With storage glycol adds a lot of money to the system and it don't transfer heat as well. Your better off to spend the money on a battery or generator backup IMO. I don't Know the formula to figure the temps you need to run in your floor but maybe someone else will chime in.
There are a few ways to handle your DHW. You can add a side arm to your electric water heater. You can use a flat plate and flow switch for an on demand type set up, you can have a coil in your boiler or storage tank, or you can get an indirect water heater. the boiler water will always be the same water and never touch your DHW.
There are a few ways to handle your DHW. You can add a side arm to your electric water heater. You can use a flat plate and flow switch for an on demand type set up, you can have a coil in your boiler or storage tank, or you can get an indirect water heater. the boiler water will always be the same water and never touch your DHW.