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04gt 12-10-2009 06:27 AM

Natural Gas Furnace Limit Switch
I have a 17 year old Tempstar 100,000 Btu input, 80% efficient natural gas furnace. Furnace works great except for when I raise thermostat setpoint by about 3 or 4 degrees and the furnace runs a while to get to the setpoint. After about 20 mins. the burners go out for about 30 sec and then reignite. I have confirmed with a voltmeter that the main limit switch is opening at this time. When the furnace runs for less time, no problems. Manifold gas pressure is 2 in h2o (3.5 in h2o is spec) and temperature rise is 50 degrees F ( 30-65 degrees F spec), clean non restrictive filter, all return and supply vents are fully open. Any other ideas?

yuri 12-10-2009 05:48 PM

Replace the limit control it is weak. When it trips it should stay off for several mins. It is weak and kaput.

04gt 12-11-2009 05:59 AM

Thanks for the reply. I'll consider replacing the limit switch. I experimented more yesterday with my issue. I installed the foam type washable air filter that was in the furnace when I bought the house. Furnace ran fine for over a 1/2 hour with a constant 50 degree temp rise and never tripped the limit. I had been running a cheap blue fiberglass filter which I thought was about as non-restrictive as you could get. The house is about 50 years old and the furnace is 17 years old. Possibly the duct work is not optimal for the retrofitted furnace and its sensitive to airflow/filters?

yuri 12-11-2009 06:40 PM

You may want to get an experienced Pro to check the heat exchanger of that furnace. They are VERY prone to cracking and popping the compression rivets which hold them together.

Yoyizit 12-11-2009 09:24 PM


Originally Posted by yuri (Post 365215)
They are VERY prone to cracking

Doesn't Tempstar get product liability or wrongful death lawsuits? Especially if they know about it?

"Lennox, Tempstar and York have supporters, including James Dulley, but reliability is below average, according to the latest survey data (which is quite outdated). Dulley commends the Lennox two-stage model for quiet operation."

yuri 12-12-2009 07:50 AM

The heat ex have a 20 yr warranty and that covers them.

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