HVAC knowers, need advice: we have an indirect water heater - runs off main high-efficiency condensing boiler. Did a normal boiler replacement, but internal switch to water heater has failed in several iterations. Help w/alternatives needed. 1) water heater with own heat source, but less efficient + fossil fuel. 2) Tankless: also fossil fuel plus lower throughput. 3) Electric: less efficient? Slower to recharge? 4) Heat pump: slow recharge + enuf on cold days? Thanks for advice.