The wiring harness itself does not have a name per se, however, the connector is what is commonly referred to as a Molex connector.

They come in various wire gauge sizes and pin counts. The indexed body is made of nylon and the male/female pins are crimped to the wires and lock into the connector body when inserted.

This is most likely a factory made harness - but it could also assembled on site.

To get the exact harness replacement you would need to contact the furnace manufacturer. To replace the Molex connector you would need to check websites such as Digikey that sell the connectors and crimp tools.

Might be simpler, if you have enough slack, to cut off the connector and wire nut the connections you wish to make.

Internal appliance wiring does not follow the NEC.

Smaller gauge conductors with higher temperature rated insulation are very common.

NEMA has guidelines, but manufacturers are pretty free to do as they please as long as they can pass UL certification (and it does not look all that much at internal practices, they are a safety organization ad look at the whole apparatus).

DigiKey would likely have compatible connectors if you want to search (and deal with a $25 minimum order).
How is the oter end of the wires in the connector terminated?

That might be an easier location to break the circuit.

You also need to be aware that internal wiring often does not follow a well standardized color code.

Take lots of pictures and notes, and watch for temperature ratings on wire insulation.

Interrupting outside the unit, then mounting the control device on the unit is likely to be far easier.

Ok, thanks guys. That is what I suspected. It looked to me more like an automotive harness than anything... I'm not going to bother with trying to find another harness through Digikey. I was able to pull 2-3 inches of wire through from the aquastat. I'll cut it and wire the switch in.

The reason for doing all of this is that I put solar hot water in last year. The solar tank feeds into the boiler (oil-fired tankless coil). This is no problem during the winter since the water in the coil in almost always hot since the boiler is running anyway to heat the house. But in the spring, summer, fall - the boiler only runs to heat the water in the coil. This is a big waste of oil since there is 150 degree water coming in right behind the water in the coil when someone uses hot water. There is no point in running the boiler to maintain temperature in the coil since the water coming into the boiler is already hot.

I could just unhook the harness every spring and then reconnect it in the fall. But if we use more than 80 gal of hot water (bath tub), then there's no hot water left till the sun comes up again. So I want a switch on the side so that anyone can turn the boiler back on with the flip of a switch. The emergency cutoff switch kills power to the thermostats around the house, which I don't want to do....

Glad you found enough free wire in the harness.

That eliminates any real considerations of temperature rating.

I'm glad you found enough slack to extend the wiring but the concerns of the temp rating here is a little bit over blown - internal high temp wiring if required is easily found and readily available.

By the way, regarding my suggestion of DigiKey - I have used them for years and they have not had a minimum order requirement in at least 5 years.

"concerns of the temp rating here is a little bit over blown - internal high temp wiring if required is easily found and readily available. "

That depends on being able to identify what wire is needed and having a source for short lengths.

There is no requirements that enough of any marking remains to ID the insulation rating.

And cut pieces are usually at least twice the price per foot of a roll, if you can actually find what you need.

In regards to temp rating on the wiring: The wiring that is currently installed from the manufacturer is labeled 105*C which seems pretty standard.

"which seems pretty standard."

Until you get something close to a higher temperature source and than wiring upwards of 150C is used.

Or you can use whatever you think it should be and cross your fingers.

dopeonplastic,

Thank you for the follow up comments on the wire temperature rating.

While most THHN/THWN is rated at 90*C (in dry locations) it is also usually rated at 105*C for AWM (appliance wiring material). It may not be listed on the wire jacket itself but is usually listed on the spool label.

Additionally, higher temperature wire would typically not be terminated by Molex connectors.