Questions on replacing 2 AC systems

[seeking444]

I own a 3500 sq/ft house with two 33 year old systems 3 ton/4 ton. One system goes to 4 bedrooms and 3 bathrooms with approximately 1500 sq/ft per one contractor. The other to living room, den, dining room, kitchen and utility with about 1600 sq/ft per the same contractor. I received estimates in 2006 when the house was purchased. The contractors I contacted recommenced the following options:

Carrier 3.5 ton/4 ton
Trane 3 ton/4 ton
Lennox 3 ton/4 ton

The systems now need to be replaced. Estimates this month are now the following:

All 4 ton/4 ton

All include attack installation of furnaces with drain pan, new copper refrigerant lines, new concrete pad, and use of existing duct work (which all contractor in 2006 and now have agreed are more than adequate).

The house is in the Texas Panhandle. Below is the climate information and cost for electricity and gas.

ave.low ave.high sunny days
January 22°F/-4°C 50°F/10°C 19 with 15 days or so less than 20
April 42°F/5°C 72°F/22°C 20
July 66°F/19°C 92°F/34°C 26 with 15 days or so above 100
October 45°F/8°C 73°F/23°C 23
ANNUAL 44°F/6°C 71°F/21°C 270
Humidity is not a problem.

Electricity: 5.5 cents/kWh
Natural Gas:$0.517 per Ccf

I find my house comfortable enough in all weather types, but there are some hot and cold spots which would be nice to smooth out to a more even temperature.

The questions I have are the following:
1. Do I really need new concrete pads?
2. From my research a 2 stage furnace is the way to go. It saves money overall and should pay for the extra cost relatively quickly. Is this true?
3. 80% vs 95%. Is the cost worth it. My research seems to indicate no.
4. From what I have read a variable speed fan will even out the temperature and maintain it more consistently. Is this correct? Will that increase the electricity/gas cost overall to operate vs a fixed speed fan?
5. Does a dual stage ac compressor also save money like the 2 stage furnace?
6. 2 stage compressor vs dual compressors. Any advantage or disadvantage?
7. Any thoughts on reliability and cost of maintenance and repair on the 2 stage furnaces vs single stage, variable vs fixed speed fans, dual stage vs single stage compressors, dual stage vs dual compressors?
8. Any other considerations I should consider?
9. Any thoughts on why the change in tonnage requirements in 7 years?

Thank you in advance for your time in helping me figure this out.

[George2]

There's a lot to reply to, I'll take the questions in order:

#1. The new units normally have a larger footprint, so yes. Concrete slabs is far better than the (junk) plastic pads so many are using now.

#2. and #3. together.......2-stage is very nice and 80% is all you'll need with your warm winters. That said, I'm very surprised no one has mentioned heat pumps! P.S. Your gas and electric rates are half of what ours are.

#4. Variable-speed equipment is a must. Make sure that they install hi-efficiency air cleaners.

#5. 2-speed condensers are for better de-humidification and quieter operation.

#6. Both are fine.

#7. No difference in reliability.

#8. Have a Manual J (to find the correct sizing) performed. If you have the house plans, there are services available (check online) that will do it very reasonably. You'll need to point out what system does what area.

#9. I question why your home even has a 4 ton in it now. Maybe a lot of windows with western exposure? Plus, if you have a 3 ton and a 4 ton now, it makes absolutely no sense to increase the 3 ton to a 4 ton.

Sidebar...........is your home a ranch or 2 story? Where is the equipment located?

[seeking444]

Thanks for the reply. The house is a single story. One system at each end of the house. 3 ton at the bedroom end 4 ton at the kitchen end

[George2]

Thanks for the reply. The house is a single story. One system at each end of the house. 3 ton at the bedroom end 4 ton at the kitchen end

Is the home on a slab? Is the ductwork in the floor or the attic? I just re-read your thread and I added the sq. ft. of the two wings and came up with 3,100 sq. ft., where did the other 400 sq. ft. go?

Also, just another F.Y.I., normally the ductwork is borderline for the original tonnage of the A/C airflow. If the contractors did not examine and measure the ductwork throughly (it takes a lot of time), then I would be very hesitant to install a 4 ton on a 3 ton system. It requires a 1/3 more airflow. Just another "red flag" in my humble opinion.

[carmon]

7 tons for a house.... wow do you leave the windows open....

[seeking444]

Slab foundation. All ductwork in attic. The missing space I would guess is hallway and entrance way. I do not believe the original estimate was measured accurately by the contractor but a guesstimate.

[ga-hvac-tech]

There's a lot to reply to, I'll take the questions in order:

#1. The new units normally have a larger footprint, so yes. Concrete slabs is far better than the (junk) plastic pads so many are using now.

#2. and #3. together.......2-stage is very nice and 80% is all you'll need with your warm winters. That said, I'm very surprised no one has mentioned heat pumps! P.S. Your gas and electric rates are half of what ours are.

#4. Variable-speed equipment is a must. Make sure that they install hi-efficiency air cleaners.

#5. 2-speed condensers are for better de-humidification and quieter operation.

#6. Both are fine.

#7. No difference in reliability.

#8. Have a Manual J (to find the correct sizing) performed. If you have the house plans, there are services available (check online) that will do it very reasonably. You'll need to point out what system does what area.

#9. I question why your home even has a 4 ton in it now. Maybe a lot of windows with western exposure? Plus, if you have a 3 ton and a 4 ton now, it makes absolutely no sense to increase the 3 ton to a 4 ton.

Sidebar...........is your home a ranch or 2 story? Where is the equipment located?

Thought I would add my thoughts:
#1: Agree
#2, 3, 4: Two stage heat and VS blowers generally come together (both or none). IMO the VS blower is a must, so take the 2 stage heat with it, you will like it.
#5: Generally 2 stage AC is both comfort and humidity control; usually a LONG (if any) payback from cost. You have really low elec rates... if comfort and humidity are not a concern (all some folks want is cold air), then with your low elec rates you probably will not get your payback. Personally, the comfort levels are more than worth the extra cost to me.
#6: Two stage compressor vs two compressors: Generally a 2 stage compressor system low is 70% of high... while a 2 compressor system low is 50% of high. Two compressor systems are expensive.
#7: Agree
#8: Agree
$9: Goes along with #8... get a load calc (get it 'room by room'). With a total number and each room number; equipment size can be closer, while some ductwork mods to even out air distribution can be done. A little over sized with a 2 stage system is not a bad thing... given sometimes we have unusually hot summers or maybe a party with 20-30 guests. Each is an excessive heat load. A little extra (which does not run all the time) is a nice thing. Might remember that 2 stage AC comes in even tons only.
As noted, media style air filters are important with a VS style furnace.

One of the things to consider is how long you plan to stay in the house. If 5 or less years... get the economy systems (not builder grade). If you plan to stay a long time, I would go for comfort. You will be thankful you did...