Last month I encounterd an oil-burner furnace, one that was itself a conversion of an older coal burner. It occurred to me that I am of the last generation to actually remember coal-fired furnaces, so I thought a little 'primer' was in order for those who do service work.

First, an overview:

[Linked Image from i143.photobucket.com]

If you look closely, just above the blower you can see doors for loading coal, as well as clearing out the 'clinkers.'

The blower has another component attached to it. That rectangular part, under the blower and to the left, is the ingnition transformer. Carefull - these generate 10,000 volts! The black coupling on the wiring actually IS an electrical coupling; electrical hardware once had a glossy black finish.

[Linked Image from i143.photobucket.com]

Now, if you followed those wires, one place they would lead is to this gizmo, mounted on one of the ducts:

[Linked Image from i143.photobucket.com]

Called a 'stack controller,' this is what tells the furnace to fire. The loose wires you see are the thermostat leads.

The proper wiring method is: dedicated circuit -> disconnect -> stack controller -> blower assembly -> neutral.

What I actually found in this basement was 'does it all circuit' -> blower assembly -> stack controller -> disconnect -> neutral. This lead to two undesireable things:
1) The basement light was out when I turned off the furnace power; and,
2) The HVAC guy got shocked even though the disconnect was opened; everything was still 'hot,' the result of having the disco LAST, rather than first.

How did this happen? Well, the original sparky wired the knob & tube up that way (white wire as 'hot'), and the various homeowner additions only made it worse. Here's the splice:

[Linked Image from i143.photobucket.com]

Finally, here's the basement lighting I was deprived of with the furnace power off:

[Linked Image from i143.photobucket.com]