"TN" wrote in message
Edwin,
Can you explain the difference between two and three zone heat? Does that
mean a bigger furnace and an extra thermostat, or something more costly
and complex?
Thanks.

Thermostat is the cheapest part of the job. You have to have separate
distribution of the heat to each zone. That means different piping or
ducting.

As for the furnace size, it would be the same. When deciding how large a
heater, the building is gone over, room by room, to determine the needs.
You take into account the type of construction, the amount of insulation,
exposure, roof type, expected minimum or maximum temperatures in the
climate. Adding it all up gives the total size needed. You can go with one
large or two smaller units to get the same effect.

Not seeing the layout of the house it is not possible to give a definite
answer. If the rooms in question are distant, it may be difficult to get
the heat from the heater to the rooms. There are losses along the way. If
the rooms are such that they need more heating that other parts of the
house, it is not efficient to fire up a large heater to do just two rooms.
It may be more economical to have a smaller supplemental unit. Only way to
decide is to see the actual house plans and work from there.