Dedicated Line Advise

Lowrider - if your panel is like mine (i.e. a two bank North American panel ) - the adjacent switches in each "bank" of switches (i.e. if you have multiple banks) alternate between the two phases - e.g.

         BANK "A"               BANK "B"
Switch #    Phase     Switch #    Phase
    1                A              15           B
    2                B              16           A
    3                A              17           B
    4                B              18           A 
    5                A              19           B
etc...

For your two phase devices you will see a  "linked switch PAIR" which takes two adjacent spots - one switch of the pair is connected to phase A and the other connected to phase B (or vice-versa)

You need to make sure the electrician knows you require both lines on the same phase.

He/she should understand how to connect to the same phase

FYI - It only takes one two phase device to "pollute" both phases - so your goose is cooked - along with most other North American homes.

The only real protection against noise introduced by two phase devices is to get
1. an Equitech transformer $$$.
2. a power re-generator

I would not be too worried - unless the power in your area is prone to brown outs or surges, the noise from the furnace switch dissipates really quickly down the power-bus

Which switch bank is immaterial, but the farther away from the two-phase device switches the better

This link shows you a typical two-phase panel layout
https://www.google.ca/search?q=two+phase+house+distribution+panel+pic&num=20&tbm=isch&tb...

Regards...

forgot to add, the most noise you will hear from two phase motors is when they turn off and on, normally via a solenoid relay. Any more than that and I would co sider having the motor replaced - it's past its prime.

My furnace has a setting that runs the fan at a low speed all the time. so there is no switching as such and no high current spikes on the supply, which can cause a brief voltage drop, depending on your supply.

regards

One last thing - while you have the electrician there, get them to check all the connections and the state of the power bus.

if the panel is 40+ years old it might be worth getting it upgraded - all things oxidize with time and bad joints can aggrivate noise issues

That's it ! :-)

willie, I appreciate your time.
First, all electrical is modern and up to code.

Second, I have a different circuit box, it's for a small row home and the layout is like this...
BANK "A"             BANK "B"
Switch #               Switch #     
1                A             2
3                B             4
5                A             6
7                B             8
9                A            10
.... up to # 20

**I found a diagram of my box online and this is what was posted.
-------------------------------------------------------

#1, 3 are a "linked switch PAIR" for furnace.
#2, 4 are linked and marked as MAIN DISCONNECT.
and BTW, the electrician placed my audio line in #5.

Are you familiar with this type of box?

Lowrider, I’ll just add to the many excellent comments that have been provided the suggestion that it may be worthwhile to re-read this thread from about two years ago. Note particularly the comments by electrician expert extraordinaire Jea48 (Jim). And in regard to ground loops, which you had asked about in your initial post above, note especially his reference to pages 31 through 37 of this paper.

It is explained in those pages that what "drives 99% of all ground loops" (to a greater or lesser degree depending on the designs of the specific components that are involved -- my words) is lack of uniform geometry in the power wiring. Which in turn results in the magnetic fields surrounding the hot and neutral conductors, that would ideally cancel each other perfectly at the mid-point between the conductors due to the currents which produce those fields being in opposite directions, cancelling less than perfectly, and therefore causing voltages to be induced in the safety ground wire. Which in turn will result in ground loop issues to a degree that depends on how safety ground and signal ground are connected to each other within any pair of interconnected components.

As you’ll see in the paper, the Romex you indicated is being used in the existing run is a good choice in that regard, given its essentially uniform geometry.

Note also the list of common sources of high frequency noise, on page 37.

Good luck. Best regards,
-- Al