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Heating & Air Conditioning

Size of replacement furnace and air conditioner

16 years ago

I live in a 20 year old home that faces south. The home is 4200 sq ft and 900 sq ft attached garage. The home is a raised ranch where the basement, which is finished is only sunk about 4 feet. The walls are 2x6 construction with an insulation value of R30. I am interested in changing the 5 ton air conditioning unit that is as old as the house and the first guy that come out did a ton of measurement and did what I think was called a heat/air load (I think) and he determined that I only need a furnance and air conditioner that is half the size of the one I have. I am not only replacing because of age but because that last 2 or 3 years the upstairs is warm and downstairs is freezing and the unit seems to run and run and never cycle off. I have had a few other guys come out and they say nothing about the size of the system and give me a quote on replacing what I have. I am concerned about spending all this money on a system that is half the size. Any advice would be great. Thank you.

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Comments (3)

daddo
16 years ago
It depends on where you live on the load calculation.
If the contractors won't or can't do a load calculation on your home- don't deal with them.
I would get at least three bids which include a heat gain/heat loss calculation and size accordingly. If the size of the system changes much, the ducting may have to be re-sized for proper airflow. 1/2 the size is suspicious.
woodinvirginia
16 years ago
1/2 the size of the 'physical' size unit you have in there
should not bother you at all. Some of these uunits A/C and furnace have combined their units so they only use one Variable speed motor to accomplish the forced air for both functions.

What you need to desicern is how any BTU's your current system is : [1 ton is 12,000 BTU's ] and what is being proposed for its replacement. Get at least 3 estimates.
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To be clear I did NOT say that they INSTALLLED the $30 thermostat for $200. I said they sold me a $30 thermostat for $200! The INSTALLATION and the SERVICE CALL were TOTALLY different charges so the whole thing was about $600 (and it conked out 3 days later so changing the thermostat didn't fix the problem,and so much for thier "diagnosis") so I DO think that the people I dealt with were thieves. I find it interesting that you would jump to such an erroneous conclusion as to me being difficult being the problem. That was REAL helpful. What I had hoped for was somewhere to start educating myself about what to look for in a good central air conditioner and what factors make one better than another, and if there ARE any REAL hazards about one kind of system over another (although I find that unlikely in refridgeration but I don't know which is why I'm asking). Otherwise the NEXT few people I deal with may try to convince me that System X will explode,and that is why Sytem Y which THEY install is better. Thanks
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Sorry, I really did not mean to post and run but other things required dealing with first. I know bsolutely nothing about air conditioners so took this picture of the Bryant unit that is there now: Answering some other questions...the place is almost gutted, insulation will need to be replaced in every single wall, from about 4 feet down, some windows that were blown out by flood water will also need to be replaced. As to build quality, it's still standing LOL. tigerdunes, not sure what AC or HP mean hence the picture. The furnace is a Bryant gas, coil is from Bryant as well. installed around 1996 and appears to be fine.
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I found this note on page 11 of the installation manual under upflow units: NOTE: Where the maximum airflow is 1800 CFM or more, both sides or the bottom must be used for return air. This is saying the bottom return by itself will be enough. There was note under the upflow unit section, but I assume it also applies to a horizontal installation. You want the filter and grill cover to be at least the size of the opening to the furnace. Making the grille bigger is not going to help if return itself is smaller. I don't know how this is going to work with the filter size you quoted. I would think you would need two 14X24 filters. It would be nice if you get get a 4 inch filter instead of the 1 inch. Otherwise you will be changing the filter every month. The 6x6 stud obstruction should be removed and you want a tightly sealed return duct. The return duct should not have a right angle. It should be a curved elbow to make the 90 degree transition. Sharp angles create more air resistance. How big was the original house and AC, and much area are you adding? Most houses don't have the properly sized duct work to handle a 5 ton AC. Do you have confidence in this HVAC contractor?
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goodgal what mdl/size/brand/age is existing furnace? you will not qualify for the tax credit with an old furnace and a new 13 SEER AC paired with new evap coil. what size AC are you replacing? for what size living area? IMO
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bob_brown
16 years ago
While I recommend doing load analysis for a new house, a remodeled house, or a house that has problems, I do not see the need for load analysis when the system works adequate. If thier are problems, and the system seems to be struggling, I look for causes, or changes to the system that may cause the problems. If the household keeps the house cold, I understand that more cooling is needed. I ask questions about cycle rate to determine field conditions. You will never get this info with a man J.

The newer systems are not the same rating. They seem to be light for fixed speed, and half the size for variable speed. The ambient does not change, so the initial conditions still need to be met. Todays trend is to use less capacity with equipment that are rated for less cooling, yet the same designator like 3 ton or 4 ton is still used. Kinda dumb.

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