The outdoor hot water furnace is usually centrally located and used to to heat
multiple buildings - house, garage, shop, barn, hot tub, swimming pool, grain drying, etc.

A typical outdoor boiler installation is the pumping of heat transfer fluid
(recommended minimum 30% propylene glycol solution)
from the outdoor boiler via insulated piping buried underground to the building to be heated.

The heat transfer fluid then typically goes through a side arm heat exchanger which is mounted
on the existing hot water tank to heat the domestic hot water and then goes through a radiator
is installed in the existing forced air furnace that is controlled by a separate thermostat.

The conceptual drawing below depicts a
"Typical Residential Furnace & DHWT Connection"
to an outdoor central heating system.