"Ivan Vegvary" wrote in message
...
Contemplating adding an ADU (Accessory Dwelling Unit) to an existing
residence in Portland, Oregon. In California this was called a
"Mother-in-law Apartment".

Unit to be in basement. Main floor already has owner and two room mates
sharing utility (gas and electric) bills. ADU will be 2 bedrooms and a
large open kitchen-dining-family room. Separate street entrance.
Approximately 800 square feet.

HVAC guy says electric heaters are out of the question due to utility
costs. Next best would be 3 mini-splits (different sizes). Down side is
the high initial equipment cost ($4-5 k) plus installation. Also it's
kind of wasteful since the air-conditioning capabilities of the
mini-splits will rarely be used.

Most favorable would be keeping the existing furnace and adding
additional ducts for a separately zoned system. Low initial cost, only
one piece of equipment to maintain, easily serviced and easily
replaceable.

Said furnace is gas and I need advice on how some of you have split the
utility bill. To date the heating (furnace-air conditioning and water
heater) averages about $70 per month. If we follow this model (splitting
cost) what prevents the basement tenant from cranking up the heat to 90°,
either on purpose or carelessly while away at work?

There are plenty of solutions to separate the electrical charges (second
service, private meter etc.). However don't see an easy way to split out
the bill for a single gas furnace that is zoned to take care of two areas.

I'm sure some of you have already solved this. Subject is new to me.

BTW, would it be legal for me to place a temperature sensor (full
disclosure to tenant) in their unit and ding them for heating above a
certain temperature?

Install a mechanical (not electronic) thermostat cut into the line that
controls the furnace. Install a security cage over it to prevent adjustments
and set it at the maximum temperature that you want the tenant to have. This
will override the thermostat that they have to control the temperature.