oil fired furnace X-10 off/on swich control

Isn't there an X-10 device that operates low-voltage relay contacts? I'd put the manual switch and the X-10 device in series, so OFF on the manual switch means OFF, so you don't surprise anyone doing maintenance.

Another possibility is to find the power supply/transformer powering the low-voltage circuit, and cut off the power there with a 110VAC X-10 device. My (gas) furnace has a 110VAC switch controlling a low voltage (12 or 24V, I think, it's been a while since I had to replace the transformer) transformer which powers the thermostat. I don't know whether oil heat is set up this way, but *something* needs to power the circuit, and my guess is it's not batteries. (Beware that smart thermostats may need power to keep the program memory functioning, although this IS often done with batteries.)

Parallel would work better...

Yes, parallel would work - but no better and, for safety reasons I think I'd stick with series which would require both switches to be on to operate. That way, as mentioned in another responder's post, turning off the original switch for service would positively shut down the furnace so that service personal could work safely. If wired parallel, the main switch could be off but if the UM-506 turned on while the service tech was performing service, the tech could be seriously burned or injured by the blower fan. I know I wouldn't want to count on others in my family - or even myself - to remember the schedule and override it when being serviced. I'd hate to see my service guy hauled away in an ambulance with severe burns or other injuries because of a poor wiring choice. Also, while I'm no expert on HVAC codes, I suspect that the parallel wiring might not meet code for the safety reasons described above.

Modern codes require a complete power disconnect to be within easy reach of all HVAC equipment. For service work the tech should shut off this switch. The low voltage switch the OP asked about is the equivalent of the thermostat wiring. NO tech would ever simply turn off the thermostat before servicing a unit.

If the switches are in series both have to close to operate the furnace. Not ideal since it would require the X-10 bit to stay closed to get heat. This is a bad idea since a X-10 glitch could leave you without heat in the middle of the winter.

In general I wire automated controls in parallel with thermostats so they provide supplemental control.

Each to his own but I strongly feel that paralleling the switches is MUCH better and would pose no safety issue. If you want the advantages of both methods simply add another another switch in series with the paralleled switches or replace the switch with a center off DPDT switch that would provide X-10 OFF ON positions.

/ power -------o o o--------- furnace |/ | o o o | | | | / | --o o-- X-10

The top switch is a center off DPDT with the center terminals connected. The lower switch is a low voltage X-10 module. In the left ON position the low voltage X-10 switch has control. The center OFF position and right ON position work like current switch.

That sounds like the ideal solution, because the OP's original specs indicated that there would be two modes of operation based on the season. I'd also add a label on the DPDT switch or nearby that clearly indicates the switch's function in case it's serviced by someone other than the OP at some point. In addition to the words

X-10 | OFF | ON

on the labels I would probably add below that

SUMMER | OFF | WINTER

to make it a no-brainer.

-- Bobby G.

Whoops!

I was done in by my own over complication. A center off SPDT switch is all that is needed. The previous DPDT drawing will not supply power to the furnace in the right position...

------ | | | / | power -------o o o ------ furnace | | | / | -o o-

The top switch is a center off SPDT switch, the lower switch is a low voltage x-10 module like the UM506.

In the right ON position the low voltage X-10 switch has control. The center OFF position and left ON position work like current switch.

So it is ON OFF X-10 or as Bobby pointed out in simplified terms SUMMER | OFF | WINTER

** Or if you want to keep the switch positions X-10 OFF ON here is a schematic:

power ---------- |/ o o o--------- furnace | | | / | --o o-- X-10

I kind of like the top one since the furnace is connected to nothing in the center off position. It is a little easier to "read" as installed. Sorry, it was late...