Did they size the furnace with a Manual J load calculation, guess, go off old size? Proper sizing is important. Though for your size home and your location, the largest version may be necessary. Did they inspect the ductwork to ensure that it was properly sized and in good condition? Great thermostat choice. As far as the attic fan goes, you may want to give it a second thought. They can sometimes create a negative pressure, sucking out conditioned air from the envelope of the home. Do you have ridge vents and attic vents? I'm not an expert on this subject, so I'll let someone else advise you better on the attic ventilation.

I also question the furnace size...more is not better.

what size is AC?

The size of the Tempstar unit was 120K BTU - I have since added 1350 sq feet by fnishing the basement and ducting every room down to the floor. That adds 5 duct runs to the existing system. THis brings the home from 2300 sq feet to 3600 sq feet.
The sizes for the G71P are 110 and 135 which is closet to my existing furnace - adding 1350 sq ft it would seem logical to up size from a 120 to a 135....is this wrong?
The guy doing the quote did not measure duct sizes...I have had 4 quotes thus far and noone has done that.
I would like to add zoning at some point to control the basement and bedrooms on the second floor. For this I would be adding Honeywells Envriazone and duct controls

No system will perform well unless the ductwork is adequate. Have it checked before you sign any contract--ensure ample supply and return as well as ductwork sizes. Your sizing change makes sense; HOWEVER, your 120k btu furnace may have only been 80% efficient, or less, meaning less of that was being output as heat.

I will add that the home is 13 years old, duct size on the supply run is 20' x 8" and runs 30 feet acros the basement feeding all the runs to the home. There is return air on the second floor and on the first floor. The duct sizing into the furnace is 20" x 14". Also, the person who added the duct runs to all the basement rooms added a large return air in the basement as well and added a curved transition where previously it was square on the supply side on the floor into the furnace to increase efficeny. This seemed to help move the air in the ducts with more force. I hope that helps on some good feed back

Would that be 20x8 going one way or two?
Any takeoffs on the plenum?

One more question-
Are you calling the Tempstar a piece of crap because you have/had problems with it or are wishing to increase your perceived efficiency and desired comfort?

I had to pay 2-seperate times $250.00 to have the main control board replaced.....the second time it was in the dead of winter and we were with out heat for almost 2-days which is not good with a little one inthe house. So I am not happy with the Temstar unit. Also, as you noted it is my intent in the big picture of being more efficient these days the 80% is a crime and an easy out by home builders to spend a few dollars and install marginal grade stuff. I think all new homes should be build with better efficiency in not only the furnace and hot water heater but in all of our appliances...just IMHO

The 20 x 8 only goes one way for 6-7 feet (feeding 4 duct runs) and runs the other way for the rest of the distance. The Furnace is centered 8-feet off of the West wall.

As far as take-offs (I hope I understand this correctly) on the supply side there is the cutout for the Humidifier which is a by-pass unit and the pipe connecting it from the furnace side. no duct runs come from the plenums.

That helps; I envisioned 1-20x8 for the entire system running the length of the house. My concerns for airflow are relieved quite a bit sight unseen.

Sorry to hear about the board, if it is any consolation that board is on Carrier units too had that been your furnace.

If you could how many take-offs in total and what sizes are they?

15 - 6" Round

Homebuilders choices for HVAC reflects what potential homebuyers are looking for. Granite countertops always trumps high efficiency equipment.

IM140

Rest assured your duct work supply is more than adequate based on the amount of 6" takeoffs. Based on common sizing parameters, typically a 6" run is capable of up to 110 CFM each, without exceeding velocity and system statics that cause system problems and noise. Being some usually serve baths and areas that are smaller, I will usually call 90 CFM as an average. Also under the same conditions above, a 6" steel pipe duct run is capable under average conditions of delivering 6,000 BTUÂs max of heat. Having 15 of these in your house, your system is capable of distributing approx 90,000 net BTUÂs max

Prior to adding the 5 runs in the basement, it is conceivable that your oversized furnace cycled frequently contributing to the failure of the circuit boards.

Couple things to keep in mind; basements donÂt usually have the same heat requirements as a first floor due to no/less windows, doors and infiltration.

Running some quick numbers for an average home in Colorado, I offer the following:
The 2,250 living space will need around 78,000 BTUÂs net, the finished basement around 27,000 BTUÂs net with a total of 105,000 BTUÂs net.

