indoor temperature. The first stage may run continuously and the second stage cycle during outside temperatures somewhat above design. Still warmer outside temperatures will have the furnace cycling on the first stage only. This IS all predicated on using a proper two-stage thermostat. Do NOT let some fast talking salesman tell you that the furnace has a "brain" and will select the proper staging without a two-stage thermostat.

Part and parcel with the two-stage furnace is to get a variable speed blower. This is the blower that moves the room air through the furnace for heating. Using the variable speed allows the proper temperature rise through the furnace regardless of the stage it is firing. On the first stage the air will move more slowly which will be quieter, often to the amount that you won't even hear it when on first stage. Probably 70% or more of the time the furnace will be firing on first stage only and while it will run longer it will not burn any more fuel but it WILL maintain a more even temperature throughout the house and this equals higher comfort levels.


There is more, much more, but my arthritic hands are hurting now. I won't be back until late Friday or maybe even Saturday so hopefully others will add more.
 
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Old 02-22-15, 02:31 PM
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Hello, thank you for all the info. To answer your questions
the furnace had been replaced in 1990, the house is insulated except the basement
we had the windows and entry door replaced in the past 5 years.

the carrier estimate had 100,000 Input Btu’s 97,000 Output Btu’s
the lennox estimate had 90,000 Btu's
I believe the estimators based the Btu's on the furnace we had now
 
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Old 02-22-15, 06:14 PM
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The Manual J heat loss/heat gain program requires the dimensions of each room, each window, each exterior door and many aspects of the house construction. If the estimator didn't take these measurements then a Manual J was not performed.

Now I'll ask a few more questions, these should have also been asked by the estimator. Generally speaking, are you satisfied with the heating throughout the house? Same question regarding cooling. Are there any areas that seem to overheat (or over cool)? Any areas that never are warm (or cool) enough to be comfortable? Do you get a blast of warm (or cool) air that changes the room temperature significantly and then the unit stops for a period of time and then repeats? On the coldest days does the furnace run continuously or almost continuously? When in cooling mode is the indoors relative humidity controlled so that you do not feel "sticky" or clammy?

The Lennox quoted is both a two-stage and variable speed model whereas the Carrier is a single stage variable speed. Both are 90+% AFUE (Annual Fuel Utilization Efficiency) rated. Did either estimate include a new thermostat? The Lennox NEEDS a two-stage thermostat for best performance. The two-stage thermostat also needs a minimum of six conductors (more is better) in the cable between the thermostat and the furnace. More than likely you have only four or five conductors so the estimate should allow for this cable replacement. The option is a "communicating" thermostat which can use only four conductors.

Look at the label inside the cabinet of your present furnace for the BTU ratings. You should see both input and output ratings. The output ratings should be about 70% to 80% of the input rating. I would not be surprised to find that your current furnace has an input of 125,000 BTUs/hour or perhaps even more. If it has a rating of 125,000 and roughly 70% AFUE then the output would be around 87,500 BTUs/hour. Installing the Carrier with a 97,000 BTUs/hour output would be installing a furnace that had roughly 10,000 BTUs/hour MORE output than your current furnace or roughly 12% LARGER than your current furnace. If your current furnace is already too large (most are) then the Carrier unit quoted would be even worse.

Even if your current furnace is rated at 100,000 BTUs/hour it is likely anywhere from 50 to 100 percent larger than needed. Without any more information than you have given I would think the 60,000 BTUs/hour two-stage unit would be a good fit. Understand that sometimes the required cooling needs will skew the results towards a larger furnace but I doubt that will be true in your situation.

Again, DEMAND that a full Manual J be performed after selecting the installer and BEFORE a specific model furnace is selected.
 
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Old 02-25-15, 01:23 PM
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^Good advice, except most 60-70% afue units have an output that's 75-80% of the input. The afue is low on these units because of the standing pilot and draft hood.
 
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