After reading many of the posts I realise that its hard to answer whether a proposal is correct or not given all the variables involved. Basically, based in Westchester County, NY 2750 sq ft house R15 insulation in all walls R38 ceiling (cathedral ceilings) air filtration is 2554 CFM50 contractor's calcs ended with a 40k BTUH @ 10 deg During the last 12 months we have used ca 1000 gallons of oil (on a 15yr+ 80% efficient oil furnace) Based on this, contractor has proposed a 3 ton system. Other feedback, and looking at other numbers posted in various forums, has suggested that a 3 ton system is not sufficient so I pushed back on the contractor. He stands behind his numbers and has offered a guarantee (in the contract) that if more than 5% of the the electricity costs in running the system comes from the back up heat source, he will, out of his own pocket, cover any expenses that increasing the tonnage would incur. My expense would be that I need to pay for a system (ca $300) to monitor the electricity usage but he will install. The contractor has been very diligent during this sales process. He is very responsive to all questions, providing references, proof of relevant licenses, insurance coverage details etc. All this contractor does is geothermal systems. I'm inclinded to go ahead with him (he wasn't the cheapest), especially since he's prepared to offer the guarantee (only good if he stays in business of course!) but have a nagging doubt about the size of the system due to other quotes we received (4-5tons) and what I have read here. Would appreciate any feedback on what other folks would do in this situation. Thanks!