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I have an old airco natural gas furnace with two burners that keeps cutting out in the middle of the night. So far, I have replaced the limit switch, thermostat, cleaned the pilot light, and checked the air filters which are clean. Someone told me it might be the thermocouple, but I'm not totally sure. I think that this furnace is the original one that came with the house back in 58 or 59 and was made in Vancouver, BC. When it doesn't come back on again, I can usually get it to work again by turning off the thermostat, then the power to the furnace, and then turn the furnace back on and the thermostat up and it will usually restart again. Go figure. Also, the pilot light is a standing one. If you have any bright ideas, please enlighten me, thank you for reading my post, Immanuel.

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Thermocouples are cheap and easy to replace. I'd just do that and see what happens.

A furnace of that vintage doesn't have much else to it. I doubt that there was a rollout switch back then, but if there was, you might try replacing that. It'll look like a little silver button just above the burners somewhere.

Of course, it's also possible that the thing is over-firing and the limit switch is just doing its job.

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Copied from earlier posts by the Airco Guru. You may wish to contact him-

Howard Pike

Location: Colquitlam, BC


Bio: Chimo Furnace owner since 1988

Airco/Olsen Product Engineer 1979 to 1988

BCIT Grad (Mech Tech) in 1979

CSB Ministries Rep since 2001

Company Information: Chimo Furnace Service Ltd

Coquitlam, BC

started building "Airco" gas furnaces under licence (that's Canadian for "license") in 1956. Basically, they were buying the heat exchangers from an American manufacturer (probably Armstrong) until they could build their own "unique" clamshells in 1957 or '58. By unique, I mean identical, until they were forced to make a small dimensional change to avoid legal problems in the early '60's.

This heat exchanger was used in the Highboy, Counterflow and Console models (AGH/AH; AGCF/ACF; and AGC/AC) from 1956 through 1985/86.

Howard P

the AGH is "A"irco "G"as "H"ighboy. It probably has three cast iron burners. It's life expectancy is unknown, for while Terasen will use photos of old Airco's to pitch their replacement programs, there has been NO failure pattern for the Airco Highboys. Sixty years is reasonable. The heat exchangers were amply wide and sufficiently tall to keep the flames well away from the metal. In a normal home, the Airco heat exchanger doesn't rust or crack. The 100-AGH is the smallest input in the Airco three burner furnaces, so it's the furnace with the longest life-expectancy. The furnace had a belt-drive 1/4 HP (was originally a 1/6 HP) motor. It should use a 24 volt thermostat.

It's a great furnace, and very reliable. However, it was rated at 80% efficiency in those days - that's steady-state efficiency. But, that was the reality plus 5%. The same model by 1986 was only 76% steady state and figured to be about 55% AFUE - Annual Fuel Utilization Efficiency - over the year.

When Airco came out with the Airco Turbo at 81% AFUE in 1981, several tests were done to compare the new mid-efficiency's to the old standard-efficiency furnaces (what you have). The average savings was 23% over the winter. That means that if you were to change out your Airco and put in a new High Efficiency furnace (which our government requires for new installations), your savings should be about 35%.

And yet, you have one of the most reliable furnaces ever built.

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  • 2 weeks later...

I have an old airco natural gas furnace with two burners that keeps cutting out in the middle of the night.

Man, yours is a challenging problem. In 1958-59, the Airco furnaces were probably using the earliest version of the Robertshaw combination gas valve. If so, then you have a red button that needs to be pushed to get the pilot light lit and held. That old valve has known electrical issues with the contacts for the push button. These repeated nuisance-problems have made replacing these valves the only remedy.

But perhaps it's something else. The problem is that there are virtually no mechanical problems that can be fixed by turning the power off then back on. It's got to be an electrical problem, but none that I'm familiar with. There just isn't enough information to diagnose this on-line. Sorry.

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