Answer
Jun 18, 2019 - 01:27 AM
The cycling of the burner during a call for heat is not a normal operating condition. The furnace can cycle off the limit switch in high ambient conditions. The limit switch is a safety device it will open to protect the furnace and the RV. It simply reports to the Module Board when it’s too hot. The Board kills power to the gas valve and electrode but keeps the fan blowing to help cool the Limit Switch. Once the Board kills the power to both those parts it starts a timer for 5 minutes. If within those 5 minutes the Limit Switch closes and there’s still a demand (thermostat is still closed) then the board will reenergize the Gas Valve and Electrode therefore reigniting the furnace.
The Limit Switch opening reporting it’s “too hot” is the Limit Switch doing it’s job. We just have to figure out WHY it’s having to do its job. So we look at 4 basic things.
Things to check with a limiting issue:
Ducting, minimum amount, correct size, no 90 degree turns, not collapsed, no extra-long runs, free and clear of debris. Register location compared to return air.Return air minimum size adequate. At least 55 square inches. Proper clearance to the nearest air register needs to be at least 18 inches away.Motor Speed (Amp draw). For the NT12S the amp draw should be 2.8 amps at 12.0VDC. Keep in mind voltage will influence the amperage. The higher the voltage the lower the amperage and vise vera. Limit switch - The limit switch is installed as a safety device to prevent overheating, if it opens, there has to be a reason why. We don’t want to punish it for doing its job, we want to find out why it is having to do its job, but at the same time the Limit Switch is a naturally CLOSED switch. If it’s had to open a lot it may become weak over a period of time therefore starts to open at a lower temperature and will need to be replaced.
The Limit Switch opening reporting it’s “too hot” is the Limit Switch doing it’s job. We just have to figure out WHY it’s having to do its job. So we look at 4 basic things.
Things to check with a limiting issue:
Ducting, minimum amount, correct size, no 90 degree turns, not collapsed, no extra-long runs, free and clear of debris. Register location compared to return air.Return air minimum size adequate. At least 55 square inches. Proper clearance to the nearest air register needs to be at least 18 inches away.Motor Speed (Amp draw). For the NT12S the amp draw should be 2.8 amps at 12.0VDC. Keep in mind voltage will influence the amperage. The higher the voltage the lower the amperage and vise vera. Limit switch - The limit switch is installed as a safety device to prevent overheating, if it opens, there has to be a reason why. We don’t want to punish it for doing its job, we want to find out why it is having to do its job, but at the same time the Limit Switch is a naturally CLOSED switch. If it’s had to open a lot it may become weak over a period of time therefore starts to open at a lower temperature and will need to be replaced.
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