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I am attempting to run some new cable on the second floor of my 100-year-old house. The cable is for a thermostat for a 240V baseboard heater, and needs to be located roughly in the center of the second floor (so that it can be placed on a wall opposite where the heater is currently installed).

I've explored a dozen ways to get the cable where I need it to be, but my most promising option seems to be using some abandoned knob and tube to pull through new cable -- I can access one run of knob and tube on each side where I need access, but there are three floor joists in between.

But there are two problems: the NM (I'm using 12-2) fits through the hole in the joist, but not through the tube. And of course the tube doesn't fit through the hole either. So if I attach the NM to the old wire and pull, it will only go as far as the middle joist, where the tubes will bunch up.

Here's a rough illustration:

Pulling wire through floor joists

What other options do I have? Could I... pull hard enough and smash the tubes? Pull a cable saw through and cut the tubes?

The exterior walls are filled with spray insulation, and the only interior wall that might offer a path has a top plate that I can't get a fish tape through, and can't access to drill through. I'd prefer not to open up the ceiling if I can avoid it, and limit holes in walls to the one spot where I'll be adding a junction box for the thermostat.

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  • Don't think you can use the tubes anyway for the cable, Too small/tight for heat to escape. All walls should have a top plate, and people usually just drill a hole big enough though them.
    – crip659
    Nov 22 '21 at 15:40
  • Are you trying to wire the heater and the thermostat? Or do you already have a heater and are just adding a thermostat? If the heater already exists (sounds like it does) and functions (not so clear) then maybe you can run a low-voltage thermostat (much thinner wire) that controls the heater via a relay? Nov 22 '21 at 16:04
  • @manassehkatz-Moving2Codidact yes the heater is wired and working using a simple dial on the unit, so I want to add a thermostat across the room to make better use of it. Running a thinner wire is a possibility, but I'd prefer something off-the-shelf. What combination of thermostat and relay could I use for this situation?
    – LShaver
    Nov 22 '21 at 16:07
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    Not sure of specifics as I am neither an expert on this nor done it myself (I have a "typical" furnace/AC with low-voltage thermostat). The basic concept would be a transformer connected near the heater to supply power, a relay wired in line with the heater to control it, and a multi-wire cable (typically 18 AWG) going from the transformer (to power it) and the relay (to control it) to the thermostat. Nov 22 '21 at 16:22
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    Yes. The transformer and relay could be installed either inside the heater, or at the service panel if you want to control all the heaters on the circuit. The wire between transformer/relay and thermostat would be 18/3. (you want that third wire to support smart thermostats). Honeywell/Aube makes a relay-transformer in one unit that would work great. Some make the transformer "common" ("C" wire) available, others do not. Nov 23 '21 at 0:51

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