Getting the Old Fraser Johnston Gas Furnace Running

This is a 70's vintage standing pilot natural gas furnace made by Fraser Johnston.

Disclaimer: I make no representations or warranties of any kind, express or implied, about the completeness, accuracy, reliability, suitability of the information contained in this video for any purpose. Any reliance you place on such information is therefore strictly at your own risk.

This Fraser-Johnston gas furnace is rated at 100,000 BTU/hr. gas input and 80,000 BTU/hr. of heat output (i.e. 80% steady state efficiency). Likely AFUE rating is in the 60% - 70% range. It's at least 2X the size needed for the 1200 sq.ft. house in my climatic zone, especially now that it actually has insulation in the walls and attic, unlike when the furnace was installed on the '70s. Not sure of the actual age of this furnace. It doesn't even have a blower door switch that I can see.

It's been at least 15 years since the furnace was actually used for heating the house on brief occasions. It was last fired up in 2011 for a full check by the gas company.

Not sure how the wiring was messed up, it was working fine in 2011 after the furnace was last operated. Opened it up in 2020 and wow, what's up with that? It's not like the wires could have slipped out of the push-in terminals. You need to insert a small screwdriver into a slot and push down hard to release a wire so it can be pulled out. The high-limit wire is a much thicker stranded cable and there's no way that can fit into the gas valve terminals. As it was, both wires from the 24V transformer weren't connected to the gas valve. The only connection on the valve were the 2 thermostat wires, effectively shorting out the gas valve.

Found the wiring diagram (link below) and corrected the problem. Simple series circuit:
24V transformer - high limit switch - thermostat - gas valve - return to transformer
For furnace to run, thermostat must be closed and the high limit switch must remain closed (it opens if high limit temperature is exceeded).

Yes, it's an old, inefficient behemoth. But it's rarely used, requires no regular maintenance and it's paid for. This furnace is only used as a 2nd backup heating source. Main heat for the house is is solar hot air, backup to that is a wood burning fireplace insert. Apparently the house originally had 2 in-wall gas heaters, one in the dining room and one in the bedroom hallway. There may have been some sort of issue with those, so they were removed and this unit was installed.

Also some sort of "approved" HVAC system, like this furnace, is likely needed to maintain an occupancy permit for the house. Ultimately I would like to replace it with a mini-split system or a central heat pump system/air handler to make use of the existing duct work under the house and get heating and cooling. Added benefit of a mini-split system would be regaining use of the only non-bedroom closet in the house by removing the old furnace.

Gas valve wiring:
https://www.hvacrschool.com/th-tr-thtr-gas-valve-terminals/

Furnace checked out by a utility gas technician following this work.

@grayfurnaceman has an excellent playlist on old gas furnaces:
https://www.youtube.com/playlist?list=PLItXrh64d2JN_IZgSy-Aim-SEo7uMlJrw

Furnace is older than 1979 as it's missing and Energy Guide sticker:
https://en.wikipedia.org/wiki/EnergyGuide

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Intro/Outro:
Louisiana Fairytale by Austin Rogers
https://web.archive.org/web/20170402222425/http://drfiddle.com/show_tune.php?id=94