Based on the ductwork, number and size of takeoffs, and home layout, if going hi-efficiency, you should be looking at a new furnace in the 115,000 to 120,000 BTU range. Any larger will put you back into the similar situation of having an oversized furnace. As you may have read, with heating equipment, especially a forced air furnace, bigger is not better.

You should as always confirm this with a pro that gives you the courtesy of a good heat loss calc.

In this furnace choiced (G71P) the choice is 110 or 135.
you say 115 - 120....so my choice will be work either way - either under sized or over sized??
You say the basement doesnt have the same heating need as the rest of the home....I would assume that is if it is zoned??
At some point we would like to bump out our family room adding maybe 200 sq feet to the home.

I would like to consider zoning - adding a thermostat in the basement and to 2-bedrooms and a bath on the second level. Does this change the furnace choice?

I would go with the 110, I tend to cover for the few days it might be extreme cold. Zoning can change the loads slightly but not enough for this application. If you bump out the family room, do you have or will this add a fireplace that is heating capable?, something too consider.

Basements often have about 50% of the heating requirements as a first floor, since they are buried. In addition they often have a 0 requirement for cooling, and the zoning dampers can help immensely with this.

In your judgement what is the point when a 135k btu furnace would be considered? Is it sq. feet or number of take off's? What would change in our home to consider the 135k btu? I know you are at a disadvanatage because you cannot see the job - but our sales guy is insisting that the 135 is the correct size for the home??

Does varabile capacity make any difference in sizing?
I choose this model (although new and not really tested in the field yet) is #1 Dual Fuel, #2 It goes behyond the standard 2-stage units by being that this is a varabible speed fan and using what Lennox calls "Precise Comfort technology" where the logic monitiors your homes heating needs as outdoor tempereture change. Then using "variable-capacity technology the furnace adjusts heat and airflow output in increments as small as 1%

Our job is to be installed in mid September

First question, they go hand in hand somewhat. You see you need to get the bonnet temp dispersed from the furnace to hit recommended temp rise out of the furnace to obtain efficiency and expected life of unit. At the same time if you add more than required heat runs to do this, the room temps will swing horribly causing discomfort. Although you may wish to have additional heat runs to deliver airflow at a lower CFM, with low FPM at the diffusers which result in a quieter operating system. At the same time depending where the registers are placed, certain velocities may be required for the air flow to efficiently get to the areas that need the conditioned air. So you see, it takes an experienced person to weigh all the factors to assure a proper balance to end up with a great system, versus what most people end up with or settle for.

Going beyond a 2-stage system,, with a system that precisely varies the input and blower speeds is always an advantage. The benefits is that it works at the capacity required which always changes because indoor and outdoor conditions are constantly different. As we all know a standard furnace is like a car in the hands of a teenager, either always on the gas or the brake. The goal of a great heating system is to be seen and not heard. The variable systems will deliver at a required rate to match conditions with the results being limited temp swings which equates to improved fuel utilization and comfort.

A quick answer on your 135 question, if all takeoffs were 6" and the correct amount were installed in the space, to match room loads, you would have about 21.

OK - I had a "Professional HVAC" person come over who measured all my ducting which is actually 8"x24 in the center section and 8x20 on the ends) Counted and measured all my windows and doors and examined my Return air and came back with the following:
According to a residential heat-loss analysis of your home (based on ACCA Manual J), the correct size of the replacement furnace is 135,000 BtuH input. This is based on a heat loss of 91,500 BtuH at altitude for the existing structure plus the potential for a 200 sq. ft. addition, bringing the total up to 96,661 BtuH. The Lennox G71MPP60D-135 furnace will yield 103,000 BtuH at this altitude.
So his recommendation was for the 135 and not the 110. He also noted ithe furnace will oerate on its lower stage almost all the time except on the below zero days.

Next year I will change all my windows to Wethershild zo-e-shild 6 and 7 windows which will make the house much tighter. With this in mind I am also adding the Lennox Pure air system that is Merv 16 and germicidal lights

I was following all along until you told us it will stay in first stage until it gets below zero.

So what you are telling us is that approx 85,000 is good enough till it hits zero.

By theory, based on a 65/35% firing rate if 87,000 is good till zero and then you will need the remaining 35%, the furnace is too big, by 25% or so.