I live in a 5000 sq foot Log home in Oregon, and received two different bids both insisting they were correct and seemed a bit sensitive when I brought this up).

The 4 ton estimate was based on a Load Calculation (not sure if it was Manual J but I assume it was) .

I don't know what R factor he used for the 14 inch Doug Fir logs, air dried now for 16 years.

The contractor who recommended a 5 ton never spoke of a load calculation but when he saw it, he and one other contractor said it would take a 5 ton "easily" .

We have ducts that are about 18 inch and three returns that are 24x36 inches

When I play around with Manual J calculations, using rough estimates, I came up with house demanding 60,000 BTU's (total was 75,000 but it said to take 15% away from that). It appears this is the max load for a Heat Pump. If it required more that the only heat pump I could use would be dual zones, or a Geothermal system, if I understand correctly. I may not have done Manual J correctly however.

Ultimately, I just find it hard to believe that this home coudl be cooled with one 4 ton, and since our last AC unit, a 4 ton Carrier SEER 15 broke down after only 8 years, it makes me nervous trying to put another 4 ton in.

Most houses this size have dual zones (which we could retrofit but it would cost a lot - would rather use a Mini-split system in my bedroom, and a regular Heat Pumps for rest of house).

A bit about my house:

It is 4900 sq feet, with :
1) 1900 sq ft finished basement on CONCRETE SLAB that is over 10 degrees cooler at all times.
2) Main level is 1900 sq feet with tons of south facing windows (16 x 16 feet wall of windows in vaulted living room alone).Main Floor above basement is NOT insulated . Ceiling is 25 feet high
3) Upper level is 1100 sq feet - two LOFT bedrooms. with huge windows
a) Loft Master BR bedroom is 27 by 22 (675 sq ft) with wall of West facing windows (approx 16 x 10 feet) and two south facing 45 degree skylights ( 4x6 each). This room is the HOTTEST place in the house yet I spend much of my time in here (I work for home and use it as an office, I work out in here, etc).
b) My son's Loft BR is 14x20, separated from mine by log walls. Master Bath is enclosed 9143 sq feet) with radiant heat flooring.
We use a total of 9 appliances between the two living spaces (Basement Apartment and Upstairs) as we have two laundry rooms, two kitchens, but only one Furnace (15 years old - Carrier 80% efficient with pilot light - not sure how many tons it is). We have overhead lighting in every room (total of 16 rooms including bathrooms) but we use High Efficiency bulbs. We have two TV's, Energy Star rated, and 4 computers.

I attached a photo of the outside of the house, for reference.

We are not currently interested in Geothermal due to the 5 foot trenches required, as there is a lot of volcanic rock under our home, which would be hard to cut into. we have a lot of volcanic rock it would be hard to excavate).

We spend $3600/yr for Propane (Gas Furnace, and Hot Water Heater use propane, as well as radiant heat in upper Bath)
We spend $1600yr for electricity (WITHOUT Air Conditionner - with it we spent around $2100 so AC added about $41 a month (does not seem like that much but the Heating costs were $300 month averaged.

Is there enough info here for others to weigh in on 4 ton vs 5 ton. The 4 ton did cool the house, but, as stated, the compressor gave out after only 8 years.