Low voltage cause furnace to run too hot?

[nathaniel]

I have an old converter in an '86 Airstream. I have ordered a new one and will install it when it arrives. My question is this: I also just installed a new furnace and it is running hot (air temp at register is about 260F - then the burner shuts off [I assume a safety switch], fan keeps running, temp drops, and it starts up again). Could low voltage from my converter (I measured 11.1V under load when the furnace is running) cause my furnace fan to run so slowly that not enough air is moving over the heat exchanger and it is overheating?
Thanks for any help and advice!
Nathaniel

[rmkrum]

Could easily be the root cause.


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[OTRA15]

Sounds possible, but isn't temp. of 260 way too high? I would consider that the furnace, high temp sensor, or something else is out of whack. Could the low voltage have affected the new furnace's circuit board?

[dznf0g]

I think the furnace is operating normally and the safety provisions are doing their job. 11.1 volts is virtually a dead battery. If you have an automotive battery charger, hook it up while the furnace is in use until your new converter arrives

[TomKirk]

Your furnace should have a low voltage cut-off switch. When operating properly, burner will not ignite but blows cold air at significantly reduced speed.

For low propane PSI, furnace will not ignite but blows cold air at normal speed.

In normal voltage vs low voltage, fan speed will vary significantly.

Q: Does furnace operate normally at full voltage??

1) Do a battery load test.

2) Also, check battery cell water levels.

3) Recharge battery, then test again.

Tom

[USAtraveler]

I installed a new furnace earlier this year. It works well. When battery voltage drops too low (down to 10v or less) , the fan will run weakly but the burner will not light off. Recharging the battery gets everything working again properly.

[OTRA15]

Originally Posted by dznf0g

I think the furnace is operating normally and the safety provisions are doing their job. 11.1 volts is virtually a dead battery. If you have an automotive battery charger, hook it up while the furnace is in use until your new converter arrives

Isn't a discharge air temp of 260 F a bit high?

[dznf0g]

I think that depends upon blower speed and airflow, both at the overheat switch and at the exchanger. Unknown unless one were to experiment

[nathaniel]

Thanks Tom. Batteries are definitely dead. The new converter should arrive soon, at which point I'll be able to test the furnace at full voltage. Hopefully all the problems disappear. My concern is just that the temp is so high. Shouldn't the burner run basically all the time the furnace is heating instead of cycling on and off on the limit switch, which it seems it's currently doing? Maybe I'm wrong here, but my understanding is that the furnace running, with burner and fan, should have a top temp of 200F until the t'stat is satisfied. Is it ok for the limit switch to be operating routinely, or will it eventually die under so much use?
Thanks so much for your help!
Nathaniel

[dznf0g]

Nathaniel,

The thermal safety switch is simply that...it goes open at a set temp, shutting off the gas flow, (I have no information on the rating spec of the specific switch) and then the blower will cool it to some temp below the "open" rating and it will close. When it closes, gas and ignition resume...and the cycle will repeat itself, if the overall blower speed and resulting airflow over the switch and exchanger is not great enough to keep the switch relatively cool and closed.

[eubank]

If your furnace has a sail switch, the low voltage would cause the fan to slow, and the lowered air flow would fail to keep the sail in the flow position. In other words, you need lots of air flow to keep propane flowing, and that requires rich voltage. (You also need a sail switch that hasn't gotten filled with dust. ��)

Lynn

[JCWDCW]

260 degrees is hot enough to melt the plastic duct used to feed to the air vents.

Years ago we used the furnace with a low battery. The hot discharge from the furnace cause release of vapors from the plastic duct which caused the Propane alarm to sound as it picked up volatiles.
JCW

[HowieE]

Hook up your car battery to the system and see if there is a difference in the fan speed when operating on full voltage. Jumper cables will work. If the trailer battery is dead lift the ground terminal cable off the post and attach the cables to the positive battery post and the floating ground cable.

Your problem is either low fan speed of a cracked heat exchanger. Place a carbon monoxide detector in front of a vent will test the heat exchanger for leakage.