You could be a real hard a$$ and demand a closed loop system able to deliver same efficiency as the SCW without additional cost. Assuming that's not feasible or reasonable and you choose to meet them part way, beware being shortlooped. Perversely enough you may even need a system with slightly increased nominal tonnage to meet the load with the less-favorable entering water temperatures likely to result from the substitution of closed loop instead of SCW. System design must take all these factors into account. We write often of Manual J and D calculations, but switching loop method requires at least a cursory glance at Manual S. In other words, make sure the selected tonnage unit will still meet the house loads given different entering water temps likely to arise with the switch to closed loop. Let me revisit the risk of being shortlooped: Whoever bears some or all of the increased cost of the well drilling stemming from the switch from SCW to closed loop will be financially motivated (potentially to the tune of 5 figures) to provide you the least possible total bore length (shortest possible loop field). That may result in your system operating at both reduced comfort and greatly increased electricity cost. We professionals here and the geo industry in general has an interest in keeping that from occurring, because one bad job might damage prospects for hundreds of others nearby. If you must switch to closed loop insist on a proper and realistic loop field design that meets your loads at high efficiency.