An outside insulated shed.
Maybe even on a trailer, so it go with you when you leave.
Yoy could heat a mass, then run air over it.
Blow through insulated duct in a window.
If there is a forced air system,
feed the heated air into the return air vent.
Draw air to be heated through an insulated duct from a basement window.
If using water, use no mass outside, just the boiler. If the boiling vessel is open to.the air,steam explosions can be avoided. Heated water can be drawn directly from.the vessel,or through a coil that passesbthrough the vessel.
The bulk of the water would be inside. I would bring it into a storage vessel,and again I would try to piggyback onto the existing system,by parking it next to or inside a return air duct.
I would like to move the heat more efficiently via steam.
By purposefully boiling the water into steam and then cooling/condensing it inside the house,we move more heat,without a pump.
We can discard the tepid water,we can use it in laundry,flush toilets, water indoor plants,pump it back out to the boiler,or out to a
greenhouse.
We could preheat our potable water by running a coil through the storage vessel.