Air-to-Water Heat Pump Backup

devvar

I live in an old, somewhat drafty house in CT (climate zone 5) that has a 20 year old oil boiler with tankless coil in fair to poor condition, vented through an unlined brick chimney in poor condition, with baseboard heat. The old cast iron radiators were ripped out in the 1960's... Total heat load of the house currently is around 100k BTU/hr on design day as calculated from actual oil use and corroborated by a Manual J. Our goal is to reduce this heat load through air sealing some insulation, but we are not doing a full energy retrofit. We have installed a 5 ton air-to-water heat pump and individual fan coils in a few rooms. Eventually this heat pump system will service the entire house, utilizing high velocity ducting in the majority of the house. We currently have one wood stove that heats about half the house well, and we plan to install another wood stove to heat the other half of the house eventually. We have propane already in the kitchen, the end goal is to remove the old oil tank, boiler, and abandon the old chimney.

My question is regarding what type of backup heat equipment we should be using with our heat pump system? I would like it to burn propane since we already have it in the house and can be direct vented through the wall. The other consideration is that I want to be able to supply the full heat load of the house on backup generator if needed. This would be much easier with a propane appliance than running the heat pump on generator. I see a couple options:

1) Decent modcon boiler (I like Viessmann, Lochinvar, HTP). Probably the most expensive option, might be overkill as a backup system
2) Cheaper modcon boiler (like Takagi or Navien), indirect DHW. Less longevity, but maybe that's OK since this will run infrequently?
3) Tankless water heater??? Since the water in my heat pump system will only reach 120F maximum, this could be an option. Again, lower reliability but much cheaper

I'm favoring an indirect tank for DHW, but would consider a standalone heat pump water heater if that made sense. We have 3 adults and 2 young kids in the house, we have had combi boilers in the past and been unimpressed with the DHW it produced.

This backup heat source will operate to (maybe) produce DHW year round, and only provide heat to the heat pump system if the heat pump can't keep up with demand or there is a mechanical or power failure. I have a hard time justifying a top of the line boiler for such intermittent use. I welcome any suggestions!

Hot_water_fan

I’d go with a minimum efficiency cast iron boiler if it’ll be used sparingly. 

devvar

Since I want direct venting I think whatever I get will have to be higher efficiency, I haven't seen anything less than 90% that direct vents. I do like the Weil McClain GV90+ for being a simple, proven design