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Old 05-22-2006, 05:17 PM   #1
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Who do I believe?

We have an approximately 1800 sq.ft. home that I just had the furnace and A/C replaced in last summer. It now has a Trane 12 SEER 3-ton A/C unit and a Trane 79k BTU variable speed 2-stage furnace. I am building an addition and adding approximately another 1400 sq.ft. I've had two HVAC guys out to bid on my job. One says that the existing system is big enough to handle the additional square footage and that he is proposing to only add duct work and install a zone control system for the existing and new areas. The other HVAC guy says that it is not big enough and is proposing adding a heat-pump, (installed in the attic), to heat and cool the new third level of the house. For obvious reasons, there is a HUGE price difference here. Whom do I believe? Neither will share their manual J-calcs with me but both swear they have done them and that they substantiate their respective proposals...




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Old 05-22-2006, 05:43 PM   #2
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Did you ask the original installer? They would be the ones I would trust. They have made their money and know the unit the best.


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Old 05-22-2006, 07:00 PM   #3
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Dave, I doubt if the return airs and vents are the proper size for the larger area. Need to check air flow.Are you going to be using the new area all of time or would zone HVAC be a better choice. For a lot of reasons, I would look at a separate unit. How is the corrent unit woprking for the house now? How long does it run between cycles.
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Old 05-22-2006, 07:00 PM   #4
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How cold does your area get in the winter time?

Did the furnace run more times than it was off last winter?

When was the house built?

How thick are the current walls? (4,6,8"...)

What R-value does the current walls and attic have?

Do you have high efficient windows?

What will the R-values of the new addition be?
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Old 05-22-2006, 08:42 PM   #5
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Heat loss/gain calculation

There's only one way to know for sure. Have the contractors perform an ACCA Manual J heat loss/gain calculation on the existing house (which should have been done when you installed your new equipment) and with the new addition. Believe me, the numbers don't lie! If they can't provide this, then they must be guessing...at your expense.
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Old 06-19-2006, 06:12 PM   #6
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I have a 2600 sq ft house and four ton ac. It aint big enough by at least a ton. Your are going to have a 3200 sq ft house on three tons. I have no idea where you are but if your area is even a little like the Texas gulf coast I bet you will need another three tons.
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Old 06-20-2006, 08:08 PM   #7
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I would get an engineering firm that does building inspections to do an appraisal of your project (it will cost a little, but they have no interests other than yours)
your bank or city department should be able to recommend one

there are just too many variables to try guessing from my desk
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Old 06-20-2006, 10:55 PM   #8
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Neither. Try this, it will take some effort, but you asked.


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