I have a heat pump system with gas backup. One pro said they always install flow-through humidifiers on duct above the furnace. The other said they prefer to install a bypass type on the return for the reason that 1) if it leaks it won't ruin the furnace (they apparently encountered that many times. 2) in summer time with cooling on they see humidifiers sweat and drip condensation when installed on duct.
What is your experience? What are pluses and minuses of each install?