Building up a large balance in a health savings account can be a smart financial move to cover medical expenses in old age. But dying with a hefty HSA can pose tax problems for heirs — specifically, non-spouse heirs like children, grandchildren, friends and others, according to financial planners. It's the "big unknown" that people don't understand about the tax-advantaged accounts, said Carolyn McClanahan, a certified financial planner and founder of Life Planning Partners in Jacksonville,