Tankless systems are engineered to heat potable water, so any antifreeze would make that water deadly to drink. Also, most are not designed for potentially continuous use 24/7, which might happen during a cold spell. Then, most do not have a huge volume capability and may contain an internal restrictor. A boiler is designed for this use...similar, but not the same. A few can be used for this application, and still have a warranty, but is some cases, that warranty is shortened significantly verses use as originally designed to heat potable water. Also, keep in mind that with any use of antifreeze, it carries with it an efficency hit as it does not transfer heat as effectively as pure water does (course, you'd want that for both a boiler and a HW source like the tankless if you're going to turn the heat off in the winter).