Assuming he's competent, he's either bound and determined to see that you are not oversized for cooling or he has determined that your ducts won't flow any more air than is needed by 4 tons. But in either case he should have spelled out the limiting factor

They just seemed VERY focused on lowering the up front cost..He didn't mention duct size at all. Rather he promoted low up front cost and long run times as the benefit (well, infinite run times during the winter without aux heat really)...

He stated operating costs for Yakima of $1800 in winter and $120 in summer..The idea seemed to be lower upfront cost with moderate operating costs with substantial use of electric resistance heat.

I don't buy that as a good solution. The 4ton/3ton combo guys clearly stated the size of the ducts and the fact they considered duct work in their bid and felt that my current duct system was adequate for the entire system. (never mind the fact they are only a little bit more expensive.)

Did the 4-ton guy indicate what size earth loop you would get? Is it a 4-ton loop for a 4-ton system? Note that geodean reminded us that the loop should be sized to the heat load of the building, NOT to the equipment tonnage (in the concurrent Loop Size thread). You can only get "full capacity" from the heat pump if the equipment can exploit the earth source, and you can only do that by having enough loop. An undersized unit connected to an undersized loop could have a number of "domino effect' consequences. As the heating load of the house increases, the unit will have longer run times and will drive down the entering and leaving loop temperature. Eventually, the equipment will not perform at the spec data. A consequence to the occupants is that - without back up heat - you will definitely perceive the cooler supply temps from the air outlets. The next domino is the auxiliary heat. With an undersized loop you will use back-up more than the initial illustrations. You need to make sure that you have enough loop for "worst case" design conditions. You indicated that loop length from both "good" bids was 780'(?)to 800'/ ton. This would account for the dry, rocky soil in your area. This gives more pipe surface area for the thermal transfer, whereas "standard" loops for damp soil are about 600' slinkies. Seven 3'-diameter slinkies can fit in a 30' wide excavation, with 1' separation between each. And 800' pipe circuits will be close to 100' long when coiled into slinkies. Seems to me that the 4 ton /3 ton guys already have the total loop covered. I realize that the earlier discussion focussed on your duct sizing. I just wanted you to have some perspective on the exterior loop issues, also.

Geogirl,
The 4 ton quote included a 4 ton loop as it was described to me (aka their standard for the area 700ft/ton x 4 ton) loop.

Of course, the 7 ton guys included a 7 ton x 800 ft/ton slinky loop (estimated excavation 100ftx30ftx6ft deep.

So, if you had the 4-ton guy increase his loop size to equal the home's load (say 3 tons more loop), you might pay another two to three grand, right? Then, wouldn't that cost nearly equal the 4 ton + 3 ton bid, the bid that gives you two Geo units AND signficiantly reduces reliance on resistive auxiliary heat...?

Yes, exactly Geogirl.

I have a 3rd IGSHPA accredited installer coming tomorrow to bid the job and then I'm going to decide (likely between newest bid and 4 ton+ 3 ton bid (which, compared to the single 4 ton bid seems VERY reasonable (and includes a full duct sealing as part of the bid--I just found that out).

Oh yeah, Yakima, where I live, is supposed to have highs between 15-25 this week with lows of 4-8dF, ALL well below the balance point of the 4 ton units 24 hours a day....

Sigh..

Interesting thread. You have two contractors that appear to be highly professional. Based upon the BIN data Engineer posted, the design temp should be 5 F. One thing to keep in mind is that the two units will give you better overall comfort than a single unit (based upon personal experience). As geo-girl points out their costs are not hugely different if the loop sizes are more similar. Also, if the loop is upsized then the unit will provide more heat during the coldest nights.

Do any of the contractors give you an energy cost analysis, this would include the bin data breakdown for how many btu's are needed at what temperature outdoor. Some contractors are not comfortable or unaware of the difference in operating costs of a 90% load match compared to a 100% load match, the operting cost for the 100% load match system is going to be much higher than a 90 or 95% match. the first guy to show you the data is the guy to look at first. He did his homework, not just a heat loss.
you need to prove to the homeowner that what you are offering is the proper system not guess at it.

Do any of the contractors give you an energy cost analysis, this would include the bin data breakdown for how many btu's are needed at what temperature outdoor. Some contractors are not comfortable or unaware of the difference in operating costs of a 90% load match compared to a 100% load match, the operting cost for the 100% load match system is going to be much higher than a 90 or 95% match. the first guy to show you the data is the guy to look at first. He did his homework, not just a heat loss.
you need to prove to the homeowner that what you are offering is the proper system not guess at it.