Hi!  Want to replace our current heat system with geothermal.  I have had a couple of estitmates -one going with a 5 or 6 ton carrier system and the other going with a 4-ton waterfurnace. I am surprised at the sizing difference.  Both contractors have been in business for several year, have good BBB records, s and have been refered to us by friends.  The carrier is a lot cheaper ($4,500) , the dealer use to install waterfurnace (what our friends have) but now only installs Carrier.  He said he would not install a 4 ton system in our house and thinks it is better to get a bigger system because we will use the back up system less.  He said we could go with the 5 ton but he thinks the 6 would be better. The waterfurnace dealer seemed very knowledgeable but how do we know who has the correct sizing?  Does one brand have a better track record over the other? Thanks!

Size is determined by loss/gain of house.
Bidders should measure your home (all doors/windows/sq ft etc.) and calculate requirements.
Ask them for those calculations if you don't already have them.
Check our shoppers checklist.
Brand matters very little if contractor is not good.
j

You will find numerous cases in this forum where the calculations were way off and using past heating bills provided a more accurate indication of what size is needed.

What jonr said- jeat loss calcs are one thing, but they don't trump fuel us. Since you have a heating history on this place, running a calculation of fuel use against heating degree days to put a stake in the ground marking upper bound on the design temp heating load is always a good sanity check.

Also, whether it makes financial sense to go all the way to the 99th or 99.9th percentile design condition on geo without having to use backup depends a lot on just how much the extra geo tonnage costs. One would also want to look at how much air-sealing & insulation upgrading would bring down the peak load. While the first ton of geo is the most expensive, the rest certainly isn't free, and the cost of peeling a ton (or even two) off the load can often be comparable to or than upsizing factor the geo. (The operating & mainenance cost of insulation & air sealing is far less than upsized geo too.) The $4500 delta in quoted cost between the 4 ton system vs. the 5-6 ton system can buy you quite a bit in the way of air sealing & insulation, especially where such upgrades are subsidized.

No reason not to employ history with load calcs. I always ask my customers to keep usage history to themselves to compare to my calcs later.
Point I don't want lost in this is I'm suspicious the bidders may not have done their home work.
I think point about envelope improvements to reduce tonnage may be a good point for shoppers checklist.
j